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TiO2 flexible nanocomposites for energy storage applications
Journal of Alloys and Compounds 729 (2017) 1072e1078
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication and characterization of ZnO/MnO2 and ZnO/TiO2 flexible nanocomposites for energy storage applications M. Yasir Rafiq a, Faisal Iqbal b, Fawad Aslam c, Muhammad Bilal a, Naeem Munir a, I. Sultana a, Fawad Ashraf d, Faisal Manzoor e, Najmul Hassan b, Aamir Razaq a, * a
Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan Materials Growth and Simulation Laboratory, Department of Physics, University of the Punjab, Lahore 54590, Pakistan Department of Physics, Hazara University Mansehra, Pakistan d Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore 54000, Pakistan e Interdisciplinary Research Centre in Biomedical Materials, COMSATS Institute of Information Technology, Lahore 54000, Pakistan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2017 Received in revised form 20 September 2017 Accepted 23 September 2017 Available online 25 September 2017
ZnO based nanostructure composites are attractive for high-tech applications of energy conversion and energy storage due to wide range in operating potential window. This study presents about fabrication of ZnO/MnO2 and ZnO/TiO2 composites via hydrothermal method in two consecutive steps. Furthermore lignocelluloses fiber directly collected form self-growing plant Monochoria Vagina, were incorporated in fabricated ZnO/MnO2 and ZnO/TiO2 composites for development of flexible and bulk paper composite electrode. The structural and morphological analysis were carried out using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), respectively. XRD analysis confirm the crystalline structures for ZnO/MnO2 and ZnO/TiO2 samples with crystallite size ~62 nm and ~63 nm, respectively. SEM images shows the sheet-like and nanorods morphology for ZnO and MnO2 nanostructures, respectively. Fourier Transform Infrared (FTIR) Spectroscopy absorbance spectrum reveal the composite formation of ZnO/ MnO2/LC and ZnO/TiO2/LC as bulk paper electrodes whereas cyclic Voltammetry measurement showed the capacitive behavior of composite paper electrodes for the suitable applications in supercapacitors and batteries. © 2017 Elsevier B.V. All rights reserved.
Keywords: ZnO composites Hydrothermal method Nanostructures Lignocelluloses Paper composites
1. Introduction Zinc oxide (ZnO) based composites has attracted the interest of researchers due to direct wide band gap of 3.37 eV and large exciton-binding energy of 60 meV [1e4]. Due to versatile applications of ZnO, several attempts are reported for development of ZnO nanostructures to enhance ion diffusion due to high surface area [5e9]. ZnO based nanostructures have a wide range of high-tech application e.g. surface acoustic wave filter [10], photonic crystal [11], photo detectors [12], light emitting diode [13], photo diodes [14], gas sensors [15] optical modulators wave guide [16], solar cells [17,18], varistors [19], electrochemical supercapacitors [20e22], nanogenerators [23e29], fuel cell and batteries [30]. To address ZnO based composite formation, coupling with metal oxide having different redox energy levels, provides an attractive
* Corresponding author. E-mail address: [email protected] (A. Razaq). https://doi.org/10.1016/j.jallcom.2017.09.253 0925-8388/© 2017 Elsevier B.V. All rights reserved.
approach to achieve efficient charge separation for enhanced life time of charge carriers and better efficiency of interfacial charge transfer [31]. Titanium dioxide (TiO2) has received significant attention due to low cost, high photocatalytic activity, and high stability [32,33]. Titanium dioxide (TiO2) and zinc oxide (ZnO) are two kinds of technologically important semiconductors because of potential applications in advanced devices/systems e.g. photonic devices, sensors, solar cells, etc [34e38]. ZnO/TiO2 based composite nanofibers are also reported as a promising candidate for exceptional antibacterial agents under UV light [39]. Polycrystalline TiO2 and ZnO both in pure form/composite, widely used in photocatalytic reaction in liquid-solid as well as in gas-solid regimes. The coupling of anatase and rutile TiO2 with ZnO are useful in order to achieve efficient electron-hole pair separation under illumination whereas presence of zinc also contributes to the stabilization of the free charge carriers [40]. Efficiency of ZnO nanorod based solar cells is mainly limited by low photocurrent [41,44] due to poor interfacial quality which is not favorable for electron injection [45,46]. In literature, capping of ZnO nanorods with small conductive
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molecules, dyes or a thin TiO2 layer are also reported to facilitate electron injection across the heterojunction [42,46]. MnO2 with the advantages of environmental friendliness, good cycle stability, natural abundance, low cost, and efficient chargedischarge appears to be promising electrode materials for supercapacitors [47,51]. Xing et al. developed the electrochemical synthesis of ZnO nanorod/MnO2 shell composites to address the low charge capacitance [52]. ZnO based cores served as mechanical supports and effective electron transport pathways to increase the electrochemical utilization of MnO2 [52]. Nanoporous MnO2 shells facilitated the ion diffusion and energy storage capacity due to relatively high electronic conductivity, and mechanical flexibility. Mao et al. reported ZnO@MnO2 core@shell nanostructure electrode on titanium substrate to obtain specific areal capacitances of 31.30 mF cm2 [52]. Yang et al. fabricated ZnO@MnO2 core shell nanocables, and authentically improved the electrochemical performance of supercapacitor [53]. The performance of MnO2 nanowire/ZnO nanorod array hybrid electrode under different bending angles demonstrates excellent mechanical stability for flexible energy storage devices [54]. Flexible energy storage devices are highly feasible for the need of modern electronic systems e.g. rollup displays, wearable devices and biologically inspired medical implements. In practical applications a flexible power source must be capable of accommodating frequent complex strains e.g. bending, twisting and deforming while retaining continuous energy supply. Lignocelluloses is the most abundant, renewable, flexible and ecofriendly material on earth. In era of modern disposable electronic technology, development of thin, flexible, light-weight and environmentally friendly electrode material are highly viable. This study presents about the novel fabrication method of ZnO/TiO2 and ZnO/MnO2 via hydrothermal method in two consecutive steps. Furthermore lignocelluloses fibers, directly collected from selfgrowing plant are used as binder for ZnO/TiO2 and ZnO/MnO2 nanostructures for bulk applications of energy storage. 2. Experimental 2.1. Material and method Zinc Nitrate Zn(NO3)2$6H2O, Hexamethylenetetramine (CH2)6N4, Potassium permanganate (KMnO4) and Titanium dioxide (TiO2) were purchased from Merck and freshly used. Lignocelluloses fibers were directly collected from stem of self-growing plant Monochornia Vagnia. 2.2. Fabrication of nanostructures 2.2.1. Preparation of ZnO ~2 g of Zinc Nitrate Zn(NO3)2$6H2O and 0.141 g of Hexamethylenetetramine (CH2)6N4 was dissolved in 20 mL of distilled water and stirred till precipitate occurred. Further slurry was placed in Teflon beaker of autoclave and kept at 80 C for 2 h in oven. Subsequently the solution was centrifuged to get the product and further placed for drying at 80 C in oven. 2.2.2. Preparation of MnO2 ~0.16 g of KMnO4 was dissolved in 35 mL deionized water for 10 min and 2.1 mL of 2 M HCl was added drop wise. Subsequently, the solution was transferred in Teflon cup and sealed in stain-less steel autoclave. The temperature of autoclave was maintained at 200 C for 4 h. Dark grey product was collected at bottom of Teflon cup and rinsed with deionized water during centrifugation to remove the alkali salt and other impurities. The final product was obtained by drying in oven at 80 C.
