- Email: [email protected]
HxMoO3@C nanobelts: Green synthesis and superior lithium storage properties
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
HxMoO3@C nanobelts: Green synthesis and superior lithium storage properties Yeping Song a,b, Hai Wang a,b,*, Zihua Li a, Naiqing Ye a, Linjiang Wang a,b, Yong Liu c a
College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, PR China Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, PR China c School of Physics and Engineering, State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, PR China b
article info
abstract
Article history:
MoO3 could be a promising high capacity anode material for lithium ion batteries (LIBs).
Received 13 November 2014
However, its applications have been hindered by its poor electronic conductivity. To
Received in revised form
address these issues, in this work, HxMoO3@C nanobelts were synthesized by mild hy-
7 January 2015
drothermal treatment of precursor MoO3 nanobelts only with the assistance of glucose and
Accepted 8 January 2015
ethanol. Subsequently, a catalysis-insertion model was proposed to describe the formation
Available online 7 February 2015
of HxMoO3 nanobelts. When tested as LIBs anodes, the superior reversible capacities of LIBs anodes were realized from HxMoO3@C nanobelts. The HxMoO3@C nanobelts electrode
Keywords:
exhibited superior reversible capacity of 480 mAh g1 retained at 200 mA g1 after 100
Lithium-ion batteries
cycles, and a superior rate capacity of 337 mAh g1 retained after 100 cycles at 500 mA g1.
Hydrogen molybdenum bronzes
The superior performance achieved in HxMoO3@C nanobelts anode system are attributed
Nanobelts
to the synergistic effect of the conductive HxMoO3, the enlargement of interplanar spacing
Anode material
and uniform carbon coating shell.
Insertion
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, one-dimensional (1D) molybdenum trioxide (aMoO3) nanobelts as promising anode materials for lithium-ion batteries (LIBs), have attracted much attention because of efficient 1D electron transport pathways, effective electrode/ electrolyte contact area [1e6]. Moreover, compared with commercial graphite anodes with a specific capacity of 372 mAh g1, MoO3 has a relatively high theoretical capacity of
1111 mA g1 [4]. However, their practical applications are still hindered by its relatively low specific capacities and rate properties that are caused by poor intrinsic ionic and electronic conductivity [4,7,8]. How to increase the specific capacities of 1D MoO3 nanobelts has been one of the most attractive topics in both scientific and industrial fields. The carbon-coating process has proved to be an effective approach, as they provided simple process [4]. Other ideal strategies, such as fabricating graphene conducting layer on the surface of MoO3 nanobelts, and using lithiation
* Corresponding author. Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, PR China. Tel.: þ86 773 5896672; fax: þ86 773 5896671. E-mail address: [email protected] (H. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.01.027 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
3614
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
technologies et al. were being continually reported in the last few years [1,2,4,5,8,9]. To date, how to increase the conductivity of the MoO3 nanobelts without the assistance of graphene or metal, however, still remains a great challenge due to its intrinsic poor conductivity. Hence, it's highly desirable to solve the above problem. As a promising candidate of the electroneproton mixed conductor, the hydrogen molybdenum bronze (HxMoO3) has attracted increasing attention due to their possible applications in fuel cells, electrochromic displays, hydrogen storage, and catalysts [10,11]. For such a structure, previous studies showed that the doped hydrogens are present in the MoO3 lattice at interstitial sites and bond to oxygen atoms of the MoO6 octahedron [12e15]. Moreover, the Mo5þ and Mo6þ cations are co-existent with mixed valences in HxMoO3 nanobelts [16]. Most importantly, the conductivity of HxMoO3 nanobelts is greatly improved due to hydrogen doping [14,16], which is quite attractive and beneficial to high specific capacity, cycling performance as well as rate capacity. In previous work, the HxMoO3 was obtained from MoO3 by the reduction-intercalation mechanism, such as employing Zn/HCl aqueous solution, or spillover method of MoO3 coated with Pt under hydrogen atmosphere [10,16,17]. For example, Qin et al. reported the preparation of HxMoO3 nanobelts using Mo powder, H2O2 as reactants, ethanol as reducing agent [18]. Very recently, Qian et al. further realized the controllable synthesis of the HxMoO3 nanobelts by employing the hydrazine solution as reducing agent in acidic media [16]. In addition, Li and co-workers recently synthesized a gel HxMoO3 solution using concentrated hydrochloric acid as proton sources [19]. Indeed, all these techniques have been well demonstrated for the successful formation of HxMoO3. Unfortunately, these synthesis routes are both tedious and complex, and other unexpected foreign impurities, such as Zn particles, are inevitably introduced to the final products. It should be noted that, as an important hydrogen-doped metal oxide, HxMoO3 seems to be an unnoticed group using as anode materials for LIBs. In previous studies, they could hardly provide effective information about their reactivity kinetics of lithium insertion. Therefore, based on the above consideration, it is essential to develop a clean and friendly method to produce the HxMoO3 electrodes in large-scale under mild conditions and investigate their electrochemical behavior. Here, we report the synthesis of HxMoO3 nanobelts with uniform carbon coating via a facile one-pot hydrothermal treatment process only by using ethanol and glucose instead of any toxic and dangerous reagents. The introduction of ethanol into this system can make hydrogen insert into MoO3 interplanar spacing, possibly due to the catalysis role of carbon derived from glucose. To the best of our knowledge, this is the first report on the preparation of HxMoO3@C nanobelts via one-step method. Furthermore, the electrochemical behaviors of HxMoO3@C nanobelts were investigated. The assynthesized HxMoO3@C nanobelts exhibit superior reversible capacity (563 mAh g1 retained after 100 cycles at 100 mAh g1) and excellent rate performance (337 mAh g1 at 500 mA g1) for lithium storage. Such superior performance can be attributed to the synergistic effect of conductive HxMoO3 nanobelts and carbon coating layer and expanded interlayer space.
Experimental sections and characterizations Materials synthesis All chemical reagents were commercial products used without further purification. Firstly, MoO3 nanobelts were prepared via a facile hydrothermal approach, according to reported studies [20]. Typically, 1 g ammonium molybdate was dissolved in 30 ml deionized water under continuous stirring. After 30 min, 6 ml HNO3 was added to the beaker dropwisely and stirred for another 30 min. The homogeneous suspension was formed. Then the resulting suspension was transferred to a Teflon-lined autoclave with a capacity of 50 ml and then kept inside an electric oven at 180 C for 24 h. To investigate the effects of the reaction conditions on the formation process of final products, 30 ml ethanol, 0.25 g glucose dissolved in 30 ml deionized water and a mixture of 30 ml ethanol and 0.25 g glucose are used as reactants, respectively, as shown in Scheme 1. The as-prepared powders, denoted as MoO3, MoO3-ethanol, MoO3-glucose, MoO3-ethanol-glucose were collected by filtration and thoroughly washed with water and ethanol and finally dried at 70 C in air.
Material characterizations The crystal structure was performed by X-ray diffraction (XRD) analysis with a PANanalytic X'Pert spectrometer using Cu Ka radiation with wavelength of 0.15405 nm. The surface morphologies of the samples were studied using a JEOL JSM6300 (Tokyo, Japan) field emission scanning electron microscope (FESEM). Energy dispersive X-ray (EDS) measurement was conducted using the EDAX system attached to the same FESEM. The samples used in the EDS tests were examined without further treatment. The thermal gravimetricdifferential scanning calorimetry (TG-DSC) analysis was performed on Netzsch Thermal Analysis STA 409 instrument in a temperature range of room temperature-800 C with a scanning rate of 5 C min1 under air atmosphere. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEM-2010F electron microscope (JEOL, Japan) operating at 200 kV. Raman measurements were carried out at room temperature, and the signals were recorded by a DXR Raman Microscope (DXR Microscope, ThermoFisher, USA). For MoO3 nanobelts, the 0.3 MW output of the 532 nm line of a Nd YAG laser was used as the excitation source. The laser power was changed to 0.1 MW due to some unknown reason. The obtained Raman spectrum was recorded with a resolution of approximately 1 cm1. The X-ray photoelectron spectra (XPS) were recorded on a VG MultiLab 2000 system with a monochromatic Al Ka X-ray source (ThermoVG Scientific). Vacuum pressure was kept at <3 109 torr during the measurements.
Electrochemical measurements The electrochemical performance of as-prepared samples was tested by employing CR2025-type coin cells at room temperature. The working electrode included active material, acetylene black and PVDF in a 50:30:20 weight ratio. A Celgard 3400
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
3615
Scheme 1 e Synthesis procedure of HxMoO3@C nanobelts via hydrothermal reaction.
membrane was used as a separator and 1.0MLiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1:1 volume, Novolyte Technologies, USA) was used as the electrolyte. The Cell was made in an argon-filled glovebox. The chargeedischarge cycling performance and cyclic voltammetry (CV) (0.01e3.0 V, 0.1 mV s1) was carried out at room temperature by using a battery testing system (NEWARE) at different current rates with a voltage window of 0.01e3.0 V and using an electrochemical workstation (CHI 860 D, Beijing), respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed at frequencies from 102 to 105 Hz.
conditions. The glucose is easily transformed into carbon under hydrothermal conditions. Therefore, we inferred reasonably that the as-formed carbon is used as a reducing agent to reduce the pristine MoO3, resulting in the formation of MoO2. Interestingly, only the mixture of glucose and ethanol can lead to the formation of a new phase, HxMoO3 (x ¼ 0.6). The presence of the experimental phenomenon
Results and discussion To investigate the effects of the reaction conditions on the formation of HxMoO3, we investigated its phase transformation behaviors using different reactants. The results were monitored via XRD patterns. XRD patterns show that the stages of evolution from MoO3 to HxMoO3 with the changes of reactants, as shown in Fig. 1. The short bars on the bottom of the plot represent the diffraction pattern of the standard PDF card: MoO3, MoO2 and H0.6MoO3, respectively. The results show that the formation of HxMoO3 phase depends strongly on the chemical environment of the solvent. The controllable experiments show that the MoO3 in ethanol preserves the initial phase without any color and structure change, but are transformed into MoO2 phase when glucose is introduced into deionized water. It is well known that the glucose is widely used in common carbon-coating method under hydrothermal
Fig. 1 e XRD patterns showing the stages of evolution from pure MoO3 to pure HxMoO3 as a function of reaction agents. The short bars on the bottom of the plot represent the diffraction pattern of the standard PDF card: MoO3, MoO2 and H0.6MoO3, respectively.
