Facile synthesis of MoO2 nanoparticles as high

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CERAMICS INTERNATIONAL

Ceramics International 42 (2016) 2198–2203 www.elsevier.com/locate/ceramint

Facile synthesis of MoO2 nanoparticles as high performance supercapacitor electrodes and photocatalysts E. Zhoua, Chenggang Wanga, Qinqin Zhaoa, Zhipeng Lib, Minghui Shaoa,n, Xiaolong Denga, Xiaojing Liua, Xijin Xua,n a

School of Physics and Technology, University of Jinan, 336 Nanxin Zhuang West Road, Jinan 250022, Shandong Province, PR China b Materials Characterization Group, Western Digital Company, 44100 Osgood Road, CA 94539, United States Received 1 September 2015; received in revised form 1 October 2015; accepted 2 October 2015 Available online 14 October 2015

Abstract Molybdenum dioxide (MoO2) nanoparticles with the size of 200 nm in diameter were synthesized by a facile hydrothermal method. The nanoparticles were directly functionalized as supercapacitors (SCs) electrodes and photocatalysts. The electrochemical studies showed that the SCs demonstrated high capacitance of 621 F g 1, which was 3 times larger than previous reports. Furthermore, they exhibited good cyclic performance with 90% capacity retention after 1000 cycles at a current density of 1 A g 1. The photocatalytic activities were evaluated by the degradation of methylene blue (MB) and rhodamine B (RhB), respectively, and the nanoparticles demonstrated preferred selectivity on the degradation of RhB (70%) than that of MB (30%). & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Molybdenum dioxide; Supercapacitors; Photocatalysis

1. Introduction Recently, pseudocapacitive materials (FeOx [1,2]) and double-layer supercapacitors (Yarn-based [3]) have been extensively investigated as promising supercapacitive materials. Especially, molybdenum dioxide (MoO2) with band gap of 3.85 eV has become a fascinating transition metal oxide because of the metallic electrical resistivity, high melting point, and high chemical stability [4]. Its high density (6.5 g cm 3) enables it to store more energy as supercapacitors with the same size of the battery compared with that of graphite (2.3 g cm 3) anode-based batteries [5]. Furthermore, its higher free electron density in the valence band will enable it to exhibit good catalytic performance; then MoO2 has the potential to be a widely-used catalytic material. To date, n

Corresponding authors. E-mail addresses: [email protected] (M. Shao), [email protected] (X. Xu). http://dx.doi.org/10.1016/j.ceramint.2015.10.008 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

various MoO2 nanostructures, including nanowires, nanospheres, nanoparticles, nanorods and nanocrystals [6–9] have been successfully synthesized by different synthetic approaches, such as electrospinning [10], solid reduction reaction [11], hydrothermal reaction and solvothermal route [12]. However, previously reported MoO2 nanostructures were rarely studied to be supercapacitor electrodes and photocatalysts, though some researchers have studied the superior lithium storage for MoO2 nanosheets [13] and the oxidation of gasoline [14]. Therefore, it is urgent to explore new MoO2 nanostructures and extend their potential applications in energy storage and environmental issues. In this paper, we reported the fabrication of MoO2 nanoparticles with a facile hydrothermal method, which showed high capacitance of 621 F g 1 and excellent cyclic performance with nearly 90% capacity retention after 1000 cycles. Besides, the photocatalytic activities have been evaluated by the degradation measurement of methylene blue (MB) and

E. Zhou et al. / Ceramics International 42 (2016) 2198–2203

rhodamine B (RhB), and a much higher selectivity on the degradation of RhB was observed. 2. Experimental The MoO2 nanoparticles were synthesized via a hydrothermal method. In a typical procedure, 0.5 g of ammonium heptamolybdate tetrahydrate (AHM) was dissolved in to 30 mL distilled water. Then, 3 mL ethylene glycol (EG) was added into this solution under vigorously stirring at room temperature for 30 min. The mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave and kept at 180 1C for 36 h. After reaction, the obtained black precipitates were collected by centrifugation and then washed with distilled water and ethanol several times. The samples were dried in a vacuum drying oven at 60 1C overnight, and then annealed at 500 1C for 6 h in argon flow. For the electrochemical properties, all the measurements are carried out with a dual unit electrochemical working station (CS2350). A three-electrode experimental cell is used in which 1.0 M H2SO4 solution served as an electrolyte, and the working electrode consists of 80 wt% MoO2 nanoparticles, 10 wt% carbon black, and 10 wt% polytetrafluoroethylene (PTFE), while a Pt foil electrode and a Ag/AgCl (satd. KCl) electrodes are used as the counter and reference electrodes, respectively. Photocatalytic degradations of organic dyes (MB and RhB) were carried out under the mercury lamp (500 W) illumination. Generally, 0.5 g

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of samples was added in 50 mL organic dye aqueous solutions (10 mg L 1) with magnetic stirring. After the adsorption– desorption equilibrium, the solutions were exposed to UV irradiation, and the degradation efficiencies were analyzed by measuring the maximum absorption wavelength using a UV–vis spectrophotometer (TU-1901).