Fig. 1. XRD pattern of {a} ZnO/TiO2 {b} f ZnO/MnO2.
2.2.3. Preparation of ZnO/MnO2 composite Recipe as described in section 2.2.2 was repeated and output sample was added as precursor in recipe of ZnO as described in section 2.2.1. 2.2.4. Preparation of ZnO/MnO2/LC sheet ~150 mg of LC fibers were dispersed in DI water and solution to obtain homogenous jel. Subsequently 300 mg of ZnO/MnO2 powder were mixed with LC fibers jel on magnetic stirrer for 30 min. Final solution was placed on Buchner funnel and pressed to obtain homogenous sheets. 2.2.5. Preparation of ZnO/TiO2 ~1 g of Titanium dioxide (TiO2)[as purchased] was dispersed in 10 mL of distilled water and used as precursor in recipe of ZnO as described in section 2.2.1. 2.2.6. Preparation of ZnO/TiO2/LC sheet ~150 mg of LC fibers were dispersed in DI water and solution to
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obtain homogenous jel. Subsequently 150 mg of ZnO/TiO2 powder were mixed with LC fibers jel on magnetic stirrer for 30 min. Final slurry was placed on Buchner funnel and pressed to obtain homogenous sheets.
2.3. Characterizations 2.3.1. X. Ray diffraction PANalytical's X-ray diffractometer (XRD) in scanning range 20 e80 by Cu Ka radiation (l value of 1.5406 Å) was used for structure analysis.
2.3.2. Scanning electron microscopy Scanning electron microscope (TESCAN Vega LMU) was used to observe the morphology of the samples. Samples were sputtered with gold to avoid charging effects.
2.3.3. Fourier Transform Infrared spectroscopy The structure and intermolecular interactions between components of samples were investigated by Fourier transform infrared (FTIR) spectroscopy. FTIR spectra of samples were recorded with Thermo Scientific Nicolet 8700 FTIR spectrometer at 8 cm1 resolution in averaging of 256 scans. The spectra were collected over 4000-400 cm1 range. 2.3.4. Electrochemical measurements Cyclic voltammetry (CV) measurements were performed in a standard three-electrode electrochemical cell utilizing a Potentiostat/Galvanostat with GPES interface (ECO Chemie, The Netherlands). Paper composites sample were employed as a working electrode, Platinum (Pt) wire as a counter electrode and Ag/AgCl as a reference electrode. CV measurements were recorded in potential window of 0.2e0.7 V at scan rate of 5 mV/s in 2.0 M solutions of sodium sulfate (Na2SO4) at different scan rates. Fresh
Fig. 2. SEM images of: {a} ZnO {b} MnO2 {c} ZnO/MnO2 {d} ZnO/MnO2/LC {e} ZnO/TiO2{f} ZnO/TiO2/LC.
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Fig. 3. FTIR Spectra of: {a} MnO2 (I), ZnO (II), ZnO/MnO2 (III), ZnO/MnO2/LC (IV). {b} TiO2 (I), ZnO (II), ZnO/TiO2 (III), ZnO/TiO2/LC (IV).