3616
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
mentioned above shows the obtained HxMoO3 is resulted from the interactions of glucose, ethanol and MoO3. According to previous literature and the comparative analysis of HxMoO3 and MoO3 crystal structure [16,18], the hydrogen can be inserted into interplanar spacing of (010) of MoO3 and we reasonably inferred that the hydrogen source in HxMoO3 may be from the ethanol rather than glucose. Actually, the HxMoO3 can be obtained only in the presence of the ethanol, according to Qian et al. reported results [18]. In our case, however, the HxMoO3 cannot be formed until the glucose was added into the mixture of MoO3 and ethanol. Therefore, we inferred that the existing carbon derived from the decomposing of glucose played a dual role, as the reduction agent, for example, Mo6þ in MoO3 is reduced to partial Mo5þ in HxMoO3, and catalyst in the process of the formation of HxMoO3. On the basis of the above observations, a “catalysis insertion” model was proposed to describe the formation of HxMoO3, as shown in Scheme S1. Obviously, the as-prepared HxMoO3 was obtained only with the assistance of glucose and ethanol, which is different from previous reports. This synthetic method takes place in one-step hydrothermal reaction, and only requires ethanol and glucose, making it an appealing method for the controllable synthesis of HxMoO3@C nanobelts, which would be expected to provide new opportunities for the hydrogen insertion of other similar layer transition metal oxides. Further, the as-prepared HxMoO3 powder is observed by FESEM. Fig. 2 shows FESEM images of HxMoO3 and pristine MoO3 powders, respectively. As shown in Fig. 2, the
morphology of HxMoO3 appears very similar to that of the pristine MoO3 nanobetls. Fig. 3 shows representative TEM and HRTEM images of the MoO3 and the HxMoO3 nanobelts, respectively. Similar to the FESEM results (Fig. 2), the lowmagnification TEM images of the MoO3 and the HxMoO3 nanobelts don't make much difference, as shown in Fig. 3a and c. Furthermore, it is found that a very thin amorphous carbon layer of ca. 4 nm is uniformly coated on the surface of HxMoO3 nanobelts (indicated by dashed line), as shown in Fig. 3cee. Combined with the observations of TEM and electron microscopy FESEM images, we found that the nanobelts are approximately 10 mm in length, 200e300 nm in width, and 100 nm in thickness. Fig. 3a is a single MoO3 nanobelt and its SAED pattern corresponding to [010] zone axis (the inset of Fig. 3a), showing that nanobelts grow in the [001] direction and their wide facet corresponds to (010) crystalline plane of HxMoO3. The HRTEM image of HxMoO3 nanobelts (Fig. 3f) and the SAED (Fig. 3g) reveals that the HxMoO3 is single-crystalline. According to the experimental results discussed above, we propose a possible mechanism in Scheme S1 to rationalize the formation pathways of the HxMoO3 nanobelts. The comparison of the crystal structures shown in Table 1 means that the phase transformation of MoO3 from orthorhombic phase to monoclinic phase (HxMoO3) occurs, moreover, the unit cell ˚ obviously during the volume increases from 202.95 to 211.12 A reduction-insertion process. In addition, it is worthwhile to note that there is significant change of (hkl), for example, for HxMoO3, there are a shrinkage of the b-axis and a significant
Fig. 2 e FESEM images of MoO3 and HxMoO3 nanobelts. (a)e(b): MoO3 nanobelts; (c)e(d): HxMoO3 nanobelts.
3617
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
Fig. 3 e TEM image of single MoO3 (a) and HxMoO3 nanobelts (c). HRTEM image of an individual MoO3 (b) and HxMoO3 nanobelts (d, e). (f) HRTEM image of top of HxMoO3 single nanobelt and their SAED patterns (MoO3: the inset of (a), HxMoO3: (g)).
increase of a-axis as compared with MoO3. As we known, lithium ion diffusion and transport suffer from a typical kinetic problem, for example, for the solid-state lithium diffusion, the increase of a-axis may improve the lithium-ion diffusion coefficient due to the 1D pore channel structure, as shown Scheme 2. A schematic image of HxMoO3 single nanobelt is shown in the inset of Fig. 3c. The sensitivity of Raman spectroscopy of carbon species and its ability to identify different forms of carbon bonding had been most demonstrated. Therefore, the carbon on the surface of HxMoO3 nanobelts can be easily identified. Fig. 4 shows the comparison Raman spectra of MoO3 and HxMoO3 nanobelts. In the Raman spectra, one obvious band at ~1600 cm1, which is attributed to the G band (graphite, sp2) [21], can be observed. These results further confirmed that the obtained sample was HxMoO3@C composite. Surprisingly, the Raman spectroscopy of the HxMoO3 appears too noise, even at the lower 0.1 MW laser power. There was a discernable difference detected. An unusual phenomenon was observed when the laser was injected on the dark blue HxMoO3 nanobelts. The dark blue was turned into white and there was no obvious spectral peak appeared during the test when the laser power increased. The specific reason is still not clear. As shown in Fig. 4, it is found that Raman features of HxMoO3 nanobelts are similar to that of the pristine MoO3. The vibration modes in the frequency ranges of 1000e600 cm1 and
600e200 cm1 correspond to the stretching and deformation modes [22,23], respectively. The 158 cm1 (Ag, B1g) band is from the translation of the rigid chains, the 666 cm1 (B2g, B3g) is an asymmetric stretching of the MoeOeMo bridge along the c axis in MoO3, while the 822 cm1 (Ag, B1g) is a symmetric stretch of the terminal oxygen atoms, and the 995 cm1 (Ag, B1g) is the asymmetric stretch of the terminal oxygen atoms [22e24]. No doubt, whether the carbon coating exists is a key issue for the analysis of the performance of LIBs. To confirm the presence and spatial distribution of carbon in the HxMoO3 nanobelts, we have investigated its Ka spectral profile by collecting a FESEM-EDS spectrum via the fine electron probe scanned along the length of the HxMoO3 nanobelts. The long electron beam was necessary to obtain an adequate signal from each line. Fig. 5a shows FESEM-EDS spectrum of HxMoO3@C nanobelts (inset shows the FESEM image showing the path of line scanning) and the corresponding Ka and La spectral profile (Fig. 5b).The Ka spectral profile of carbon suggests that the carbon are almost uniformly distributed in the samples. Furthermore, the elemental composition of the samples was obtained (the inset in Fig. 5a). The elemental composition analysis strongly supports the presence of carbon in HxMoO3 nanobelts. Furthermore, to determine the carbon presence of the HxMoO3@C nanobelts, TG-DSC curves are shown in Fig. S6. TG curve exhibits a slight weight loss at approximately 262 C, which can be assigned to the
Table 1 e The comparison of crystal structures of MoO3 and H0.6MoO3. Unit cells parameters MoO3 H0.6MoO3
Space-group
˚) a (A
˚) b (A
˚) c (A
˚ 3) V (A
d (nm)
Pbnm(62)-orthorhombic C2/m(12)-monoclinic
3.963 14.543
13.855 3.852
3.696 3.769
202.95 211.12
1.38 (010) 1.45 (100)
3618
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
Scheme 2 e Schematic illustration of the change of MoO3 crystal structure into HxMoO3 and the lithium-ion insertion-extraction process for HxMoO3.
dehydrogenation of HxMoO3 [25]. The weight loss at approximately 429 C can be attributed to the oxidation of carbon [26]. The abrupt weight loss at approximately 742 C can be assigned to the sublimation of the sample [25]. The XPS analysis has been used to confirm the presence, contents and chemical states of Mo and O in the samples. From the comparison of XPS survey spectra for MoO3 and HxMoO3 nanobelts, it can be seen that the XPS survey spectra of the two samples are similar, as shown in Fig. S1. With molybdenum element being concerned, the doublets of 235.6
Fig. 4 e Raman spectra of the pristine MoO3 and the carbon coated HxMoO3 nanobelts: HxMoO3@C.
and 232.5 eV are attributed to the binding energies of the 3d3/2 and 3d5/2 electrons of Mo6þ [27], respectively, which are present in MoO3 nanobelts, as shown in Fig. 6a. For HxMoO3 nanobelts, two sets of obvious doublets are detected, one being ascribed to Mo6þ (235.7 and 232.6 eV corresponding to 3d3/2 and 3d5/2 of Mo6þ) and the other for Mo5þ (234.2 and 231.1 eV corresponding to 3d3/2 and 3d5/2 of Mo5þ) [28], as shown in Fig. 6b. The Mo5þ cation is obviously detected in the XPS spectrum of HxMoO3. Based on the previous literature analysis [16], for our case, the catalysis-insertion method creates a heavy hydrogen intercalation. For MoO3, the deconvolution of the O 1s peak resulted in two peaks located at the peak 530.63 eV and 532.08 eV (Fig. S2a) corresponds to O2 in normal MoO3 matrix and the O bind of MoeOeMo, respectively [29]. After incorporation of hydrogen into the MoO3, it was observed that the higher binding energy O1s peak increased to 532.23 eV, indicating the significant enrichment of oxygen vacancies (Fig. S2b) [30]. This further confirmed that the Mo6þ was partially reduced to Mo5þ, which is accompanied by the formation of oxygen vacancies. For HxMoO3, the researchers had shown that the more hydrogen intercalated, the more electrons injected in the meantime. Moreover, the additional electron in Mo5þ relative to Mo6þ may not be strictly localized. In fact, HxMoO3 exhibits some metallic characteristics, probably due to these additional nonlocalized electrons. No doubt, the conductivity of HxMoO3 will be beneficial for charge transfer in LIBs applications. Based on a comparative analysis of the structural characteristics and conductivity between MoO3 and HxMoO3, it is expected that the lithiumstorage properties of the HxMoO3 should be more superior to that of the MoO3.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
3619
Fig. 5 e FESEM-EDS spectrum of HxMoO3@C nanobelts (a) and the corresponding Ka and La spectral profile (b).
