Synthesis of self-assembled mesoporous 3D In2O3 hierarchical micro flowers composed of nanosheets and their electrochemical properties
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This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY-NC 3.0 https://creativecommons.org/licenses/by-nc/3.0/ Synthesis of self-assembled mesoporous 3D In2O3 hierarchical micro flowers composed of nanosheets and their electrochemical properties © the Authors, 2018. Published version Prakasam, Balasubramaniam Arul; Lahtinen, Manu; Peuronen, Anssi; Manikandan, Govindhasamy; Muruganandham, Manickavachagam; Sillanpää, Mika Prakasam, B. A., Lahtinen, M., Peuronen, A., Manikandan, G., Muruganandham, M., & Sillanpää, M. (2018). Synthesis of self-assembled mesoporous 3D In2O3 hierarchical micro flowers composed of nanosheets and their electrochemical properties. RSC Advances, 8(45), 2585625865. https://doi.org/10.1039/c8ra03573c 2018
Synthesis of self-assembled mesoporous 3D In 2 O 3 hierarchical micro flowers composed of nanosheets and their electrochemical properties† Balasubramaniam Arul Prakasam, * ab Manu Lahtinen, c Anssi Peuronen, c Govindhasamy Manikandan, b Manickavachagam Muruganandham a and Mika Sillanp¨ a¨ a ad This report describes the methodology for the fabrication of mesoporous In 2 O 3 microflowers by hydrothermal and calcination procedures in which In(OH) 3 /In 2 S 3 acts as an intermediate. Both In 2 O 3 and its precursor were analyzed with scanning electron microscopy, energy dispersive X-ray spectrophotometry, transmission electron microscopy and powder X-ray diffraction. BET surface area, pore size and pore volume analyses were also carried out. Electron microscopy images clearly evidence the self-assembly of 2D nanosheets into the micro flower structure. The mechanism of self-assembly and calcination is reported. Electrochemical properties of the synthesized In 2 O 3 micro flowers were studied. 1. Introduction The synthesis of semiconductor nanomaterials with extraordinary shapes and structures are of great interest since these properties can signicantly inuence the semiconducting properties of such materials. Therefore, the development of synthetic pathways that give rise to certain nanoscopic features is important. Decomposition of simple precursor components in benign hydrothermal conditions presents an interesting strategy to yield novel nanosized 3D structures with a large surface area. One semiconducting material, which represents a suitable candidate for such an approach, is indium(III) oxide, In 2 O 3 , which has a wide band gap of 3.55–3.75 eV (analogous to GaN). In 2 O 3 has been shown to have many promising utilization areas such as gas sensors, solar cells and at-panel displays. 1–6 Versatile morphological properties of In 2 O 3 are demonstrated in several studies, wherein various shapes, such as nanocubes, nanorods and microspheres, have been reported. 7–23 Generally, In(OH) 3 and InOOH are used as precursors for the synthesis of In 2 O 3 . 7–20 Synthesis of In 2 O 3 via In(OH) 3 /In 2 S 3 as an intermediate mesostructure, followed by calcination, presents an interesting option which allows variation of morphology and porosity due to the intermediate structure which is eventually oxidized to In 2 O 3 . 21–33 There are some latest reports on the fabrication and characterization of morphologically interesting nano materials. 34–37 Variation of the nanostructure using this synthetic pathway arises largely from the selection of the precursor components. Although many organic compounds can be used as a sulfur source, careful selection is obligatory as the organic compounds (if the process is free from calcination) can be incorporated with the positively charged metal ions on the surface of nanoparticles and thereby limit the mobility of the charge carrier in electronic devices. Dithiobiuret is an interesting candidate in this regard as the decomposition products are volatile and hence the formed nanoparticles will be free from organic impurities. One of our particular areas of interest is morphosynthesis of porous hierarchical structures such as 3D microowers. 