3. Results and discussion 3.1. Morphology and structural properties The morphologies of MoO2 nanoparticles are shown in Fig. 1. Typical low magnification SEM image (Fig. 1a) shows that MoO2 mainly exhibit spherical morphology, which are composed of loosely aggregated nanoparticles with size around 200 nm. The enlarged SEM image (Fig. 1b) indicates that the annealed MoO2 nanoparticles are adhered with each other. This special construction provides good physical contact between the nanoparticles, which is benefical to the electron conduction. XRD pattern (Fig. 1c) indicates that that all the diffraction peaks can be exclusively indexed to a pure MoO2 monoclinic phase, matching well with JCPDS Card no. 32-0671. Furthermore, the Raman spectroscopy (Fig. 1d) clearly shows that the vibration modes of MoO2 are clearly confirmed at 987, 819, and 666 cm 1 and the finger bands at 341, 287 and 196 cm 1 can be assigned to the phonon vibration modes of MoO2 [15,16].

Fig. 1. SEM images of the MoO2 samples with (a) low and (b) higher magnifications; (c) XRD pattern and (d) Raman spectrum of MoO2 nanoparticles.

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Fig. 2. XPS spectra for the annealed MoO2 nanoparticles, (a) survey XPS spectrum,(b) high resolution XPS of Mo 3d, (c) N2 adsorption–desorption isotherm, and (d) BJH pore size distribution calculated from the desorption branch.

The peak positions and relative intensities are consistent with those previously reported [17]. We identify the deposited molybdenum oxide in these experiments as MoO2 using X-ray photoelectron spectroscopy (XPS). A typical survey XPS spectrum for the annealed MoO2 is shown in Fig. 2a. The XPS spectrum involves four distinct peaks at 232.1 eV (Mo 3d), 398.1 eV (Mo 3p3/2), 416.1 eV (Mo 3p1/2), and 531.1 eV (O 1 s) of characteristic of MoO2 [17]. In this study, the core level spectrum of Mo3d is further investigated by a high resolution XPS (see in Fig. 2b). The Mo 3d5/2 peak is centered at 230.2 eV whereas the Mo 3d3/2 peak is observed at 233.1 eV, with a spin energy separation of 2.9 eV. The peak at 235.8 eV for Mo 3d3/2, arising from the possible surface oxidation of the MoO2 in air [18] is also slightly detected. Fig. 2c and d shows the N2 adsorption and desorption isotherms of the annealed MoO2 nanoparticles as well as corresponding Barret–Joyner–Halenda (BJH) pore size distribution curve. The MoO2 has a Brunauer–Emmett–Teller (BET) surface area of 70.33 m2 g 1, a total pore volume of 0.071 cm3 g 1, and a relatively broad pore size distribution centered at 4.95 nm. These results are consistent with the SEM observations as shown in Fig. 1a and b, which confirms the strengthened interconnection of the annealed MoO2 nanoparticles [19]. This kind of unique network construction provides

a good physical contact between the nanoparticles and is beneficial for ionic and electronic transfer [17].

3.2. Electrochemical studies Their electrochemical performances are evaluated as working supercapacitor electrodes. Fig. 3a presents the CV curves at scan rates of 2, 5, 10, 20, 30, 40 and 50 mV s 1 in a potential range of 0.1 to 0.45 V versus Ag/AgCl (satd. KCl). The CV curves possess good symmetry and no redox peaks are observed, which was contributed by double layer capacitors and pseudocapacitors in their structure [20]. From Fig. 3a, it can be observed that CV curves current density increases gradually with the increase of the scan rate, and the quasirectangular is maintained rather well even at the scan rate of 50 mV s 1 [21], suggesting that both the adsorption and intercalation processes of MoO2 are kinetically facile. The galvanostatic charge–discharge curves are presented in Fig. 3b at different current densities. The slight curvature character indicates that the supercapacitor behavior can be influenced by electrical double-layer contributions along with the redox reactions [22]. From the charge–discharge curves, the specific