Fig. 7a shows the representative cyclic voltammetry (CV) curves of the HxMoO3 nanobelts at a scan rate of 0.1 mV s1 in the range of 3e0.01 V. Evidently, lithium-ion insertion/desertion in HxMoO3 nanobelts electrode takes place as a multistep process, which is similar to pristine MoO3 nanobelts electrode (Fig. S3). Furthermore, it is found that the CV curves of the pristine MoO3 and HxMoO3 are very similar. Considering the similarity of their structures, therefore, we may explain the electrochemical mechanism of HxMoO3 electrodes by means of the MoO3 model proposed by most previous report studies. The first cathodic scan of the MoO3 presented an irreversible reduction peak at around 0.23 and 2.28 V attributed to electrolyte decomposition and formation of an SEI layer. Interestingly, for the HxMoO3@C nanobelts, the corresponding value is 0.19 and 2.09 V, both of which are lower than the of MoO3 nanobelts. It should be noted that the intensity of the peak position at 2.09 V in HxMoO3 is lower than that of MoO3, indicating a different SEI film caused possibly by the intrinsic
conductivity of HxMoO3. It is known that a-MoO3 exhibits electrochemical redox behavior and lithium-ion insertion/ desertion according to the following reaction [31e33]: þ
discharge
xLi þMoO3 þxe ! Lix MoO3 þ
(1) discharge
Liy MoO3 þ ð6 yÞLi þ ð6 yÞe % Mo þ 3Li2 O charge
(2)
For HxMoO3 electrode, during the first anodic scanning process, the two peaks at 1.2 and 2.5 V in the first cycle were then assigned to the desertion of lithium ions from the inner structure of the MoO6 octahedron layers and the van der Waals gaps between the MoO6 bilayers [34]. While for MoO3, there are not obvious anodic peak observed at 2.5 V except for the anodic peak at 1.2 V. From the 1st CV curves, we therefore draw that the outstanding electrochemical kinetics of HxMoO3@C nanobelts. During the lithium insertion, two
Fig. 6 e XPS investigation. XPS spectra of MoO3 and HxMoO3 nanobelt for Mo3d core level. The experiment data are fitted with the Gaussian/Lorentzian mixed functions.
3620
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
Fig. 7 e Electrochemical properties of HxMoO3 nanobelts. (a) CV of HxMoO3 nanobelts at a scan rate of 0.1 mV s¡1 in the range of 3e0.01 V. (b) Discharge and charge curves of HxMoO3 nanobelts at a current density of 200 mA g¡1 in the range of 3e0.01 V. (c) Cycling performance of pristine MoO3 and HxMoO3 nanobelts at a current density of 200 mA g¡1. (d) Rate performance of MoO3 and HxMoO3 nanobelts.
electrodes underwent an irreversible structural change from a phase to incomplete amorphous phase LixMoO3, while the long plateau at 0.43 V in the first cycle was related to the lithium insertion into LixMoO3 by a unique conversion reaction to form disordered Mo metal and Li2O, and the formation of the SEI layer [35]. This is further confirmed by 1st chargeedischarge profile of HxMoO3, as shown in Fig. 7b. Obviously, the intensity of peak of HxMoO3 is higher than that of MoO3 under the same test conditions, indicating a better kinetics of chargeedischarge. The superior conductivity of HxMoO3 and expanded crystal interplanar spacing create many opportunities for lithium ion insertion/desertion in interplanar spacing of MoO6 and the inner structure of the MoO6 octahedron layers.
Subsequently, the chargeedischarge curve lithium storage performance of the HxMoO3 and MoO3 nanobelts is investigated, respectively, as shown in Fig. 7b and Fig. S4 at a current density of 200 mA g1. It is observed that the initial chargeedischarge capacities of the HxMoO3@C nanobelts electrode are 1193 and 686 mAh g1, respectively. The capacity of the HxMoO3@C nanobelts electrode does not decay seriously, and it can deliver a reversible capacity of 480 mAh g1 even after 100 cycles (Fig. 7b). It is worthy of noticing that the capacity of HxMoO3@C nanobelts first decreases, then becomes stable until 25 cycles, and finally increases gradually with the increasing number of cycles, as shown in Fig. 7c. For metallic conductive MoO2 electrodes, anomalous behaviors have been reported in previous report [36]. In our case, two possible
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
factors may be ascribed to this unconventional phenomenon. On one hand, the LiyMoO3 have converted into Mo in the equation (2), while the deconstruction of the HxMoO3 nanobelts and emergence of lithiated amorphous LixMoO3 or LiyMoO3 may lower the conductivity of the electrodes, which will synergistically affect the lithium storage and cause capacity fading at the early stage; on the other hand, the remaining unreacted active Mo would continually increase active sites as the lithium ion insertion, resulting in an increased capacity with the increasing number of cycles. For more details, it will be discussed in the future. It can be predicted that the capacity will reach a maximum, and then gradually decreased if more cycles are applied in the cycling performance for the HxMoO3@C nanobelts, owing to the full use of active components and the deteriorating conductivity. There are also some similar electrochemical behavior for electrodes at the current of 100 and 500 mA g1 (Fig. S5). Importantly, the HxMoO3@C nanobelts electrode still exhibits an excellent cyclic performance at a higher current density of 500 mA g1, and the capacity still reaches 337 mAh g1 after 100 cycles (Fig. S5b). In contrast, the pristine MoO3 electrode only exhibits a lower capacity (initial discharge and charge capacities of 1138 and 438 mAh g1 at 500 mA g1). To better understand the advantage of the HxMoO3@C nanobelts in lithium storage, the rate performance of the HxMoO3@C nanobelts is investigated. The rate performances of the HxMoO3@C and MoO3 are shown in Fig. 7d, in which the current density increased from 50 to 500 mA g1 in a stepwise manner and then returned to 50 mA g1. Obviously, the HxMoO3@C exhibits a superior rate properties in comparison with that of MoO3 nanobelts. For example, the HxMoO3@C nanobelts exhibit much superior rate performance with 377 and 291 mAh g1, which is about two times larger than that of pristine MoO3 nanobelts electrode (159 and 127 mAh g1), at the current densities of 100 and 200 mA g1, respectively. Even at the high current rate (500 mA g1), the HxMoO3@C nanobelts can still be retained 213 mAh g1. Importantly, the high capacity of the hierarchical composite electrode can be recovered to the nearly initial values even after 30 discharge and charge cycles, implying their good reversibility. Based on the comparison experimental results of the above electrochemical properties, it is easy to raise a question: Why can HxMoO3@C nanobelts get superior performance than MoO3 counterparts. In fact, such a unique structure has several features that enhance the Li-storage properties of these the HxMoO3@C nanobelts. First, the carbon shell and metallic HxMoO3 nanobelts can improve the electron transfer capacity. The enhanced conductivity of the electrodes is a key factor for rate properties of LIBs. Second, the interplanar spacing of HxMoO3 for lithium ion diffusion and the electron transfer channel is increased, which is beneficial for lithium ion insertion/desertion in the interplanar MoO6 octahedron and further diffusion into MoO6 octahedron. This structural feature can provide a sufficient channel to enable more lithium ion access to the composites nanobelts facilely. Third, the oxygen vacancies created a number of electrochemical active sites and ensure a high utilization of electrode materials, thus leads to a higher capacity [37]. In addition to the improved cycling stability, the HxMoO3@C nanobelts also exhibit excellent rate capability with respect to the pristine
3621
MoO3 nanobelts. It's well-known that the conductivity of the electrodes, as a key factor, has an important effect on their rate performance. Herein, to reveal the lithium ion diffusion and electron transfer kinetic properties of the HxMoO3@C nanobelts, we take the electrochemical impedance spectroscopy (EIS) measurements. As shown in Fig. 8, electrochemical impedance spectroscopy reveals that the HxMoO3@C nanobelts electrode undergoes an obvious reduction in solidelectrolyte interfacial and charge transfer resistances Rct, with a simultaneous increase in the lithium-ions diffusion coefficient in comparison to an electrode made of MoO3 nanobelts, which indicates that the metallic HxMoO3 and carbon coating could enable much easier charge transfer at the electrode/electrolyte interface (compared with MoO3 nanobelts) and boost the electronic conductivity. In short, the metallic HxMoO3 nanobelts and the presence of carbon coating not only help to enhanced specific capacity and superior cycling stability, but also reduce the electrode resistances Rct and improve the lithium-ions diffusion kinetics during the cycling process.