38 Within this scope, we herein demonstrate the self-assembly of 3D structural template consisting of indium sulde and hydroxide synthesized in hydrothermal conditions using dithiobiuret as the sulfur source and its subsequent calcination to a In 2 O 3 nanostructure. The 3D In 2 O 3 microowers are formed by the aggregation of 2D nanoakes with a high degree of compositional control. The structural and physical properties of these complex hierarchical structures are analyzed with SEM, EDX and TEM as well as by nitrogen adsorption analysis. Furthermore, the phase identication of samples was conrmed by powder X-ray diffraction (PXRD). An electrochemical study of the synthesized In 2 O 3 micro owers with an aim to understand the charge storage property has been carried out in detail. a Department of Green Chemistry, School of Engineering Science, Lappeenranta University of Technology, Sammonkatu 12, FI-50130 Mikkeli, Finland. E-mail: [email protected] b Department of Chemistry, Annamalai University, Annamalainagar, 608002, India c Department of Chemistry, Laboratories of Inorganic and Analytical Chemistry, University of Jyv¨ askyl¨ a, P. O. Box 35, FI-40014, Finland d Department of Civil and Environmental Engineering, Florida International University, Miami, FL-33174, USA †Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra03573c Cite this: RSC Adv.,2018,8,25856 Received 25th April 2018 Accepted 2nd July 2018 DOI: 10.1039/c8ra03573c rsc.li/rsc-advances 25856 |RSC Adv.,2018,8, 25856–25865 This journal is © The Royal Society of Chemistry 2018 RSC Advances PAPER Open Access Article. Published on 18 July 2018. Downloaded on 7/19/2018 6:22:52 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online View Journal | View Issue
2. Experimental details 2.1. Synthesis of indium oxide Milli Q-Plus water (resistance ¼18.2 M U) was used for all the experimental work. In 2 S 3 /In(OH) 3 micro owers were prepared using indium nitrate (1.5 g, 5 mmol) and dithiobiuret (1.0 g, 7.5 mmol). Indium nitrate and dithiobiuret were separately dissolved in about 50 mL of water and heated till boiling. Hot solutions were mixed under stirring (200 rpm) and aer 30 min the mixture was transferred into a 250 mL Teon cup, which was further lled with 50 mL of water. Sealed Teon vessel was put into a stainless steel shell and was autoclaved at 150 Cfor15h.Aer the hydrothermal reaction, the ask was allowedtocooldowntoroomtemperatureandtheprecipitate formed was ltered, washed with water and ethanol in order to eliminate possible impurities. It was then dried in an air oven at 120 Cfor2h.In 2 O 3 microowers were prepared in hot air oven by calcinating the precursors (In 2 S 3 /In(OH) 3 ) at 600 Cfor 6 h, followed by cooling down to room temperature before collecting the product. In this work, we aimed at the synthesis of structures with intriguing morphology, thus only the optimized reaction conditions are reported and the reaction conditions which resulted in inhomogeneous or broken morphologies are deliberately leout from the discussion. 2.2. SEM, TEM and powder PXRD measurements The morphology of prepared bulk was examined using Hitachi S-4100 scanning electron microscope (SEM). Prior to SEM measurements, the samples were mounted on a carbon platform that was then coated with platinum using a magnetron sputter for 10 minutes. The ratio between In, S and O within the samples were analyzed using an energy-dispersive X-ray spectrophotometer installed in the scanning electron microscope. For transmission electron microscope (TEM) study, the sample was dispersed onto a Cu grid with holey carbon supporting lms and studied at room temperature in a Tecnai F20 microscope operated at 200 kV. Powder X-ray diffraction (PXRD) was carried out using PANalytical X'Pert PRO diffractometer with Cu K a1 radiation (1.5406 ˚ A;45kV,40mA).A sample was placed on a silicon-made (producing zerobackground signal) sample holder using petrolatum jelly as an adhesive. Data was recorded with X'Celerator detector in 2qrange of 8–80using step size of 0.02and counting time of 140 s per step. Average crystal sizes of the substances were determined using Scherrer equation (eqn (1)) and values were corrected by the instrumental broadening. D¼ Kl bcos q(1) where in Dstands for average size of the crystals, Kas shapedependent Scherrer's constant (0.9), las radiation wavelength (1.5406 ˚ A), and bas integral breath given in radians subtracted by instrumental broadening (determined with a silicon standard measured using the above said experimental settings). 