samples were used for each measurement. The weights of the samples were in the range of 5e8 mg and results were normalized with respect to the active mass in the sample. 3. Results and discussion XRD measurements were performed to confirm the crystalline structure of fabricated composite ZnO/TiO2 and ZnO/MnO2. Fig. 1a displays XRD pattern of ZnO/TiO2 composite where all sharp and clear diffraction peaks are well indexed based on anatase phase of TiO2 (JCPDS No.01-084-1285). In Fig. 1a, existence of strong crystalline peaks at 2Ѳ values of 25.3 , 37.8 , 48.0 , 53.8 , 55.0 and 62.6 correspond to crystal planes of (101), (004), (200), (105), (211) and (204), respectively. The appearance of other peaks at 2Ѳ values of 34.3 , 35.3 and 46.7 correspond to crystal planes of (002), (101) and (102), respectively. Fig. 1a confirm the formation of ZnO (JCPDS No.01-075-1533) in presence of TiO2 whereas crystallite size were calculated aprox. 63 nm by Scherer equation. Fig. 1b displays XRD pattern of ZnO/MnO2 composite where existence of strong crystalline peaks at 2Ѳ values of 19.0 , 37.0 , 45.1, 49.4 , 59.7 and 65.6 correspond to crystal planes of (111), (311), (400), (331), (511) and (440), respectively and confirm the formation of MnO2 (JCPDS No.00-042-1169). In Fig. 2b, emergence of peaks at 30.7, 34.3 , 35.3 and 46.7 are due to hexagonal crystal system of ZnO (JCPDS No.01-075-1533) and correspond to crystal planes of (100), (002), (101) and (102), respectively. Unexpectedly, ZnO during the hydrothermal process react with MnO2 to produce rhombohedral crystal structure ZnMn3O7 (H2O)3 compound. In Fig. 1b, peaks at 23.5 , 25.3 , 26.8 , 28.6 , 34.8 and 35.1 correspond the crystal planes of (110), (015), (113), (202), (205) and (116) representing the
pattern of ZnMn3O7 (H2O)3 (JCPDS No.01-084-1692)[labeled with squares]. In Fig. 2b, crystallite size of ZnO/MnO2 is calculated approx. 62 nm by using Scherer equation. Fig. 2 shows the morphology of fabricated nanostructures characterized by scanning electron microscopy (SEM). Fig. 1a shows the morphology of stacking ZnO nanoplatelets consisted of fine nanoparticles whereas similar morphology is reported in literature as well [55]. Fig. 2b shows the nanorods morphology of MnO2 having average length of aprox. 2.17 mm and diameter 147 nm. Fig. 2c reveals the binary composite morphology of ZnO nanoplatelets and MnO2 nanorods of ZnO/MnO2 having uniform distribution. Fig. 2d shows the morphology of tertiary composite ZnO/ MnO2/LC. It can clearly observe the presence of ZnO, MnO2 and LC fibers, respectively in Fig. 2d. Embedded image of ZnO/MnO2/LC in Fig. 2d confirm that LC fibers acted as binder for ZnO/MnO2 to use as flexible and bulk paper electrode for energy storage and energy conversion applications. Fig. 2e shows SEM images of ZnO/TiO2 and ZnO/TiO2/LC composites. It can clearly seen in Fig. 2e that morphology of ZnO in platelets form and TiO2 is in particle form, respectively. Fig. 2e reveals the spherical shape of TiO2 with average diameter of̴ 157 nm. Fig. 2f displays the morphology of ZnO/TiO2/LC tertiary composite paper sheet. It can clearly observe in Fig. 2b that LC fibers acted as binder by wrapping of TiO2 nanoparticles and ZnO nanoplatelets. Embedded image (in Fig. 2f) of ZnO/TiO2/LC confirm the SEM results in the form of composite pare sheet which can be cut with help of scissor in any shape for high-tech application of energy storage and dye-synthesized solar cell. Fig. 3 shows FTIR spectrum of all presented nanostructures and supplements the information obtained from XRD and SEM. In FTIR spectrum, frequencies at which absorption occurs indicate the type
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of functional groups present in the sample under studies. Fig. 3a shows FTIR spectra of MnO2 (i), ZnO (ii), ZnO/MnO2 (iii) and ZnO/ MnO2/LC (iv), respectively. Fig. 3a illustrates a series of absorption bands in the range of 500e4000 cm1 in which region below 1500 cm1 is important for metal oxides i.e. absorption bands due to inter-atomic vibrations. The characteristic absorption band of MnO2 occurs at 690 cm1 corresponds to MneO stretching vibrations [56] whereas absorption band from vibration properties of ZnO is observed in the range of 650e1000 cm1 [57,58]. In Fig. 3a, curves (iii) and (iv) display the peaks at 1632 and 3340 cm1 which correspond to OeH bending and stretching for ZnO/MnO2 and ZnO/ MnO2/LC, respectively. In Fig. 3a, curve (iv) revealed the peak at 1326 cm1 which is attributed to symmetric CeH bending due to incorporation of LC fibers in ZnO/MnO2/LC [59]. Fig. 3a shows FTIR results of TiO2 (i), ZnO (ii), ZnO/TiO2 (iii) and ZnO/TiO2/LC (iv), respectively. Fig. 3b shows the same results as discussed in SEM results, apart from the peak observed at 2360 cm1 which indicate CeO bending to the absorption of CO2 molecule present in air. Fig. 4 shows cyclic voltammetry (CV) measurements of ZnO/ MnO2/LC and ZnO/TiO2/LC to confirm the electrochemical characteristics for energy storage. Fig. 4a shows cyclic voltammograms of ZnO/MnO2/LC (as working electrode) in 2.0 M sodium sulfate solution. The reversible redox prosperities of ZnO/MnO2/LC can
Fig. 5. Stability Cycles of {a} ZnO/MnO2/LC and {b} ZnO/TiO2/LC. Inset image shows comparison of second and hundredth cycles of ZnO/MnO2/LC and ZnO/TiO2/LC, respectively.
Fig. 4. Cyclic voltammograms of {a} ZnO/MnO2/LC and {b} ZnO/TiO2/LC.
clearly observe in Fig. 4 by employing wide potential range from 0.05 V to þ0.8 V at different scan rates. In Fig. 4a, current values are normalized with respect to active material of Zn/MnO2. In Fig. 4a, ratio of the reverse-to-forward peak currents, Ipred/Ipoxd, is unity for a simple reversible couple which approaches the limit for a reversible one electron transfer which confirm the reversible behavior in presence of ZnO/MnO2. Fig. 4b shows CV measurements of ZnO/TiO2/LC composite sheet in 2.0 M sodium sulfate solution by employing working potential window form 1.0 to 1.0 at different scan rates. It can clearly seen in Fig. 4b that process is quasi reversible and two oxidation peaks at 0.1 and 0.6 and two reduction peaks at 0.1 and 0.5, respectively which correspond to ZnO and TiO2. CV measurements confirmed the electro-active behavior of ZnO/MnO2/LC and ZnO/TiO2/LC flexible composite sheets which is highly feasible for energy storage and energy conversion devices. Fig. 5 shows the stability measurements of ZnO/MnO2/LC and ZnO/ TiO2/LC whereas inset images shows the comparison of 2nd and 100th cycles, respectively. It can seen in Fig. 5a that ZnO/MnO2/LC composite sheet retain approximately̴ 70% specific capacitance. Fig. 5b shows that ZnO/TiO2/LC composite paper sheet reveals loss of 13% specific capacitance after 100 cycles whereas inset image confirm the presented results.