Conclusions In summary, a simple, green and effective catalysis-insertion strategy is developed to fabricate conductive HxMoO3@C nanobelts only through the decomposition of glucose and hydrogen insertion. The intimate conductive contact between the HxMoO3 nanobelts and the carbon shell not only provides an expressway of electron transfer for lithium-ion insertion/ desertion but also expands the interplanar spacing during the chargeedischarge process. As promising anode materials for LIBs, the as-formed HxMoO3@C nanobelts exhibit high specific capacity, cycling stability and rate capability as compared with the pristine MoO3 nanobelts. As for the increased capacity of HxMoO3@C nanobelts compared with pristine MoO3 nanobelts, the increased unit cell volume and d(100) distance in HxMoO3 and synergies in the conductive HxMoO3 and carbon coating coreeshell structure are considered to contribute to the higher capacity and rate performance. The proposed
Fig. 8 e Nyquist plots of MoO3 and HxMoO3 nanobelts.
3622
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
synthesis strategy will open up new opportunities in the hydrogen insertion of other layer transition metal oxides and their applications in lithium-ion batteries.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 51462007, 41272064) and Guangxi Natural Science Foundation (No. 2014GXNSFAA118349).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.01.027.
references
[1] Mai LQ, Hu B, Chen W, Qi YY, Lao CS, Yang RS, et al. Lithiated MoO3 nanobelts with greatly improved performance for lithium batteries. Adv Mater 2007;19:3712e6. [2] Xue XY, Chen ZH, Xing LL, Yuan S, Chen YJ. SnO2/alphaMoO3 coreeshell nanobelts and their extraordinarily high reversible capacity as lithium-ion battery anodes. Chem Commun 2011;47:5205e7. [3] Hashem AM, Groult H, Mauger A, Zaghib K, Julien CM. Electrochemical properties of nanofibers alpha-MoO3 as cathode materials for Li batteries. J Power Sources 2012;219:126e32. [4] Hassan MF, Guo ZP, Chen Z, Liu HK. Carbon-coated MoO3 nanobelts as anode materials for lithium-ion batteries. J Power Sources 2010;195:2372e6. [5] Reddy CVS, Walker EH, Wen C, Mho SI. Hydrothermal synthesis of MoO3 nanobelts utilizing poly(ethylene glycol). J Power Sources 2008;183:330e3. [6] Cai LL, Rao PM, Zheng XL. Morphology-controlled flame synthesis of single, branched, and flower-like alpha-MoO3 nanobelt arrays. Nano Lett 2011;11:872e7. [7] Meduri P, Clark E, Kim JH, Dayalan E, Sumanasekera GU, Sunkara MK. MoO3x nanowire arrays as stable and highcapacity anodes for lithium ion batteries. Nano Lett 2012;12:1784e8. [8] Dong YF, Li S, Xu HM, Yan MY, Xu XM, Tian XC, et al. Wrinkled-graphene enriched MoO3 nanobelts with increased conductivity and reduced stress for enhanced electrochemical performance. Phys Chem Chem Phys 2013;15:17165e70. [9] Reddy CVS, Qi YY, Jin W, Zhu QY, Deng ZR, Chen W, et al. An electrochemical investigation on (MoO3 þ PVP þ PVA) nanobelts for lithium batteries. J Solid State Electr 2007;11:1239e43. [10] Braida B, Adams S, Canadell E. Concerning the structure of hydrogen molybdenum bronze phase III. A combined theoretical e experimental study. Chem Mater 2005;17:5957e69. [11] Lu J, Li WS, Du JH, Fu JM. Co-deposition of Pt-HXMoO3 and its catalysis on methanol oxidation in sulfuric acid solution. J New Mat Electr Syst 2005;8:5e14. [12] Sha XW, Chen L, Cooper AC, Pez GP, Cheng HS. Hydrogen absorption and diffusion in bulk alpha-MoO3. J Phys Chem C 2009;113:11399e407.
[13] Hirata T, Ishioka K, Kitajima M. Raman spectra of MoO3 implanted with protons. Appl Phys Lett 1996;68:458e60. [14] Zeng HC, Ng WK, Cheong LH, Xie F, Xu R. Insertion direction of hydrogen in protonation of alpha-MoO3. J Phys Chem B 2001;105:7178e81. [15] Adams S. CDW superstructures in hydrogen molybdenum bronzes HxMoO3. J Solid State Chem 2000;149:75e87. [16] Hu XK, Qian YT, Song ZT, Huang JR, Cao R, Xiao JQ. Comparative study on MoO3 and HxMoO3 nanobelts: structure and electric transport. Chem Mater 2008;20:1527e33. [17] Noh H, Wang D, Luo S, Flanagan TB, Balasubramaniam R, Sakamoto Y. Hydrogen bronze formation within Pd/MoO3 composites. J Phys Chem B 2004;108:310e9. [18] Hu XK, Ma DK, Xu LQ, Zhu YC, Qian YT. Selective preparation of MoO3 and HxMoO3 nanobelts in molybdenum-hydrogen peroxide system. Chem Lett 2006;35:962e3. [19] Xiang XD, Huang QM, Fu Z, Lin YL, Wu W, Hu SJ, et al. HxMoO3-assisted deposition of platinum nanoparticles on MIATNTs for electrocatalytic oxidation of methanol. Int J Hydrogen Energy 2012;37:4710e6. [20] Phuruangrat A, Chen JS, Lou XW, Yayapao O, Thongtem S, Thongtem T. Hydrothermal synthesis and electrochemical properties of alpha-MoO3 nanobelts used as cathode materials for Li-ion batteries. Appl Phys A Mater 2012;107:249e54. [21] Pimenta MA, Dresselhaus G, Dresselhaus MS, Cancado LG, Jorio A, Saito R. Studying disorder in graphite-based systems by Raman spectroscopy. Phys Chem Chem Phys 2007;9:1276e91. [22] Mestl G, Ruiz P, Delmon B, Knozinger H. In situ Raman spectroscopy characterization of 18O exchange in physical mixtures of antimony oxides and molybdenum oxide. J Phys Chem U S 1994;98:11283e92. [23] Mestl G, Ruiz P, Delmon B, Knozinger H. Oxygen-exchange properties of MoO3: an in-situ Raman-spectroscopy study. J Phys Chem U S 1994;98:11269e75. [24] Windom BC, Sawyer WG, Hahn DWA. Raman spectroscopic study of MoS2 and MoO3: applications to tribological systems. Tribol Lett 2011;42:301e10. [25] Eda Kazuo. Thermal decomposition of hydrogen molybdenum bronze, H0.25MoO3, in a nitrogen atmosphere: defects and phase transformations. J Mater Chem 1992;5:533e8. [26] Feng C, Gao H, Zhang C, Guo Z, Liu H. Synthesis and electrochemical properties of MoO3/C nanocomposite. Electrochim Acta 2013;93:101e6. [27] de Moraes MAB, Trasferetti BC, Rouxinol FP, Landers R, Durrant SF, Scarminio J, et al. Molybdenum oxide thin films obtained by the hot-filament metal oxide deposition technique. Chem Mater 2004;16:513e20. [28] Choi JG, Thompson LT. XPS study of as-prepared and reduced molybdenum oxides. Appl Surf Sci 1996;93:143e9. [29] Chen YP, Lu CL, Xu L, Ma Y, Hou WH, Zhu JJ. Singlecrystalline orthorhombic molybdenum oxide nanobelts: synthesis and photocatalytic properties. Crystengcomm 2010;12:3740e7. [30] Naeem M, Hasanain SK, Kobayashi M, Ishida Y, Fujimori A, Buzby S, et al. Effect of reducing atmosphere on the magnetism of Zn1xCoxO (0 x 0.10) nanoparticles. Nanotechnology 2006;17:2675e80. [31] Dickens PG, Reynolds GJ. Transport and equilibrium properties of some oxide insertion compounds. Solid State Ionics 1981;5:331e4. [32] Lee SH, Kim YH, Deshpande R, Parilla PA, Whitney E, Gillaspie DT, et al. Reversible lithium-ion insertion in
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
molybdenum oxide nanoparticles. Adv Mater 2008;20:3627e32. [33] Yu AS, Kumagai N, Liu ZL, Lee JY. Preparation of sodium molybdenum oxides by a solution technique and their electrochemical performance in lithium intercalation. Solid State Ionics 1998;106:11e8. [34] Zhou L, Yang LC, Yuan P, Zou J, Wu YP, Yu CZ. Alpha-MoO3 nanobelts: a high performance cathode material for lithium ion batteries. J Phys Chem C 2010;114:21868e72.
3623
[35] Aragon MJ, Leon B, Vicente CP, Tirado JL. On the use of transition metal oxysalts as conversion electrodes in lithium-ion batteries. J Power Sources 2009;189:823e7. [36] Tang QW, Shan ZQ, Wang L, Qin X. MoO2-graphene nanocomposite as anode material for lithium-ion batteries. Electrochim Acta 2012;79:148e53. [37] Jung YS, Lee S, Ahn D, Dillon AC, Lee SH. Electrochemical reactivity of ball-milled MoO(3y) as anode materials for lithium-ion batteries. J Power Sources 2009;188:286e91.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
HxMoO3@C nanobelts: Green synthesis and superior lithium storage properties Yeping Song a,b, Hai Wang a,b,*, Zihua Li a, Naiqing Ye a, Linjiang Wang a,b, Yong Liu c a
College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, PR China Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, PR China c School of Physics and Engineering, State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, PR China b
article info
abstract
Article history:
MoO3 could be a promising high capacity anode material for lithium ion batteries (LIBs).