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Fig. 3. (a) Cyclic voltammograms of annealed MoO2 at different scan rate from 2 mV s 1 to 50 mV s 1 and (b) Galvanostatic charge–discharge curves at different current densities from 1 A g 1 to 7 A g 1. (c) Variation of specific capacitance at different current densities. (d) Cycle performance of annealed MoO2 electrode at a current density of 1 A g 1.

capacitance can be calculated from the formula [22,23]: IΔt C F=g ¼ ΔV

ð1Þ

where I (A g 1) is the current density used for charge– discharge, Δt(s) is the time elapsed for the discharge cycle and ΔV is the voltage interval of the discharge (after correcting for iR drop). According to Eq. (1), their specific capacitances are calculated to vary in the range from 620 to 441 F g 1 as the current densities change from 1 to 7 A g 1, as shown in Fig. 3c. The results we reported are much higher than the previously reports 140 F g 1 and 146 F g 1 [24,25]. We ascribe this improvement of specific capacitances to be the unique morphologies of our samples, the adherence for our samples ensure the ideal physical contact between the nanoparticles, and this is benefical to the electron conduction, herein the results of specific capacitances are well improved. Besides, the specific capacitances decrease with the increase of current density. We think that the inner active sites or the pores of the electrode can be fully accessed due to the low ohmic drop at low current densities; hence high specific capacitance values can be achieved [26]. When the current density increases, the capacitance value will drop induced by the slow redox reactions rate [27]. The charge–discharge studies are performed at constant current density of 1 A g 1 to evaluate the stability of the MoO2

nanoparticles, and the variation of specific capacitance for 1000 cycles is shown in Fig. 3d. It can be seen that the electrode exhibits good reversibility with cycling efficiency of 80% at 600 cycles, and then the specific capacitance curve rises up to 90% at 1000 cycles, which can be attributed to the improved wettability over time [28]. The energy density (W h kg 1), power density (W kg 1) were calculated according to the following Equations [29]: E¼

1 CðΔVÞ2 2

ð2Þ

P¼

E td

ð3Þ

where ΔV(V) is the applied potential window (Vato V b ), C (F g 1) is the capacitance, td is the discharging time. The energy density and power densities plots (Fig. 4a) reflected the optimal hybrid capacitor were also examined, in which the capacitor exhibit promising energy and power densities of from 7.73 to 4.09 W h kg 1 and power densities from 90 to 866.72 kW kg 1, indicating that the capacitor is capable of delivering high power and energy simultaneously. Besides, the highest energy density of 7.73 W h kg 1 at 90 kW kg 1 (1 A g 1) gradually reduces to 5.75 W h kg 1 at 165.61 kW kg 1 (2 A g 1) [30]. In order to further investigate

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Fig. 4. (a) Energy density and power densities plots and (b) Nyquist plots of MoO2 electrodes.

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the electrochemical behaviors of the nanostructured device, electrochemical impedance spectroscopy (EIS) (Fig. 4b) is also employed to characterize the MoO2 electrodes, and it can be calculated that the equivalent series resistance (ESR) of MoO2 electrode is about 1.7 Ω, indicating a lower diffusion resistance and charge-transfer resistance.

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3.3. Photocatalytic activity

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Photocatalytic activities were evaluated by the degradation test of MB and RhB. Fig. 5 shows the absorption spectra of RhB and MB of annealed MoO2 under mercury lamp irradiation at different time intervals. It can be observed that the intensities of absorption peaks rapidly decrease as a result of the degradation of RhB and MB, Fig. 5c shows the degredation rate of RhB for the as-made and annealed MoO2 nanoparticles. It can be seen that about 30% of MB and 70% of RhB were degraded for the annealed and as-made samples. When the MoO2 nanoparticles are annealed, the crystallinity will become better; such crystallization raises the photocatalytic activity as our previous report on TiO2 [31,32]. In order to illustrate the selectivity difference of MoO2 nanoparticles among those dyes, the degradation efficiencies are compared for different dyes in 100 min as shown in Fig. 5c. The results show the degradation efficiencies of MB and RhB are about 30% and 70% in 100 min, respectively,

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Fig. 5. Absorption spectra of RhB (a) and MB (b) aqueous solution, the C/C0 vs. time curves of RhB and MB (c).