2.3. Electrochemical studies Cyclic voltammetry studies were performed using SP-150, BioLogic Science Instruments. A three electrode cell conguration comprising active material coated glassy carbon electrode (area 0.071 cm 2 ), Pt wire and Ag/AgCl were used as the working, counter and reference electrodes respectively. For the fabrication of electrode, a mixture of 1 mg of active material (In 2 O 3 ) and 1 ml of water was sonicated for 15 minutes and from the resultant suspension 5 microlitre was casted on GCE (working electrode). 3. Results and discussion 3.1. SEM analysis The surface morphologies of the hydrothermal and calcinated products were further analyzed by SEM. From the SEM images of the hydrothermal product (Fig. 1) it was observed that the microstructures with ower-like morphology are formed with the size distribution of around 0.8 to 1.2 mm. Microowers are built from small 2D nanosheets with thickness of 20–30 nm (Fig. 1e and f). These thin nanosheets, that are analogous to petals of owers, are aligned to the spherical surface by pointing toward a common center. The microower structures are highly dispersed and assembled by many interleaving and bending nanosheets with the thickness of less than 30 nm. From SEM or EDX we were unable to differentiate In(OH) 3 from In 2 S 3 structures as the micro owers exhibit peaks due to both oxygen and sulfur in EDX (Fig. S1†). SEM images of the calcinated product (Fig. 2) showed that the morphology was conserved aer calcination. However, thinning of nanosheet with curly structures, which are porous in nature, was observed. SEM image of In 2 O 3 microowers (Fig. 2) suggests that the petals with the sharp edges in the precursors are transformed into nanochain-like structures composed of small beads with the size of <10 nm (Fig. 2h). This is easily visualized by comparing the Fig. 1f and Fig. 2f–h. It is important to note that the overall dimensions of the micro- owers stay intact during the calcination process; however, the formation of bead-like structures with the thinning of petals is evidenced. The petal thinning in microowers as well as the deformation of sharp edges to bead-like structures are due to the removal of SO 2 from In 2 S 3 /In 3 S 4 and H 2 O from In(OH) 3 during calcination. Hence, the resulting In 2 O 3 seems to exhibit porosity (Fig. 2f–h). 3.2. EDX analysis EDX spectrum of the hydrothermal product (Fig. S1†) showed peaks arising from the presence of indium, sulfur and oxygen. The In, S and O atomic ratio was not in line with the general formula In(OH) 3 /In 2 S 3 as the hydrothermal product was a mixture of indium suldes and indium oxide which is evident from the EDX analysis and was further conrmed from powder PXRD analysis (vide infra). The remaining peak observed at 1.5 keV is due to Al holder over which the sample was placed. EDX spectrum of calcinated product (Fig. S2†) is devoid of peak due to sulfur at 2.33 keV (KR1), which was intensely present in the This journal is © The Royal Society of Chemistry 2018 RSC Adv.,2018,8, 25856–25865 | 25857 Paper RSC Advances Open Access Article. Published on 18 July 2018. Downloaded on 7/19/2018 6:22:52 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
EDX of hydrothermal product. This shows the complete conversion of indium suldes to indium oxide during calcination. The stoichiometry of indium and oxygen in the calcinated product was found to be closer to 2 : 3 indicating the formation of In 2 O 3 . This was further conrmed by PXRD analysis. 3.3. TEM analysis TEM images of the hydrothermal products were shown in Fig. 3. The dark and light contrast (Fig. 3b, d, e and f) of the petals of the microowers in TEM images are due to the difference in orientations of petals with respect to the electron beam. When the edges of the petals are parallel to electron beam it appears dark in contrast (Fig. 3d and e) whereas, when the surface of the petals is parallel to the electron beam they appear light in contrast (Fig. 3f). Similarly, the folded areas are also dark in contrast. TEM images of hydrothermal product clearly evidence the self-assembly of 2D nanosheets into microower structure. The annular electron diffraction pattern indicates that the hydrothermal product is polycrystalline. Aer calcination, retention of morphology (Fig. 4) and the thinning of petals or nanoakes into wafer like structure were observed. In the calcinated product, the akes are curved with wavy margins and the magnied view is shown in Fig. 4e and f, which reveals that the structure is highly porous as discussed above. It has been reported earlier 23 that the formation of porous structures in In 2 O 3 is due to the distortion of crystal lattice by internal tensile stress as a result of change in bond parameters during the conversion of In 2 S 3 to In 2 O 3 . The loss of H 2 O and SO 2 molecules from the precursors (In(OH) 3 or In 2 S 3 respectively) results in the formation of In 2 O 3 nanoparticles with the size of 20 to 25 nm which are the basic building units of mesoporous hierarchical microower structure. The TEM images Fig. 4e and f showed that the material is composed of particles. 