Received 13 November 2014
However, its applications have been hindered by its poor electronic conductivity. To
Received in revised form
address these issues, in this work, HxMoO3@C nanobelts were synthesized by mild hy-
7 January 2015
drothermal treatment of precursor MoO3 nanobelts only with the assistance of glucose and
Accepted 8 January 2015
ethanol. Subsequently, a catalysis-insertion model was proposed to describe the formation
Available online 7 February 2015
of HxMoO3 nanobelts. When tested as LIBs anodes, the superior reversible capacities of LIBs anodes were realized from HxMoO3@C nanobelts. The HxMoO3@C nanobelts electrode
Keywords:
exhibited superior reversible capacity of 480 mAh g1 retained at 200 mA g1 after 100
Lithium-ion batteries
cycles, and a superior rate capacity of 337 mAh g1 retained after 100 cycles at 500 mA g1.
Hydrogen molybdenum bronzes
The superior performance achieved in HxMoO3@C nanobelts anode system are attributed
Nanobelts
to the synergistic effect of the conductive HxMoO3, the enlargement of interplanar spacing
Anode material
and uniform carbon coating shell.
Insertion
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Nowadays, one-dimensional (1D) molybdenum trioxide (aMoO3) nanobelts as promising anode materials for lithium-ion batteries (LIBs), have attracted much attention because of efficient 1D electron transport pathways, effective electrode/ electrolyte contact area [1e6]. Moreover, compared with commercial graphite anodes with a specific capacity of 372 mAh g1, MoO3 has a relatively high theoretical capacity of
1111 mA g1 [4]. However, their practical applications are still hindered by its relatively low specific capacities and rate properties that are caused by poor intrinsic ionic and electronic conductivity [4,7,8]. How to increase the specific capacities of 1D MoO3 nanobelts has been one of the most attractive topics in both scientific and industrial fields. The carbon-coating process has proved to be an effective approach, as they provided simple process [4]. Other ideal strategies, such as fabricating graphene conducting layer on the surface of MoO3 nanobelts, and using lithiation
* Corresponding author. Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Ministry of Education, Guilin University of Technology, Guilin 541004, PR China. Tel.: þ86 773 5896672; fax: þ86 773 5896671. E-mail address: [email protected] (H. Wang). http://dx.doi.org/10.1016/j.ijhydene.2015.01.027 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
3614
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
technologies et al. were being continually reported in the last few years [1,2,4,5,8,9]. To date, how to increase the conductivity of the MoO3 nanobelts without the assistance of graphene or metal, however, still remains a great challenge due to its intrinsic poor conductivity. Hence, it's highly desirable to solve the above problem. As a promising candidate of the electroneproton mixed conductor, the hydrogen molybdenum bronze (HxMoO3) has attracted increasing attention due to their possible applications in fuel cells, electrochromic displays, hydrogen storage, and catalysts [10,11]. For such a structure, previous studies showed that the doped hydrogens are present in the MoO3 lattice at interstitial sites and bond to oxygen atoms of the MoO6 octahedron [12e15]. Moreover, the Mo5þ and Mo6þ cations are co-existent with mixed valences in HxMoO3 nanobelts [16]. Most importantly, the conductivity of HxMoO3 nanobelts is greatly improved due to hydrogen doping [14,16], which is quite attractive and beneficial to high specific capacity, cycling performance as well as rate capacity. In previous work, the HxMoO3 was obtained from MoO3 by the reduction-intercalation mechanism, such as employing Zn/HCl aqueous solution, or spillover method of MoO3 coated with Pt under hydrogen atmosphere [10,16,17]. For example, Qin et al. reported the preparation of HxMoO3 nanobelts using Mo powder, H2O2 as reactants, ethanol as reducing agent [18]. Very recently, Qian et al. further realized the controllable synthesis of the HxMoO3 nanobelts by employing the hydrazine solution as reducing agent in acidic media [16]. In addition, Li and co-workers recently synthesized a gel HxMoO3 solution using concentrated hydrochloric acid as proton sources [19]. Indeed, all these techniques have been well demonstrated for the successful formation of HxMoO3. Unfortunately, these synthesis routes are both tedious and complex, and other unexpected foreign impurities, such as Zn particles, are inevitably introduced to the final products. It should be noted that, as an important hydrogen-doped metal oxide, HxMoO3 seems to be an unnoticed group using as anode materials for LIBs. In previous studies, they could hardly provide effective information about their reactivity kinetics of lithium insertion. Therefore, based on the above consideration, it is essential to develop a clean and friendly method to produce the HxMoO3 electrodes in large-scale under mild conditions and investigate their electrochemical behavior. Here, we report the synthesis of HxMoO3 nanobelts with uniform carbon coating via a facile one-pot hydrothermal treatment process only by using ethanol and glucose instead of any toxic and dangerous reagents. The introduction of ethanol into this system can make hydrogen insert into MoO3 interplanar spacing, possibly due to the catalysis role of carbon derived from glucose. To the best of our knowledge, this is the first report on the preparation of HxMoO3@C nanobelts via one-step method. Furthermore, the electrochemical behaviors of HxMoO3@C nanobelts were investigated. The assynthesized HxMoO3@C nanobelts exhibit superior reversible capacity (563 mAh g1 retained after 100 cycles at 100 mAh g1) and excellent rate performance (337 mAh g1 at 500 mA g1) for lithium storage. Such superior performance can be attributed to the synergistic effect of conductive HxMoO3 nanobelts and carbon coating layer and expanded interlayer space.
Experimental sections and characterizations Materials synthesis All chemical reagents were commercial products used without further purification. Firstly, MoO3 nanobelts were prepared via a facile hydrothermal approach, according to reported studies [20]. Typically, 1 g ammonium molybdate was dissolved in 30 ml deionized water under continuous stirring. After 30 min, 6 ml HNO3 was added to the beaker dropwisely and stirred for another 30 min. The homogeneous suspension was formed. Then the resulting suspension was transferred to a Teflon-lined autoclave with a capacity of 50 ml and then kept inside an electric oven at 180 C for 24 h. To investigate the effects of the reaction conditions on the formation process of final products, 30 ml ethanol, 0.25 g glucose dissolved in 30 ml deionized water and a mixture of 30 ml ethanol and 0.25 g glucose are used as reactants, respectively, as shown in Scheme 1. The as-prepared powders, denoted as MoO3, MoO3-ethanol, MoO3-glucose, MoO3-ethanol-glucose were collected by filtration and thoroughly washed with water and ethanol and finally dried at 70 C in air.
Material characterizations The crystal structure was performed by X-ray diffraction (XRD) analysis with a PANanalytic X'Pert spectrometer using Cu Ka radiation with wavelength of 0.15405 nm. The surface morphologies of the samples were studied using a JEOL JSM6300 (Tokyo, Japan) field emission scanning electron microscope (FESEM). Energy dispersive X-ray (EDS) measurement was conducted using the EDAX system attached to the same FESEM. The samples used in the EDS tests were examined without further treatment. The thermal gravimetricdifferential scanning calorimetry (TG-DSC) analysis was performed on Netzsch Thermal Analysis STA 409 instrument in a temperature range of room temperature-800 C with a scanning rate of 5 C min1 under air atmosphere. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained with a JEM-2010F electron microscope (JEOL, Japan) operating at 200 kV. Raman measurements were carried out at room temperature, and the signals were recorded by a DXR Raman Microscope (DXR Microscope, ThermoFisher, USA). For MoO3 nanobelts, the 0.3 MW output of the 532 nm line of a Nd YAG laser was used as the excitation source. The laser power was changed to 0.1 MW due to some unknown reason. The obtained Raman spectrum was recorded with a resolution of approximately 1 cm1. The X-ray photoelectron spectra (XPS) were recorded on a VG MultiLab 2000 system with a monochromatic Al Ka X-ray source (ThermoVG Scientific). Vacuum pressure was kept at <3 109 torr during the measurements.
Electrochemical measurements The electrochemical performance of as-prepared samples was tested by employing CR2025-type coin cells at room temperature. The working electrode included active material, acetylene black and PVDF in a 50:30:20 weight ratio. A Celgard 3400
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
3615
Scheme 1 e Synthesis procedure of HxMoO3@C nanobelts via hydrothermal reaction.
membrane was used as a separator and 1.0MLiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1:1 volume, Novolyte Technologies, USA) was used as the electrolyte. The Cell was made in an argon-filled glovebox. The chargeedischarge cycling performance and cyclic voltammetry (CV) (0.01e3.0 V, 0.1 mV s1) was carried out at room temperature by using a battery testing system (NEWARE) at different current rates with a voltage window of 0.01e3.0 V and using an electrochemical workstation (CHI 860 D, Beijing), respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed at frequencies from 102 to 105 Hz.
conditions. The glucose is easily transformed into carbon under hydrothermal conditions. Therefore, we inferred reasonably that the as-formed carbon is used as a reducing agent to reduce the pristine MoO3, resulting in the formation of MoO2. Interestingly, only the mixture of glucose and ethanol can lead to the formation of a new phase, HxMoO3 (x ¼ 0.6). The presence of the experimental phenomenon
Results and discussion To investigate the effects of the reaction conditions on the formation of HxMoO3, we investigated its phase transformation behaviors using different reactants. The results were monitored via XRD patterns. XRD patterns show that the stages of evolution from MoO3 to HxMoO3 with the changes of reactants, as shown in Fig. 1. The short bars on the bottom of the plot represent the diffraction pattern of the standard PDF card: MoO3, MoO2 and H0.6MoO3, respectively. The results show that the formation of HxMoO3 phase depends strongly on the chemical environment of the solvent. The controllable experiments show that the MoO3 in ethanol preserves the initial phase without any color and structure change, but are transformed into MoO2 phase when glucose is introduced into deionized water. It is well known that the glucose is widely used in common carbon-coating method under hydrothermal
Fig. 1 e XRD patterns showing the stages of evolution from pure MoO3 to pure HxMoO3 as a function of reaction agents. The short bars on the bottom of the plot represent the diffraction pattern of the standard PDF card: MoO3, MoO2 and H0.6MoO3, respectively.