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4. Conclusion ZnO/MnO2 and ZnO/TiO2 were prepared via facile hydrothermal method in two consecutive steps at relatively low temperature. XRD measurements revealed that ZnO/TiO2 and ZnO/MnO2 were in pure crystalline form with crystallite size of̴ 62 nm (ZnO/MnO2) and̴ 63 nm (ZnO/TiO2). SEM images shows the morphology of ZnO as nano-platelets, TiO2 as nanoparticles and MnO2 as nanorods, respectively. Furthermore lignocelluloses fibers, directly collected form self-growing plant, Monochornia Vagina employed as binders for ZnO/TiO2 and ZnO/MnO2 nanostructures for development of bulk, flexible working electrodes for energy storage applications. FTIR results confirm the formation of binary (ZnO/TiO2 and ZnO/ MnO2) and tertiary composites (ZnO/MnO2/LC and ZnO/TiO2/LC). CV measurements confirmed the capacitive behavior of ZnO/MnO2/ LC and ZnO/TiO2/LC paper composite sheet at different scan rate shows with complete reversibility. Presented flexible ZnO/MnO2/LC and ZnO/TiO2/LC composite paper electrode can be cut with the help of scissor in any shape which is highly feasible in high-tech application of energy storage and energy conversion applications. Acknowledgement The financial support from NRPU grant no: 5334/Federal/NRPU/ R&D/HEC/2016 awarded by Higher Education Commission (HEC), Pakistan are greatly acknowledged. References [1] Sunandan Baruah, Chanchana Thanachayanont, Joydeep Dutta, Growth of ZnO nanowires on nonwoven polyethylene fibers, Sci. Technol. Adv. Mater. 9 (2008) 1e8. [2] Bonamali Pal, Maheshwar Sharon, Enhanced photocatalytic activity of highly porous ZnO thin films prepared by solegel process, Mater. Chem. Phys. 76 (2002) 82e87. [3] Eriko Oshima, Hiraku Ogina, Ikuo Niikura, Katsumi Maeda, Mitsuru Sato, Masumi Ito, Tsuguo Fukuda, Growth of the 2-in-size bulk ZnO single crystals by the hydrothermal method, J. Cryst. Growth 260 (2004) 166e170. [4] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, High temperature excitonic stimulated emission from ZnO epitaxial layers, Appl. Phys. Lett. 73 (1998) 1038e1040. [5] Mingpeng Yu, Hongtao Sun, Xiang Sun, Fengyuan Lu, Gongkai Wang, Tao Hu, Hong Qiu, Jie Lian, Hierarchical Al-doped and hydrogenated ZnO Nanowire@ MnO2 ultra-thin nanosheet core/shell arrays for high-performance supercapacitor electrode, Int. J. Electrochem. Sci. 8 (2013) 2313e2329. [6] H. Jiang, L. Yang, C. Li, C. Yan, P.S. Lee, J. Ma, Higherate electrochemical capacitors from highly graphitic carbonetipped manganese oxide/mesoporous carbon/manganese oxide hybrid nanowires, Energy Environ. Sci. 4 (2011) 1813e1819. [7] C.H. Hsu, D.H. Chen, Synthesis and conductivity enhancement of Al-doped ZnO nanorod array thin films, Nanotechnology 21 (2010) 1e8. [8] B.L. Zhu, J. Wang, S.J. Zhu, J. Wu, R. Wu, D.W. Zeng, C.S. Xie, Influence of hydrogen introduction on structure and properties of ZnO thin films during sputtering and post-annealing, Thin Solid Films 519 (2011) 3809e3815. [9] Sunandon Baruah, Joydeep Dutta, Hydrothermal growth of ZnO nanostructures, Sci. Technol. Adv. Mater. 10 (2009) 1e18. [10] N.W. Emanetoglu, C. Gorla, Y. Liu, S. Liang, Y. Lu, Epitaxial ZnO piezoelectric thin films for saw filters, Mater. Sci. Semicond. Process. 2 (1999) 247e252. [11] Y. Chen, D. Bagnall, T. Yao, ZnO as a novel photonic material for the UV region, Mater. Sci. Eng. B 75 (2000) 190e198. [12] S. Liang, H. Sheng, Y. Liu, Z. Hio, Y. Lu, H. Chen, ZnO Schottky ultraviolet photodetectors, J. Cryst. Growth 225 (2001) 110e113. [13] Noriko Saito, Hajime Haneda, Takashi Sekiguchi, Naoki Ohashi, Isao Sakaguchi, Kunihito Koumoto, Low temperature fabrication of light-emitting zinc oxide micropatterns using self-assembled monolayers, Adv. Mater. 14 (2002) 418e421. [14] J.Y. Lee, Y.S. Choi, J.H. Kim, M.O. Park, S. Im, Optimizing n-ZnO@p-Si heterojunctions for photodiode applications, Thin Solid Films 403 (2002) 553e557. [15] P. Mitra, A.P. Chatterjee, H.S. Maiti, ZnO thin film sensor, Mater. Lett. 35 (1998) 33e38. [16] M.H. Koch, P.Y. Timbrell, R.N. Lamb, The influence of film crystallinity on the coupling efficiency of ZnO optical modulator waveguides, Semicond. Sci. Technol. 10 (1995) 1523e1527. [17] M. Gratzel, Dye-Sensitized solid-state heterojunction solar cells, MRS Bull. 30 (2005) 23e27. [18] J.B. Baxter, A.M. Walker, K. van Ommering, E.S. Aydil, Synthesis and
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication and characterization of ZnO/MnO2 and ZnO/TiO2 flexible nanocomposites for energy storage applications M. Yasir Rafiq a, Faisal Iqbal b, Fawad Aslam c, Muhammad Bilal a, Naeem Munir a, I. Sultana a, Fawad Ashraf d, Faisal Manzoor e, Najmul Hassan b, Aamir Razaq a, * a
Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan Materials Growth and Simulation Laboratory, Department of Physics, University of the Punjab, Lahore 54590, Pakistan Department of Physics, Hazara University Mansehra, Pakistan d Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore 54000, Pakistan e Interdisciplinary Research Centre in Biomedical Materials, COMSATS Institute of Information Technology, Lahore 54000, Pakistan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 June 2017 Received in revised form 20 September 2017 Accepted 23 September 2017 Available online 25 September 2017
ZnO based nanostructure composites are attractive for high-tech applications of energy conversion and energy storage due to wide range in operating potential window. This study presents about fabrication of ZnO/MnO2 and ZnO/TiO2 composites via hydrothermal method in two consecutive steps. Furthermore lignocelluloses fiber directly collected form self-growing plant Monochoria Vagina, were incorporated in fabricated ZnO/MnO2 and ZnO/TiO2 composites for development of flexible and bulk paper composite electrode. The structural and morphological analysis were carried out using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), respectively. XRD analysis confirm the crystalline structures for ZnO/MnO2 and ZnO/TiO2 samples with crystallite size ~62 nm and ~63 nm, respectively. SEM images shows the sheet-like and nanorods morphology for ZnO and MnO2 nanostructures, respectively. Fourier Transform Infrared (FTIR) Spectroscopy absorbance spectrum reveal the composite formation of ZnO/ MnO2/LC and ZnO/TiO2/LC as bulk paper electrodes whereas cyclic Voltammetry measurement showed the capacitive behavior of composite paper electrodes for the suitable applications in supercapacitors and batteries. © 2017 Elsevier B.V. All rights reserved.