3.4. X-ray diffraction analysis The phase identication of samples was made by powder X-ray diffraction (PXRD). In the PXRD pattern of the hydrothermal product (Fig. S3†), characteristic diffraction peaks of In 2 S 3 , In 3 S 4 and In 2 O 3 phases can be identied indicating phase Fig. 1 SEM images of hydrothermal products; (a–d) mono-dispersed micro flower structures of hydrothermal products; (e) micro flowers composed of nano sheet (petals); (f) nano sheet with the breadth of 21.5 nm. 25858 |RSC Adv.,2018,8, 25856–25865 This journal is © The Royal Society of Chemistry 2018 RSC Advances Paper Open Access Article. Published on 18 July 2018. Downloaded on 7/19/2018 6:22:52 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
mixture of all three. Similarly, aer calcination at 400 C for 2 h the PXRD pattern shows a mixture of In 3 S 4 and In 2 O 3 (Fig. S4†) indicating the conversion from In 2 S 3 or In 3 S 4 to In 2 O 3 has occurred only partially. However, based on further calcination tests during which hydrothermal products were calcined at 600 C for 6 h, In 2 O 3 is the sole component as identied in the acquired PXRD pattern (Fig. 5). 21,22 It was reported 24 that for the formation of 3D porous structures from 2D nanosheets, surfactants are indispensable as they can cap the crystal surface selectively and the face with high density of surface atoms is blocked by the adsorption of surfactants during crystal growth. However, in our present study surfactants or other reagents are not used and the 3D microowers were observed with superior control over shape and size. Scherrer analysis for the average crystal size of the nal calcination product at 600 C, gave size range of 20–23 nm. 3.5. Surface area analysis Fig. 6 shows the typical sorption isotherms of In 2 O 3 micro- owers. The prepared In 2 O 3 microowers exhibited a hysteresis Fig. 2 SEM images of In 2 O 3 microflowers; (a and b) homogeneous structure of microflowers; (c–e) microflowers with different dimensions (f and g) basic building units of microflowers (nanosheet) (h) bead-like structure in nanosheets. This journal is © The Royal Society of Chemistry 2018 RSC Adv.,2018,8, 25856–25865 | 25859 Paper RSC Advances Open Access Article. Published on 18 July 2018. Downloaded on 7/19/2018 6:22:52 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
loop in a high relative pressure range of 0.77 to 0.98, which is characteristic of type IV isotherm according to IUPAC classication and evidences the presence of mesopores. The observed BET surface area is 70.28 m 2 g 1 for In 2 O 3 microowers which is higher compared to the reported In 2 O 3 porous materials. 7,9 The total pore volume is 0.154 cm 3 g 1 and the average pore diameter is 1.518 nm. The observed results exemplify the mesoporous nature of In 2 O 3 microowers. Since the surface area is very high it may have potential application as a catalyst or in gas storage. 3.6. Hydrothermal reactions, growth mechanism and calcination reactions In the described synthetic process we have used dithiobiuret as a sulfur source which, together with indium salt (In(NO 3 ) 3 ), was expected to give indium sulde during hydrothermal process. To our surprise we observed the formation of In(OH) 3 as a major hydrothermal product along with the formation of In 2 S 3 and In 3 S 4 as minor products. The whole hydrothermal and calcination process can be explained by the following eqn (2)–(6). NH 2 CSNHCSNH 2 +5H 2 O/3NH 4+ +OH +S 2 + 2CO 2 +H 2 S(2) In 3+ + 3OH /In(OH) 3 (3) 2In 3+ +3S 2 /In 2 S 3 /In 3 S 4 (4) 2In(OH) 3 /In 2 O 3 +3H 2 O(5) 2In 2 S 3 +6O 2 /2In 2 O 3 + 3SO 2 (6) Fig. 3 TEM images of hydrothermal product; (a–c) overall images of microflowers; (d–f) images of thin petals of microflowers; (g and h) images showing sharp edges of petals (i) SAED pattern of hydrothermal product. 25860 |RSC Adv.,2018,8,25856–25865 This journal is © The Royal Society of Chemistry 2018 RSC Advances Paper Open Access Article. Published on 18 July 2018. Downloaded on 7/19/2018 6:22:52 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