3616
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
mentioned above shows the obtained HxMoO3 is resulted from the interactions of glucose, ethanol and MoO3. According to previous literature and the comparative analysis of HxMoO3 and MoO3 crystal structure [16,18], the hydrogen can be inserted into interplanar spacing of (010) of MoO3 and we reasonably inferred that the hydrogen source in HxMoO3 may be from the ethanol rather than glucose. Actually, the HxMoO3 can be obtained only in the presence of the ethanol, according to Qian et al. reported results [18]. In our case, however, the HxMoO3 cannot be formed until the glucose was added into the mixture of MoO3 and ethanol. Therefore, we inferred that the existing carbon derived from the decomposing of glucose played a dual role, as the reduction agent, for example, Mo6þ in MoO3 is reduced to partial Mo5þ in HxMoO3, and catalyst in the process of the formation of HxMoO3. On the basis of the above observations, a “catalysis insertion” model was proposed to describe the formation of HxMoO3, as shown in Scheme S1. Obviously, the as-prepared HxMoO3 was obtained only with the assistance of glucose and ethanol, which is different from previous reports. This synthetic method takes place in one-step hydrothermal reaction, and only requires ethanol and glucose, making it an appealing method for the controllable synthesis of HxMoO3@C nanobelts, which would be expected to provide new opportunities for the hydrogen insertion of other similar layer transition metal oxides. Further, the as-prepared HxMoO3 powder is observed by FESEM. Fig. 2 shows FESEM images of HxMoO3 and pristine MoO3 powders, respectively. As shown in Fig. 2, the
morphology of HxMoO3 appears very similar to that of the pristine MoO3 nanobetls. Fig. 3 shows representative TEM and HRTEM images of the MoO3 and the HxMoO3 nanobelts, respectively. Similar to the FESEM results (Fig. 2), the lowmagnification TEM images of the MoO3 and the HxMoO3 nanobelts don't make much difference, as shown in Fig. 3a and c. Furthermore, it is found that a very thin amorphous carbon layer of ca. 4 nm is uniformly coated on the surface of HxMoO3 nanobelts (indicated by dashed line), as shown in Fig. 3cee. Combined with the observations of TEM and electron microscopy FESEM images, we found that the nanobelts are approximately 10 mm in length, 200e300 nm in width, and 100 nm in thickness. Fig. 3a is a single MoO3 nanobelt and its SAED pattern corresponding to [010] zone axis (the inset of Fig. 3a), showing that nanobelts grow in the [001] direction and their wide facet corresponds to (010) crystalline plane of HxMoO3. The HRTEM image of HxMoO3 nanobelts (Fig. 3f) and the SAED (Fig. 3g) reveals that the HxMoO3 is single-crystalline. According to the experimental results discussed above, we propose a possible mechanism in Scheme S1 to rationalize the formation pathways of the HxMoO3 nanobelts. The comparison of the crystal structures shown in Table 1 means that the phase transformation of MoO3 from orthorhombic phase to monoclinic phase (HxMoO3) occurs, moreover, the unit cell ˚ obviously during the volume increases from 202.95 to 211.12 A reduction-insertion process. In addition, it is worthwhile to note that there is significant change of (hkl), for example, for HxMoO3, there are a shrinkage of the b-axis and a significant
Fig. 2 e FESEM images of MoO3 and HxMoO3 nanobelts. (a)e(b): MoO3 nanobelts; (c)e(d): HxMoO3 nanobelts.
3617
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
Fig. 3 e TEM image of single MoO3 (a) and HxMoO3 nanobelts (c). HRTEM image of an individual MoO3 (b) and HxMoO3 nanobelts (d, e). (f) HRTEM image of top of HxMoO3 single nanobelt and their SAED patterns (MoO3: the inset of (a), HxMoO3: (g)).
increase of a-axis as compared with MoO3. As we known, lithium ion diffusion and transport suffer from a typical kinetic problem, for example, for the solid-state lithium diffusion, the increase of a-axis may improve the lithium-ion diffusion coefficient due to the 1D pore channel structure, as shown Scheme 2. A schematic image of HxMoO3 single nanobelt is shown in the inset of Fig. 3c. The sensitivity of Raman spectroscopy of carbon species and its ability to identify different forms of carbon bonding had been most demonstrated. Therefore, the carbon on the surface of HxMoO3 nanobelts can be easily identified. Fig. 4 shows the comparison Raman spectra of MoO3 and HxMoO3 nanobelts. In the Raman spectra, one obvious band at ~1600 cm1, which is attributed to the G band (graphite, sp2) [21], can be observed. These results further confirmed that the obtained sample was HxMoO3@C composite. Surprisingly, the Raman spectroscopy of the HxMoO3 appears too noise, even at the lower 0.1 MW laser power. There was a discernable difference detected. An unusual phenomenon was observed when the laser was injected on the dark blue HxMoO3 nanobelts. The dark blue was turned into white and there was no obvious spectral peak appeared during the test when the laser power increased. The specific reason is still not clear. As shown in Fig. 4, it is found that Raman features of HxMoO3 nanobelts are similar to that of the pristine MoO3. The vibration modes in the frequency ranges of 1000e600 cm1 and
600e200 cm1 correspond to the stretching and deformation modes [22,23], respectively. The 158 cm1 (Ag, B1g) band is from the translation of the rigid chains, the 666 cm1 (B2g, B3g) is an asymmetric stretching of the MoeOeMo bridge along the c axis in MoO3, while the 822 cm1 (Ag, B1g) is a symmetric stretch of the terminal oxygen atoms, and the 995 cm1 (Ag, B1g) is the asymmetric stretch of the terminal oxygen atoms [22e24]. No doubt, whether the carbon coating exists is a key issue for the analysis of the performance of LIBs. To confirm the presence and spatial distribution of carbon in the HxMoO3 nanobelts, we have investigated its Ka spectral profile by collecting a FESEM-EDS spectrum via the fine electron probe scanned along the length of the HxMoO3 nanobelts. The long electron beam was necessary to obtain an adequate signal from each line. Fig. 5a shows FESEM-EDS spectrum of HxMoO3@C nanobelts (inset shows the FESEM image showing the path of line scanning) and the corresponding Ka and La spectral profile (Fig. 5b).The Ka spectral profile of carbon suggests that the carbon are almost uniformly distributed in the samples. Furthermore, the elemental composition of the samples was obtained (the inset in Fig. 5a). The elemental composition analysis strongly supports the presence of carbon in HxMoO3 nanobelts. Furthermore, to determine the carbon presence of the HxMoO3@C nanobelts, TG-DSC curves are shown in Fig. S6. TG curve exhibits a slight weight loss at approximately 262 C, which can be assigned to the
Table 1 e The comparison of crystal structures of MoO3 and H0.6MoO3. Unit cells parameters MoO3 H0.6MoO3
Space-group
˚) a (A
˚) b (A
˚) c (A
˚ 3) V (A
d (nm)
Pbnm(62)-orthorhombic C2/m(12)-monoclinic
3.963 14.543
13.855 3.852
3.696 3.769
202.95 211.12
1.38 (010) 1.45 (100)
3618
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
Scheme 2 e Schematic illustration of the change of MoO3 crystal structure into HxMoO3 and the lithium-ion insertion-extraction process for HxMoO3.
dehydrogenation of HxMoO3 [25]. The weight loss at approximately 429 C can be attributed to the oxidation of carbon [26]. The abrupt weight loss at approximately 742 C can be assigned to the sublimation of the sample [25]. The XPS analysis has been used to confirm the presence, contents and chemical states of Mo and O in the samples. From the comparison of XPS survey spectra for MoO3 and HxMoO3 nanobelts, it can be seen that the XPS survey spectra of the two samples are similar, as shown in Fig. S1. With molybdenum element being concerned, the doublets of 235.6
Fig. 4 e Raman spectra of the pristine MoO3 and the carbon coated HxMoO3 nanobelts: HxMoO3@C.
and 232.5 eV are attributed to the binding energies of the 3d3/2 and 3d5/2 electrons of Mo6þ [27], respectively, which are present in MoO3 nanobelts, as shown in Fig. 6a. For HxMoO3 nanobelts, two sets of obvious doublets are detected, one being ascribed to Mo6þ (235.7 and 232.6 eV corresponding to 3d3/2 and 3d5/2 of Mo6þ) and the other for Mo5þ (234.2 and 231.1 eV corresponding to 3d3/2 and 3d5/2 of Mo5þ) [28], as shown in Fig. 6b. The Mo5þ cation is obviously detected in the XPS spectrum of HxMoO3. Based on the previous literature analysis [16], for our case, the catalysis-insertion method creates a heavy hydrogen intercalation. For MoO3, the deconvolution of the O 1s peak resulted in two peaks located at the peak 530.63 eV and 532.08 eV (Fig. S2a) corresponds to O2 in normal MoO3 matrix and the O bind of MoeOeMo, respectively [29]. After incorporation of hydrogen into the MoO3, it was observed that the higher binding energy O1s peak increased to 532.23 eV, indicating the significant enrichment of oxygen vacancies (Fig. S2b) [30]. This further confirmed that the Mo6þ was partially reduced to Mo5þ, which is accompanied by the formation of oxygen vacancies. For HxMoO3, the researchers had shown that the more hydrogen intercalated, the more electrons injected in the meantime. Moreover, the additional electron in Mo5þ relative to Mo6þ may not be strictly localized. In fact, HxMoO3 exhibits some metallic characteristics, probably due to these additional nonlocalized electrons. No doubt, the conductivity of HxMoO3 will be beneficial for charge transfer in LIBs applications. Based on a comparative analysis of the structural characteristics and conductivity between MoO3 and HxMoO3, it is expected that the lithiumstorage properties of the HxMoO3 should be more superior to that of the MoO3.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
3619
Fig. 5 e FESEM-EDS spectrum of HxMoO3@C nanobelts (a) and the corresponding Ka and La spectral profile (b).