Keywords: ZnO composites Hydrothermal method Nanostructures Lignocelluloses Paper composites
1. Introduction Zinc oxide (ZnO) based composites has attracted the interest of researchers due to direct wide band gap of 3.37 eV and large exciton-binding energy of 60 meV [1e4]. Due to versatile applications of ZnO, several attempts are reported for development of ZnO nanostructures to enhance ion diffusion due to high surface area [5e9]. ZnO based nanostructures have a wide range of high-tech application e.g. surface acoustic wave filter [10], photonic crystal [11], photo detectors [12], light emitting diode [13], photo diodes [14], gas sensors [15] optical modulators wave guide [16], solar cells [17,18], varistors [19], electrochemical supercapacitors [20e22], nanogenerators [23e29], fuel cell and batteries [30]. To address ZnO based composite formation, coupling with metal oxide having different redox energy levels, provides an attractive
* Corresponding author. E-mail address: [email protected] (A. Razaq). https://doi.org/10.1016/j.jallcom.2017.09.253 0925-8388/© 2017 Elsevier B.V. All rights reserved.
approach to achieve efficient charge separation for enhanced life time of charge carriers and better efficiency of interfacial charge transfer [31]. Titanium dioxide (TiO2) has received significant attention due to low cost, high photocatalytic activity, and high stability [32,33]. Titanium dioxide (TiO2) and zinc oxide (ZnO) are two kinds of technologically important semiconductors because of potential applications in advanced devices/systems e.g. photonic devices, sensors, solar cells, etc [34e38]. ZnO/TiO2 based composite nanofibers are also reported as a promising candidate for exceptional antibacterial agents under UV light [39]. Polycrystalline TiO2 and ZnO both in pure form/composite, widely used in photocatalytic reaction in liquid-solid as well as in gas-solid regimes. The coupling of anatase and rutile TiO2 with ZnO are useful in order to achieve efficient electron-hole pair separation under illumination whereas presence of zinc also contributes to the stabilization of the free charge carriers [40]. Efficiency of ZnO nanorod based solar cells is mainly limited by low photocurrent [41,44] due to poor interfacial quality which is not favorable for electron injection [45,46]. In literature, capping of ZnO nanorods with small conductive
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molecules, dyes or a thin TiO2 layer are also reported to facilitate electron injection across the heterojunction [42,46]. MnO2 with the advantages of environmental friendliness, good cycle stability, natural abundance, low cost, and efficient chargedischarge appears to be promising electrode materials for supercapacitors [47,51]. Xing et al. developed the electrochemical synthesis of ZnO nanorod/MnO2 shell composites to address the low charge capacitance [52]. ZnO based cores served as mechanical supports and effective electron transport pathways to increase the electrochemical utilization of MnO2 [52]. Nanoporous MnO2 shells facilitated the ion diffusion and energy storage capacity due to relatively high electronic conductivity, and mechanical flexibility. Mao et al. reported ZnO@MnO2 core@shell nanostructure electrode on titanium substrate to obtain specific areal capacitances of 31.30 mF cm2 [52]. Yang et al. fabricated ZnO@MnO2 core shell nanocables, and authentically improved the electrochemical performance of supercapacitor [53]. The performance of MnO2 nanowire/ZnO nanorod array hybrid electrode under different bending angles demonstrates excellent mechanical stability for flexible energy storage devices [54]. Flexible energy storage devices are highly feasible for the need of modern electronic systems e.g. rollup displays, wearable devices and biologically inspired medical implements. In practical applications a flexible power source must be capable of accommodating frequent complex strains e.g. bending, twisting and deforming while retaining continuous energy supply. Lignocelluloses is the most abundant, renewable, flexible and ecofriendly material on earth. In era of modern disposable electronic technology, development of thin, flexible, light-weight and environmentally friendly electrode material are highly viable. This study presents about the novel fabrication method of ZnO/TiO2 and ZnO/MnO2 via hydrothermal method in two consecutive steps. Furthermore lignocelluloses fibers, directly collected from selfgrowing plant are used as binder for ZnO/TiO2 and ZnO/MnO2 nanostructures for bulk applications of energy storage. 2. Experimental 2.1. Material and method Zinc Nitrate Zn(NO3)2$6H2O, Hexamethylenetetramine (CH2)6N4, Potassium permanganate (KMnO4) and Titanium dioxide (TiO2) were purchased from Merck and freshly used. Lignocelluloses fibers were directly collected from stem of self-growing plant Monochornia Vagnia. 2.2. Fabrication of nanostructures 2.2.1. Preparation of ZnO ~2 g of Zinc Nitrate Zn(NO3)2$6H2O and 0.141 g of Hexamethylenetetramine (CH2)6N4 was dissolved in 20 mL of distilled water and stirred till precipitate occurred. Further slurry was placed in Teflon beaker of autoclave and kept at 80 C for 2 h in oven. Subsequently the solution was centrifuged to get the product and further placed for drying at 80 C in oven. 2.2.2. Preparation of MnO2 ~0.16 g of KMnO4 was dissolved in 35 mL deionized water for 10 min and 2.1 mL of 2 M HCl was added drop wise. Subsequently, the solution was transferred in Teflon cup and sealed in stain-less steel autoclave. The temperature of autoclave was maintained at 200 C for 4 h. Dark grey product was collected at bottom of Teflon cup and rinsed with deionized water during centrifugation to remove the alkali salt and other impurities. The final product was obtained by drying in oven at 80 C.