In this process dithiobiuret decomposes slowly leading to a low degree of supersaturation in solution and resulting in the nucleation between S 2 and In 3+ ions at the initial period of the hydrothermal treatment as shown in eqn (2) and (4). Although the dithiobiuret releases S 2 ions thus generating In 2 S 3 /In 3 S 4 , the formation of In(OH) 3 under hydrothermal conditions (eqn (3)) is prevalent over indium sulde(s) formation due to the presence of excess OH . The formation of In(OH) 3 has not been previously reported in the synthesis of In 2 S 3 under hydrothermal conditions with various other sulfur sources such as thioacetamide, L-cysteine, thiourea, sodium thiosulfate, elemental sulfur and dithiocarbamate. 21–33 On the other hand, Fig. 4 TEM images of In 2 O 3 microflowers; (a–c) low magnification TEM images; (d–f) high magnification TEM images. Fig. 5 X-ray powder diffraction pattern of the product calcinated at 600 C for 2 h. Characteristic peak positions of In 2 O 3 are indicated by their Miller indices. Fig. 6 Nitrogen adsorption–desorption isotherm of In 2 O 3 micro flowers. This journal is © The Royal Society of Chemistry 2018 RSC Adv.,2018,8, 25856–25865 | 25861 Paper RSC Advances Open Access Article. Published on 18 July 2018. Downloaded on 7/19/2018 6:22:52 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
the formation of In(OH) 3 has been shown to occur in the absence of sulfur sources via direct hydrolysis of simple indium salts, such as In(NO 3 ) 3 , under hydrothermal conditions. 13 Hence, in our system, the degradation of dithiobiuret into OH and S 2 ions results into a competition between the formation of In(OH) 3 and In 2 S 3 /In 3 S 4 from which the predominant species is probably In(OH) 3 since its formation is favored under hydrothermal conditions. In addition to acting as a sulde and hydroxide source, dithiobiuret is as a shape controlling agent and an organic base and has a clear impact on the formation of the observed uniform and visually appealing morphology of microowers with sharp edges. Microowers are formed during the hydrothermal process from nanoscopic petals (nanoakes) in the absence of surfactants or templates. The process could be explained in terms of Ostwald ripening mechanism according to which the nanoakes are formed at the expense of the smaller nuclei that are formed initially (Fig. S5†). In nano particles different crystal planes have different surface energies, and of those the planes having the higher surface energies are preferred by crystal growth mechanism dened as Oswald ripening (wherein larger particles tend to grow further by expense of smaller particles). 39 For the minimization of surface energy, akes aggregate to form a spherical structure during the surface kinetics process, nally resulting in the formation of hierarchical microowers. The lateral growth through the spherical diffusion model was proposed already 23 for the formation of ake-like morphologies from smaller nuclei. 36 Subsequently, the akes act as building blocks and point towards the common center thereby creating the obtained ower-like morphology. The lateral growth through the mass transfer process at the hydrothermal conditions has been discussed already. 39 During the calcination process, the precursors were converted into In 2 O 3 as shown in eqn (5) and (6). The exhaustion of sulfur atoms, which has been shown to occur in oxidation of In 2 S 3 to In 2 O 3 , 21 was ruled out in our case as we have not observed any formation of elemental sulfur during the calcination process. In such case, SO 2 would have been liberated and the evaporation of SO 2 molecules would then result in the destruction of ake-like structure and to the formation of small indium oxide nanoparticles. However, the absence of such particles was unequivocally proved by SEM and TEM measurements. The conversion of the hydrothermal product to In 2 O 3 was partial aer 2 h at 400 C (Fig. S4†). The precursors are completely converted into In 2 O 3 aer 6 h at 600 C which was evidenced by PXRD wherein the obtained pattern lacks diffraction peaks originating from other crystalline phases (Fig. 5). It is noteworthy that In(OH) 3 /In 2 O 3 conversion is straightforward and other products like oxyhydroxides (InOOH) are not formed during the calcination. 