Fig. 7a shows the representative cyclic voltammetry (CV) curves of the HxMoO3 nanobelts at a scan rate of 0.1 mV s1 in the range of 3e0.01 V. Evidently, lithium-ion insertion/desertion in HxMoO3 nanobelts electrode takes place as a multistep process, which is similar to pristine MoO3 nanobelts electrode (Fig. S3). Furthermore, it is found that the CV curves of the pristine MoO3 and HxMoO3 are very similar. Considering the similarity of their structures, therefore, we may explain the electrochemical mechanism of HxMoO3 electrodes by means of the MoO3 model proposed by most previous report studies. The first cathodic scan of the MoO3 presented an irreversible reduction peak at around 0.23 and 2.28 V attributed to electrolyte decomposition and formation of an SEI layer. Interestingly, for the HxMoO3@C nanobelts, the corresponding value is 0.19 and 2.09 V, both of which are lower than the of MoO3 nanobelts. It should be noted that the intensity of the peak position at 2.09 V in HxMoO3 is lower than that of MoO3, indicating a different SEI film caused possibly by the intrinsic
conductivity of HxMoO3. It is known that a-MoO3 exhibits electrochemical redox behavior and lithium-ion insertion/ desertion according to the following reaction [31e33]: þ
discharge
xLi þMoO3 þxe ! Lix MoO3 þ
(1) discharge
Liy MoO3 þ ð6 yÞLi þ ð6 yÞe % Mo þ 3Li2 O charge
(2)
For HxMoO3 electrode, during the first anodic scanning process, the two peaks at 1.2 and 2.5 V in the first cycle were then assigned to the desertion of lithium ions from the inner structure of the MoO6 octahedron layers and the van der Waals gaps between the MoO6 bilayers [34]. While for MoO3, there are not obvious anodic peak observed at 2.5 V except for the anodic peak at 1.2 V. From the 1st CV curves, we therefore draw that the outstanding electrochemical kinetics of HxMoO3@C nanobelts. During the lithium insertion, two
Fig. 6 e XPS investigation. XPS spectra of MoO3 and HxMoO3 nanobelt for Mo3d core level. The experiment data are fitted with the Gaussian/Lorentzian mixed functions.
3620
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
Fig. 7 e Electrochemical properties of HxMoO3 nanobelts. (a) CV of HxMoO3 nanobelts at a scan rate of 0.1 mV s¡1 in the range of 3e0.01 V. (b) Discharge and charge curves of HxMoO3 nanobelts at a current density of 200 mA g¡1 in the range of 3e0.01 V. (c) Cycling performance of pristine MoO3 and HxMoO3 nanobelts at a current density of 200 mA g¡1. (d) Rate performance of MoO3 and HxMoO3 nanobelts.
electrodes underwent an irreversible structural change from a phase to incomplete amorphous phase LixMoO3, while the long plateau at 0.43 V in the first cycle was related to the lithium insertion into LixMoO3 by a unique conversion reaction to form disordered Mo metal and Li2O, and the formation of the SEI layer [35]. This is further confirmed by 1st chargeedischarge profile of HxMoO3, as shown in Fig. 7b. Obviously, the intensity of peak of HxMoO3 is higher than that of MoO3 under the same test conditions, indicating a better kinetics of chargeedischarge. The superior conductivity of HxMoO3 and expanded crystal interplanar spacing create many opportunities for lithium ion insertion/desertion in interplanar spacing of MoO6 and the inner structure of the MoO6 octahedron layers.
Subsequently, the chargeedischarge curve lithium storage performance of the HxMoO3 and MoO3 nanobelts is investigated, respectively, as shown in Fig. 7b and Fig. S4 at a current density of 200 mA g1. It is observed that the initial chargeedischarge capacities of the HxMoO3@C nanobelts electrode are 1193 and 686 mAh g1, respectively. The capacity of the HxMoO3@C nanobelts electrode does not decay seriously, and it can deliver a reversible capacity of 480 mAh g1 even after 100 cycles (Fig. 7b). It is worthy of noticing that the capacity of HxMoO3@C nanobelts first decreases, then becomes stable until 25 cycles, and finally increases gradually with the increasing number of cycles, as shown in Fig. 7c. For metallic conductive MoO2 electrodes, anomalous behaviors have been reported in previous report [36]. In our case, two possible
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
factors may be ascribed to this unconventional phenomenon. On one hand, the LiyMoO3 have converted into Mo in the equation (2), while the deconstruction of the HxMoO3 nanobelts and emergence of lithiated amorphous LixMoO3 or LiyMoO3 may lower the conductivity of the electrodes, which will synergistically affect the lithium storage and cause capacity fading at the early stage; on the other hand, the remaining unreacted active Mo would continually increase active sites as the lithium ion insertion, resulting in an increased capacity with the increasing number of cycles. For more details, it will be discussed in the future. It can be predicted that the capacity will reach a maximum, and then gradually decreased if more cycles are applied in the cycling performance for the HxMoO3@C nanobelts, owing to the full use of active components and the deteriorating conductivity. There are also some similar electrochemical behavior for electrodes at the current of 100 and 500 mA g1 (Fig. S5). Importantly, the HxMoO3@C nanobelts electrode still exhibits an excellent cyclic performance at a higher current density of 500 mA g1, and the capacity still reaches 337 mAh g1 after 100 cycles (Fig. S5b). In contrast, the pristine MoO3 electrode only exhibits a lower capacity (initial discharge and charge capacities of 1138 and 438 mAh g1 at 500 mA g1). To better understand the advantage of the HxMoO3@C nanobelts in lithium storage, the rate performance of the HxMoO3@C nanobelts is investigated. The rate performances of the HxMoO3@C and MoO3 are shown in Fig. 7d, in which the current density increased from 50 to 500 mA g1 in a stepwise manner and then returned to 50 mA g1. Obviously, the HxMoO3@C exhibits a superior rate properties in comparison with that of MoO3 nanobelts. For example, the HxMoO3@C nanobelts exhibit much superior rate performance with 377 and 291 mAh g1, which is about two times larger than that of pristine MoO3 nanobelts electrode (159 and 127 mAh g1), at the current densities of 100 and 200 mA g1, respectively. Even at the high current rate (500 mA g1), the HxMoO3@C nanobelts can still be retained 213 mAh g1. Importantly, the high capacity of the hierarchical composite electrode can be recovered to the nearly initial values even after 30 discharge and charge cycles, implying their good reversibility. Based on the comparison experimental results of the above electrochemical properties, it is easy to raise a question: Why can HxMoO3@C nanobelts get superior performance than MoO3 counterparts. In fact, such a unique structure has several features that enhance the Li-storage properties of these the HxMoO3@C nanobelts. First, the carbon shell and metallic HxMoO3 nanobelts can improve the electron transfer capacity. The enhanced conductivity of the electrodes is a key factor for rate properties of LIBs. Second, the interplanar spacing of HxMoO3 for lithium ion diffusion and the electron transfer channel is increased, which is beneficial for lithium ion insertion/desertion in the interplanar MoO6 octahedron and further diffusion into MoO6 octahedron. This structural feature can provide a sufficient channel to enable more lithium ion access to the composites nanobelts facilely. Third, the oxygen vacancies created a number of electrochemical active sites and ensure a high utilization of electrode materials, thus leads to a higher capacity [37]. In addition to the improved cycling stability, the HxMoO3@C nanobelts also exhibit excellent rate capability with respect to the pristine
3621
MoO3 nanobelts. It's well-known that the conductivity of the electrodes, as a key factor, has an important effect on their rate performance. Herein, to reveal the lithium ion diffusion and electron transfer kinetic properties of the HxMoO3@C nanobelts, we take the electrochemical impedance spectroscopy (EIS) measurements. As shown in Fig. 8, electrochemical impedance spectroscopy reveals that the HxMoO3@C nanobelts electrode undergoes an obvious reduction in solidelectrolyte interfacial and charge transfer resistances Rct, with a simultaneous increase in the lithium-ions diffusion coefficient in comparison to an electrode made of MoO3 nanobelts, which indicates that the metallic HxMoO3 and carbon coating could enable much easier charge transfer at the electrode/electrolyte interface (compared with MoO3 nanobelts) and boost the electronic conductivity. In short, the metallic HxMoO3 nanobelts and the presence of carbon coating not only help to enhanced specific capacity and superior cycling stability, but also reduce the electrode resistances Rct and improve the lithium-ions diffusion kinetics during the cycling process.