indicating good degredation selectivity on RhB. We ascribed these different degradation ratios to be caused by the different molecular structures of the two dyes [33]. 4. Conclusions In summary, MoO2 nanoparticles have been successfully synthesized via a facile method. They exhibit much higher specific capacitance of 621 F g 1. The excellent cyclic performance is exhibited after 1000 charge–discharge cycles. The MoO2 nanoparticles also present good photocatalytic


performance and exhibit degredation selectivity on RhB. This work demonstrates that the MoO2 nanoparticles have potential applications as supercapacitors and photocatalytic materials. Acknowledgments Thanks University of Jinan (UJn) for the support on new staff, and the project supported by the Taishan Scholar (No. TSHW20120210), the National Natural Science Foundation of China (Grant nos. 11304120 and 61205175), the Encouragement Foundation for Excellent Middle-aged and Young Scientist of Shandong Province (Grant nos. BS2013CL020 and BS2014CL012), and the ScienceTechnology Program of Higher Education Institutions of Shandong Province (Grant no. J15LJ06). References [1] Y. Huang, Y. Huang, M. Zhu, W. Meng, Z. Pei, C. Liu, Magneticassisted, self-healable, yarn-based supercapacitor, ACS Nano 9 (2015) 6242–6251. [2] W. Meng, W. Chen, L. Zhao, Y. Huang, M. Zhu, Y. Huang, Y. Fu, F. Geng, J. Yu, X. Chen, C. Zhi, Porous Fe3O4/carbon composite electrode material prepared from metal–organic framework template and effect of temperature on its capacitance, Nano Energy 8 (2014) 133–140. [3] Y. Huang, D. Ding, M. Zhu, W. Meng, Y. Huang, F. Geng, J. Li, J. Lin, C. Tang, Z. Lei, Z. Zhang, C. Zhi, Facile synthesis of α-Fe2O3 nanodisk with superior photocatalytic performance and mechanism insight, Sci. Technol. Adv. Mater. 16 (2015) 014801. [4] Y.F. Shi, B.K. Guo, S.A. Corr, Q.H. Shi, Y.S. Hu, K.R. Heier, L. Chen, R. Seshadri, G.D. Stucky, Ordered mesoporous metallic MoO2 materials with highly reversible lithium storage capacity, Nano Lett. 12 (2009) 4215–4220. [5] B. Guo, X. Fang, B. Li, Y. Shi, C. Ouyang, Y. Hu, Z.X. Wang, G. D. Stucky, L. Chen, Synthesis and lithium storage mechanism of ultrafine MoO2 nanorods, Chem. Mater. 24 (2012) 457–463. [6] W. Shutao, Z. Yang, W. Weizhi, et al., Template-assisted synthesis of porous molybdenum dioxide nanofibers and nanospheres by redox etching methed, J. Cryst. Growth 290 (2006) 96–103. [7] Q. Yang, Q. Liang, J. Liu, S.Q. Liang, S.S. Tang, P.J. Lu, Y.K. Lu, Ultrafine MoO2 nanoparticles grown on graphene sheets as anode materials for lithium-ion batteries, Mater. Lett. 127 (2014) 32–35. [8] J.G. Liu, Z.J. Zhang, C.Y. Pan, Y. Zhao, X. Su, Y. Zhou, D.P. Yu, Enhanced field emission properties of MoO2 nanorods with controllable shape and orientation, Mater. Lett. 58 (2004) 3812–3815. [9] Z.X. Yan, J.M. Xie, J.J. Jing, M.M. Zhang, W. Wei, S.B. Yin, MoO2 nanocrystals down to 5 nm as Pt electrocatalyst promoter for stable oxygen reduction reaction, Int. J. Hydrog. Energy 37 (2012) I5948–I5955. [10] W. Luo, X. Hu, Y. Sun, Y. Huang, Electrospinning of carbon-coated MoO2 nanofibers with enhanced lithium-storage properties, Phys. Chem. Chem. Phys. 13 (2011) 16735–16740. [11] Y. Sun, X. Hu, J. Yu, Q. Li, W. Luo, L. Yuan, W. Zhang, Y. Huang, Morphosynthesis of a hierarchical MoO2 nanoarchitecture as a binderfree anode for lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2870–2877. [12] D. Koziej, M. Rossell, B. Ludi, A. Hintennach, P. Novák, J. Grunwaldt, M. Niederberger, Interplay between size and crystal structure of molybdenum dioxide nanoparticles-synthesis, growth mechanism, and electrochemical performance, Small 7 (2011) 377–387.

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