Fig. 1. XRD pattern of {a} ZnO/TiO2 {b} f ZnO/MnO2.
2.2.3. Preparation of ZnO/MnO2 composite Recipe as described in section 2.2.2 was repeated and output sample was added as precursor in recipe of ZnO as described in section 2.2.1. 2.2.4. Preparation of ZnO/MnO2/LC sheet ~150 mg of LC fibers were dispersed in DI water and solution to obtain homogenous jel. Subsequently 300 mg of ZnO/MnO2 powder were mixed with LC fibers jel on magnetic stirrer for 30 min. Final solution was placed on Buchner funnel and pressed to obtain homogenous sheets. 2.2.5. Preparation of ZnO/TiO2 ~1 g of Titanium dioxide (TiO2)[as purchased] was dispersed in 10 mL of distilled water and used as precursor in recipe of ZnO as described in section 2.2.1. 2.2.6. Preparation of ZnO/TiO2/LC sheet ~150 mg of LC fibers were dispersed in DI water and solution to
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obtain homogenous jel. Subsequently 150 mg of ZnO/TiO2 powder were mixed with LC fibers jel on magnetic stirrer for 30 min. Final slurry was placed on Buchner funnel and pressed to obtain homogenous sheets.
2.3. Characterizations 2.3.1. X. Ray diffraction PANalytical's X-ray diffractometer (XRD) in scanning range 20 e80 by Cu Ka radiation (l value of 1.5406 Å) was used for structure analysis.
2.3.2. Scanning electron microscopy Scanning electron microscope (TESCAN Vega LMU) was used to observe the morphology of the samples. Samples were sputtered with gold to avoid charging effects.
2.3.3. Fourier Transform Infrared spectroscopy The structure and intermolecular interactions between components of samples were investigated by Fourier transform infrared (FTIR) spectroscopy. FTIR spectra of samples were recorded with Thermo Scientific Nicolet 8700 FTIR spectrometer at 8 cm1 resolution in averaging of 256 scans. The spectra were collected over 4000-400 cm1 range. 2.3.4. Electrochemical measurements Cyclic voltammetry (CV) measurements were performed in a standard three-electrode electrochemical cell utilizing a Potentiostat/Galvanostat with GPES interface (ECO Chemie, The Netherlands). Paper composites sample were employed as a working electrode, Platinum (Pt) wire as a counter electrode and Ag/AgCl as a reference electrode. CV measurements were recorded in potential window of 0.2e0.7 V at scan rate of 5 mV/s in 2.0 M solutions of sodium sulfate (Na2SO4) at different scan rates. Fresh
Fig. 2. SEM images of: {a} ZnO {b} MnO2 {c} ZnO/MnO2 {d} ZnO/MnO2/LC {e} ZnO/TiO2{f} ZnO/TiO2/LC.
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Fig. 3. FTIR Spectra of: {a} MnO2 (I), ZnO (II), ZnO/MnO2 (III), ZnO/MnO2/LC (IV). {b} TiO2 (I), ZnO (II), ZnO/TiO2 (III), ZnO/TiO2/LC (IV).