Conclusions In summary, a simple, green and effective catalysis-insertion strategy is developed to fabricate conductive HxMoO3@C nanobelts only through the decomposition of glucose and hydrogen insertion. The intimate conductive contact between the HxMoO3 nanobelts and the carbon shell not only provides an expressway of electron transfer for lithium-ion insertion/ desertion but also expands the interplanar spacing during the chargeedischarge process. As promising anode materials for LIBs, the as-formed HxMoO3@C nanobelts exhibit high specific capacity, cycling stability and rate capability as compared with the pristine MoO3 nanobelts. As for the increased capacity of HxMoO3@C nanobelts compared with pristine MoO3 nanobelts, the increased unit cell volume and d(100) distance in HxMoO3 and synergies in the conductive HxMoO3 and carbon coating coreeshell structure are considered to contribute to the higher capacity and rate performance. The proposed
Fig. 8 e Nyquist plots of MoO3 and HxMoO3 nanobelts.
3622
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
synthesis strategy will open up new opportunities in the hydrogen insertion of other layer transition metal oxides and their applications in lithium-ion batteries.
Acknowledgments This work was supported by National Natural Science Foundation of China (No. 51462007, 41272064) and Guangxi Natural Science Foundation (No. 2014GXNSFAA118349).
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2015.01.027.
references
[1] Mai LQ, Hu B, Chen W, Qi YY, Lao CS, Yang RS, et al. Lithiated MoO3 nanobelts with greatly improved performance for lithium batteries. Adv Mater 2007;19:3712e6. [2] Xue XY, Chen ZH, Xing LL, Yuan S, Chen YJ. SnO2/alphaMoO3 coreeshell nanobelts and their extraordinarily high reversible capacity as lithium-ion battery anodes. Chem Commun 2011;47:5205e7. [3] Hashem AM, Groult H, Mauger A, Zaghib K, Julien CM. Electrochemical properties of nanofibers alpha-MoO3 as cathode materials for Li batteries. J Power Sources 2012;219:126e32. [4] Hassan MF, Guo ZP, Chen Z, Liu HK. Carbon-coated MoO3 nanobelts as anode materials for lithium-ion batteries. J Power Sources 2010;195:2372e6. [5] Reddy CVS, Walker EH, Wen C, Mho SI. Hydrothermal synthesis of MoO3 nanobelts utilizing poly(ethylene glycol). J Power Sources 2008;183:330e3. [6] Cai LL, Rao PM, Zheng XL. Morphology-controlled flame synthesis of single, branched, and flower-like alpha-MoO3 nanobelt arrays. Nano Lett 2011;11:872e7. [7] Meduri P, Clark E, Kim JH, Dayalan E, Sumanasekera GU, Sunkara MK. MoO3x nanowire arrays as stable and highcapacity anodes for lithium ion batteries. Nano Lett 2012;12:1784e8. [8] Dong YF, Li S, Xu HM, Yan MY, Xu XM, Tian XC, et al. Wrinkled-graphene enriched MoO3 nanobelts with increased conductivity and reduced stress for enhanced electrochemical performance. Phys Chem Chem Phys 2013;15:17165e70. [9] Reddy CVS, Qi YY, Jin W, Zhu QY, Deng ZR, Chen W, et al. An electrochemical investigation on (MoO3 þ PVP þ PVA) nanobelts for lithium batteries. J Solid State Electr 2007;11:1239e43. [10] Braida B, Adams S, Canadell E. Concerning the structure of hydrogen molybdenum bronze phase III. A combined theoretical e experimental study. Chem Mater 2005;17:5957e69. [11] Lu J, Li WS, Du JH, Fu JM. Co-deposition of Pt-HXMoO3 and its catalysis on methanol oxidation in sulfuric acid solution. J New Mat Electr Syst 2005;8:5e14. [12] Sha XW, Chen L, Cooper AC, Pez GP, Cheng HS. Hydrogen absorption and diffusion in bulk alpha-MoO3. J Phys Chem C 2009;113:11399e407.
[13] Hirata T, Ishioka K, Kitajima M. Raman spectra of MoO3 implanted with protons. Appl Phys Lett 1996;68:458e60. [14] Zeng HC, Ng WK, Cheong LH, Xie F, Xu R. Insertion direction of hydrogen in protonation of alpha-MoO3. J Phys Chem B 2001;105:7178e81. [15] Adams S. CDW superstructures in hydrogen molybdenum bronzes HxMoO3. J Solid State Chem 2000;149:75e87. [16] Hu XK, Qian YT, Song ZT, Huang JR, Cao R, Xiao JQ. Comparative study on MoO3 and HxMoO3 nanobelts: structure and electric transport. Chem Mater 2008;20:1527e33. [17] Noh H, Wang D, Luo S, Flanagan TB, Balasubramaniam R, Sakamoto Y. Hydrogen bronze formation within Pd/MoO3 composites. J Phys Chem B 2004;108:310e9. [18] Hu XK, Ma DK, Xu LQ, Zhu YC, Qian YT. Selective preparation of MoO3 and HxMoO3 nanobelts in molybdenum-hydrogen peroxide system. Chem Lett 2006;35:962e3. [19] Xiang XD, Huang QM, Fu Z, Lin YL, Wu W, Hu SJ, et al. HxMoO3-assisted deposition of platinum nanoparticles on MIATNTs for electrocatalytic oxidation of methanol. Int J Hydrogen Energy 2012;37:4710e6. [20] Phuruangrat A, Chen JS, Lou XW, Yayapao O, Thongtem S, Thongtem T. Hydrothermal synthesis and electrochemical properties of alpha-MoO3 nanobelts used as cathode materials for Li-ion batteries. Appl Phys A Mater 2012;107:249e54. [21] Pimenta MA, Dresselhaus G, Dresselhaus MS, Cancado LG, Jorio A, Saito R. Studying disorder in graphite-based systems by Raman spectroscopy. Phys Chem Chem Phys 2007;9:1276e91. [22] Mestl G, Ruiz P, Delmon B, Knozinger H. In situ Raman spectroscopy characterization of 18O exchange in physical mixtures of antimony oxides and molybdenum oxide. J Phys Chem U S 1994;98:11283e92. [23] Mestl G, Ruiz P, Delmon B, Knozinger H. Oxygen-exchange properties of MoO3: an in-situ Raman-spectroscopy study. J Phys Chem U S 1994;98:11269e75. [24] Windom BC, Sawyer WG, Hahn DWA. Raman spectroscopic study of MoS2 and MoO3: applications to tribological systems. Tribol Lett 2011;42:301e10. [25] Eda Kazuo. Thermal decomposition of hydrogen molybdenum bronze, H0.25MoO3, in a nitrogen atmosphere: defects and phase transformations. J Mater Chem 1992;5:533e8. [26] Feng C, Gao H, Zhang C, Guo Z, Liu H. Synthesis and electrochemical properties of MoO3/C nanocomposite. Electrochim Acta 2013;93:101e6. [27] de Moraes MAB, Trasferetti BC, Rouxinol FP, Landers R, Durrant SF, Scarminio J, et al. Molybdenum oxide thin films obtained by the hot-filament metal oxide deposition technique. Chem Mater 2004;16:513e20. [28] Choi JG, Thompson LT. XPS study of as-prepared and reduced molybdenum oxides. Appl Surf Sci 1996;93:143e9. [29] Chen YP, Lu CL, Xu L, Ma Y, Hou WH, Zhu JJ. Singlecrystalline orthorhombic molybdenum oxide nanobelts: synthesis and photocatalytic properties. Crystengcomm 2010;12:3740e7. [30] Naeem M, Hasanain SK, Kobayashi M, Ishida Y, Fujimori A, Buzby S, et al. Effect of reducing atmosphere on the magnetism of Zn1xCoxO (0 x 0.10) nanoparticles. Nanotechnology 2006;17:2675e80. [31] Dickens PG, Reynolds GJ. Transport and equilibrium properties of some oxide insertion compounds. Solid State Ionics 1981;5:331e4. [32] Lee SH, Kim YH, Deshpande R, Parilla PA, Whitney E, Gillaspie DT, et al. Reversible lithium-ion insertion in
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 3 6 1 3 e3 6 2 3
molybdenum oxide nanoparticles. Adv Mater 2008;20:3627e32. [33] Yu AS, Kumagai N, Liu ZL, Lee JY. Preparation of sodium molybdenum oxides by a solution technique and their electrochemical performance in lithium intercalation. Solid State Ionics 1998;106:11e8. [34] Zhou L, Yang LC, Yuan P, Zou J, Wu YP, Yu CZ. Alpha-MoO3 nanobelts: a high performance cathode material for lithium ion batteries. J Phys Chem C 2010;114:21868e72.
3623
[35] Aragon MJ, Leon B, Vicente CP, Tirado JL. On the use of transition metal oxysalts as conversion electrodes in lithium-ion batteries. J Power Sources 2009;189:823e7. [36] Tang QW, Shan ZQ, Wang L, Qin X. MoO2-graphene nanocomposite as anode material for lithium-ion batteries. Electrochim Acta 2012;79:148e53. [37] Jung YS, Lee S, Ahn D, Dillon AC, Lee SH. Electrochemical reactivity of ball-milled MoO(3y) as anode materials for lithium-ion batteries. J Power Sources 2009;188:286e91.