3.7. Effect of reaction parameters and additives It was clear that the increase in reaction time resulted in slight increase in the dimensions of microowers and also thickening of petals. 28 This could be due to the deposition of smaller nuclei to the larger crystals formed already. Similarly, changing the reactants ratio does not have a meaningful inuence on the morphology of microowers. Whereas, extending the calcination to 10 h results in the breaking of microower structure by creating holes with the size range of 100 to 200 nm [Fig. 7a–c]. Interestingly, the addition of ethylenediamine seems to have a spectacular effect on the nanoparticle morphology as spherical and oval shapes are formed [Fig. 7d] whereas the Fig. 7 SEM images of (a–c) broken In 2 O 3 microstructures; (d) In 2 O 3 nanoparticles formed with the addition of EDA during hydrothermal process. 25862 |RSC Adv.,2018,8, 25856–25865 This journal is © The Royal Society of Chemistry 2018 RSC Advances Paper Open Access Article. Published on 18 July 2018. Downloaded on 7/19/2018 6:22:52 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online
microower morphology was not anymore observed. We are currently exploring the effects of different additives on morphology and report these results at a later time. 3.8. Electrochemical studies Fig. 8 shows the typical cyclic voltammogram (at various scan rates) of In 2 O 3 on glassy carbon electrode (GCE) in 1 M Na 2 SO 4 between 0.6 to 0.6 V vs. Ag/AgCl. In general, the area under the CV curve represents the amount of charge stored in the electrode material. In the CV curve, two quasi reversible redox peaks are observed which illustrate the sequential transitions of In 3+ to In + . The main pseudocapacitance behaviour of the sample can be attributed to the quasi-reversible redox transition of In 3+ to In + and vice versa. 40 The capacitive behaviour of In 2 O 3 is clearly seen from the gure with near rectangular shaped voltammograms and larger CV currents. Nevertheless, In 2 O 3 has various reduction potential ranges from 0.14 to 1.034 (SHE), the one observed at 0.52 V against Ag/AgCl indicates that the synthesized In 2 O 3 is in In 3+ which is reduced to In + under the given potential. The possible redox reaction can be written as In 3+ +2e 4In + The specic capacitance values as a function of scan rate of CV are shown in Fig. 9. The SC value of 75 F g 1 and 37 F g 1 has been obtained at the scan rate of 10 mV s 1 and 100 mV s 1 respectively. The specic capacitance of the electrode is decreased with increase of scan rate. The reason for the variation of specic capacitance with scan rate can be explained with the following points; (i) at high scan rate, the ions from electrolyte have less accessible time to diffuse into the electrode material resulting in lower specic capacitance. Hence, only limited amount of charge is stored (surface) which leads to decrease of specic capacitance, (ii) at low scan rate, the ions from electrolyte have the ability to access both inner and outer surface of the electrode material which leads to higher specic capacitance. Comparison of specic capacitance values for In 2 O 3 materials reported in literature and from this work is shown in Table 1. The reduced specic capacitance for the synthesized In 2 O 3 micro ower (in this work) may be due to the mesoporous nature of 3D sheets of In 2 O 3 . The mesoporous material may hold the ions for a longer time and hence the amount of charge on the surface gets decreased and also the specic capacitance. Fig. 8 Cyclic voltammograms (CVs) of bare GCE and In 2 O 3 /GCE electrodes in 1 M Na 2 SO 4 electrolyte. Fig. 9 Specific capacitance of In 2 O 3 /GCE electrodes against the scan rate. Table 1 Comparison of specific capacitance values of In 2 O 3 materials In 2 O 3 materials Specic capacitance Porous indium oxide hollow spheres (PIOHS) 41 320 F g 1 and 185 F g 1 @1Ag 1 In 2 O 3 nanoparticles/CNT 42 64 F g 1 In 2 O 3 -impregnated mesoporous carbon 42 275 F g 1 Nano rod In 2 O 3 (ref. 43) 104.9 F g 1 @8Ag 1 Nanostructured indium oxide 44 190 F g 1 @10mVs 1 80 F g 1 @ 1000 mV s 1 In 2 O 3 nanostructures 45 (a) Nanotowers 10.1 mF cm 2 (b) Nanobouquets 12.5 mF cm 2 (c) Nanocones 4.9 mF cm 2 (d) Nanowires 16.6 mF cm 2 In this work 75 F g 1 @10mVs 1 37 F g 1 @ 100 mV s 1 This journal is © The Royal Society of Chemistry 2018 RSC Adv.,2018,8, 25856–25865 | 25863 Paper RSC Advances Open Access Article. Published on 18 July 2018. Downloaded on 7/19/2018 6:22:52 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online