samples were used for each measurement. The weights of the samples were in the range of 5e8 mg and results were normalized with respect to the active mass in the sample. 3. Results and discussion XRD measurements were performed to confirm the crystalline structure of fabricated composite ZnO/TiO2 and ZnO/MnO2. Fig. 1a displays XRD pattern of ZnO/TiO2 composite where all sharp and clear diffraction peaks are well indexed based on anatase phase of TiO2 (JCPDS No.01-084-1285). In Fig. 1a, existence of strong crystalline peaks at 2Ѳ values of 25.3 , 37.8 , 48.0 , 53.8 , 55.0 and 62.6 correspond to crystal planes of (101), (004), (200), (105), (211) and (204), respectively. The appearance of other peaks at 2Ѳ values of 34.3 , 35.3 and 46.7 correspond to crystal planes of (002), (101) and (102), respectively. Fig. 1a confirm the formation of ZnO (JCPDS No.01-075-1533) in presence of TiO2 whereas crystallite size were calculated aprox. 63 nm by Scherer equation. Fig. 1b displays XRD pattern of ZnO/MnO2 composite where existence of strong crystalline peaks at 2Ѳ values of 19.0 , 37.0 , 45.1, 49.4 , 59.7 and 65.6 correspond to crystal planes of (111), (311), (400), (331), (511) and (440), respectively and confirm the formation of MnO2 (JCPDS No.00-042-1169). In Fig. 2b, emergence of peaks at 30.7, 34.3 , 35.3 and 46.7 are due to hexagonal crystal system of ZnO (JCPDS No.01-075-1533) and correspond to crystal planes of (100), (002), (101) and (102), respectively. Unexpectedly, ZnO during the hydrothermal process react with MnO2 to produce rhombohedral crystal structure ZnMn3O7 (H2O)3 compound. In Fig. 1b, peaks at 23.5 , 25.3 , 26.8 , 28.6 , 34.8 and 35.1 correspond the crystal planes of (110), (015), (113), (202), (205) and (116) representing the
pattern of ZnMn3O7 (H2O)3 (JCPDS No.01-084-1692)[labeled with squares]. In Fig. 2b, crystallite size of ZnO/MnO2 is calculated approx. 62 nm by using Scherer equation. Fig. 2 shows the morphology of fabricated nanostructures characterized by scanning electron microscopy (SEM). Fig. 1a shows the morphology of stacking ZnO nanoplatelets consisted of fine nanoparticles whereas similar morphology is reported in literature as well [55]. Fig. 2b shows the nanorods morphology of MnO2 having average length of aprox. 2.17 mm and diameter 147 nm. Fig. 2c reveals the binary composite morphology of ZnO nanoplatelets and MnO2 nanorods of ZnO/MnO2 having uniform distribution. Fig. 2d shows the morphology of tertiary composite ZnO/ MnO2/LC. It can clearly observe the presence of ZnO, MnO2 and LC fibers, respectively in Fig. 2d. Embedded image of ZnO/MnO2/LC in Fig. 2d confirm that LC fibers acted as binder for ZnO/MnO2 to use as flexible and bulk paper electrode for energy storage and energy conversion applications. Fig. 2e shows SEM images of ZnO/TiO2 and ZnO/TiO2/LC composites. It can clearly seen in Fig. 2e that morphology of ZnO in platelets form and TiO2 is in particle form, respectively. Fig. 2e reveals the spherical shape of TiO2 with average diameter of̴ 157 nm. Fig. 2f displays the morphology of ZnO/TiO2/LC tertiary composite paper sheet. It can clearly observe in Fig. 2b that LC fibers acted as binder by wrapping of TiO2 nanoparticles and ZnO nanoplatelets. Embedded image (in Fig. 2f) of ZnO/TiO2/LC confirm the SEM results in the form of composite pare sheet which can be cut with help of scissor in any shape for high-tech application of energy storage and dye-synthesized solar cell. Fig. 3 shows FTIR spectrum of all presented nanostructures and supplements the information obtained from XRD and SEM. In FTIR spectrum, frequencies at which absorption occurs indicate the type
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of functional groups present in the sample under studies. Fig. 3a shows FTIR spectra of MnO2 (i), ZnO (ii), ZnO/MnO2 (iii) and ZnO/ MnO2/LC (iv), respectively. Fig. 3a illustrates a series of absorption bands in the range of 500e4000 cm1 in which region below 1500 cm1 is important for metal oxides i.e. absorption bands due to inter-atomic vibrations. The characteristic absorption band of MnO2 occurs at 690 cm1 corresponds to MneO stretching vibrations [56] whereas absorption band from vibration properties of ZnO is observed in the range of 650e1000 cm1 [57,58]. In Fig. 3a, curves (iii) and (iv) display the peaks at 1632 and 3340 cm1 which correspond to OeH bending and stretching for ZnO/MnO2 and ZnO/ MnO2/LC, respectively. In Fig. 3a, curve (iv) revealed the peak at 1326 cm1 which is attributed to symmetric CeH bending due to incorporation of LC fibers in ZnO/MnO2/LC [59]. Fig. 3a shows FTIR results of TiO2 (i), ZnO (ii), ZnO/TiO2 (iii) and ZnO/TiO2/LC (iv), respectively. Fig. 3b shows the same results as discussed in SEM results, apart from the peak observed at 2360 cm1 which indicate CeO bending to the absorption of CO2 molecule present in air. Fig. 4 shows cyclic voltammetry (CV) measurements of ZnO/ MnO2/LC and ZnO/TiO2/LC to confirm the electrochemical characteristics for energy storage. Fig. 4a shows cyclic voltammograms of ZnO/MnO2/LC (as working electrode) in 2.0 M sodium sulfate solution. The reversible redox prosperities of ZnO/MnO2/LC can
Fig. 5. Stability Cycles of {a} ZnO/MnO2/LC and {b} ZnO/TiO2/LC. Inset image shows comparison of second and hundredth cycles of ZnO/MnO2/LC and ZnO/TiO2/LC, respectively.
Fig. 4. Cyclic voltammograms of {a} ZnO/MnO2/LC and {b} ZnO/TiO2/LC.
clearly observe in Fig. 4 by employing wide potential range from 0.05 V to þ0.8 V at different scan rates. In Fig. 4a, current values are normalized with respect to active material of Zn/MnO2. In Fig. 4a, ratio of the reverse-to-forward peak currents, Ipred/Ipoxd, is unity for a simple reversible couple which approaches the limit for a reversible one electron transfer which confirm the reversible behavior in presence of ZnO/MnO2. Fig. 4b shows CV measurements of ZnO/TiO2/LC composite sheet in 2.0 M sodium sulfate solution by employing working potential window form 1.0 to 1.0 at different scan rates. It can clearly seen in Fig. 4b that process is quasi reversible and two oxidation peaks at 0.1 and 0.6 and two reduction peaks at 0.1 and 0.5, respectively which correspond to ZnO and TiO2. CV measurements confirmed the electro-active behavior of ZnO/MnO2/LC and ZnO/TiO2/LC flexible composite sheets which is highly feasible for energy storage and energy conversion devices. Fig. 5 shows the stability measurements of ZnO/MnO2/LC and ZnO/ TiO2/LC whereas inset images shows the comparison of 2nd and 100th cycles, respectively. It can seen in Fig. 5a that ZnO/MnO2/LC composite sheet retain approximately̴ 70% specific capacitance. Fig. 5b shows that ZnO/TiO2/LC composite paper sheet reveals loss of 13% specific capacitance after 100 cycles whereas inset image confirm the presented results.
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4. Conclusion ZnO/MnO2 and ZnO/TiO2 were prepared via facile hydrothermal method in two consecutive steps at relatively low temperature. XRD measurements revealed that ZnO/TiO2 and ZnO/MnO2 were in pure crystalline form with crystallite size of̴ 62 nm (ZnO/MnO2) and̴ 63 nm (ZnO/TiO2). SEM images shows the morphology of ZnO as nano-platelets, TiO2 as nanoparticles and MnO2 as nanorods, respectively. Furthermore lignocelluloses fibers, directly collected form self-growing plant, Monochornia Vagina employed as binders for ZnO/TiO2 and ZnO/MnO2 nanostructures for development of bulk, flexible working electrodes for energy storage applications. FTIR results confirm the formation of binary (ZnO/TiO2 and ZnO/ MnO2) and tertiary composites (ZnO/MnO2/LC and ZnO/TiO2/LC). CV measurements confirmed the capacitive behavior of ZnO/MnO2/ LC and ZnO/TiO2/LC paper composite sheet at different scan rate shows with complete reversibility. Presented flexible ZnO/MnO2/LC and ZnO/TiO2/LC composite paper electrode can be cut with the help of scissor in any shape which is highly feasible in high-tech application of energy storage and energy conversion applications. Acknowledgement The financial support from NRPU grant no: 5334/Federal/NRPU/ R&D/HEC/2016 awarded by Higher Education Commission (HEC), Pakistan are greatly acknowledged. References [1] Sunandan Baruah, Chanchana Thanachayanont, Joydeep Dutta, Growth of ZnO nanowires on nonwoven polyethylene fibers, Sci. Technol. Adv. Mater. 9 (2008) 1e8. [2] Bonamali Pal, Maheshwar Sharon, Enhanced photocatalytic activity of highly porous ZnO thin films prepared by solegel process, Mater. Chem. Phys. 76 (2002) 82e87. [3] Eriko Oshima, Hiraku Ogina, Ikuo Niikura, Katsumi Maeda, Mitsuru Sato, Masumi Ito, Tsuguo Fukuda, Growth of the 2-in-size bulk ZnO single crystals by the hydrothermal method, J. Cryst. Growth 260 (2004) 166e170. [4] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, High temperature excitonic stimulated emission from ZnO epitaxial layers, Appl. Phys. Lett. 73 (1998) 1038e1040. [5] Mingpeng Yu, Hongtao Sun, Xiang Sun, Fengyuan Lu, Gongkai Wang, Tao Hu, Hong Qiu, Jie Lian, Hierarchical Al-doped and hydrogenated ZnO Nanowire@ MnO2 ultra-thin nanosheet core/shell arrays for high-performance supercapacitor electrode, Int. J. Electrochem. Sci. 8 (2013) 2313e2329. [6] H. Jiang, L. Yang, C. Li, C. Yan, P.S. Lee, J. Ma, Higherate electrochemical capacitors from highly graphitic carbonetipped manganese oxide/mesoporous carbon/manganese oxide hybrid nanowires, Energy Environ. Sci. 4 (2011) 1813e1819. [7] C.H. Hsu, D.H. Chen, Synthesis and conductivity enhancement of Al-doped ZnO nanorod array thin films, Nanotechnology 21 (2010) 1e8. [8] B.L. Zhu, J. Wang, S.J. Zhu, J. Wu, R. Wu, D.W. Zeng, C.S. Xie, Influence of hydrogen introduction on structure and properties of ZnO thin films during sputtering and post-annealing, Thin Solid Films 519 (2011) 3809e3815. [9] Sunandon Baruah, Joydeep Dutta, Hydrothermal growth of ZnO nanostructures, Sci. Technol. Adv. Mater. 10 (2009) 1e18. [10] N.W. Emanetoglu, C. Gorla, Y. Liu, S. Liang, Y. Lu, Epitaxial ZnO piezoelectric thin films for saw filters, Mater. Sci. Semicond. Process. 2 (1999) 247e252. [11] Y. Chen, D. Bagnall, T. Yao, ZnO as a novel photonic material for the UV region, Mater. Sci. Eng. B 75 (2000) 190e198. [12] S. Liang, H. Sheng, Y. Liu, Z. Hio, Y. Lu, H. Chen, ZnO Schottky ultraviolet photodetectors, J. Cryst. Growth 225 (2001) 110e113. [13] Noriko Saito, Hajime Haneda, Takashi Sekiguchi, Naoki Ohashi, Isao Sakaguchi, Kunihito Koumoto, Low temperature fabrication of light-emitting zinc oxide micropatterns using self-assembled monolayers, Adv. Mater. 14 (2002) 418e421. [14] J.Y. Lee, Y.S. Choi, J.H. Kim, M.O. Park, S. Im, Optimizing n-ZnO@p-Si heterojunctions for photodiode applications, Thin Solid Films 403 (2002) 553e557. [15] P. Mitra, A.P. Chatterjee, H.S. Maiti, ZnO thin film sensor, Mater. Lett. 35 (1998) 33e38. [16] M.H. Koch, P.Y. Timbrell, R.N. Lamb, The influence of film crystallinity on the coupling efficiency of ZnO optical modulator waveguides, Semicond. Sci. Technol. 10 (1995) 1523e1527. [17] M. Gratzel, Dye-Sensitized solid-state heterojunction solar cells, MRS Bull. 30 (2005) 23e27. [18] J.B. Baxter, A.M. Walker, K. van Ommering, E.S. Aydil, Synthesis and
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