COMPOSICIÓN Y ACLARACIONES SOBRE RUBROS DE LOS ESTADOS CONTABLES

Currently, there are a variety of inorganic nanotubes with one-dimensional structure [99].Generally, one of the most important issues in a wide variety of one- dimensional nanostructured materials, such as nanowires, nanotubes, and others, is strong bonding between the nanomaterials and their substrates. This strong bond gives the nanostructured materials a more stable structure, which leads to higher efficiency, and long service life of nanomaterials-based energy generation devices [100]. This type of stable structure has attracted the particular attention of researches in recent years due to the special physicochemical properties that are related to this

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type of good structure, which can provide high surface area, high porosity, and cavities in hollow structures [101].Thus, nanotubes with stable structure can be utilized in many applications such as energy storage and biomedicine due to their higher strength [102, 103], chemical stability [104], high thermal conductivity, and high efficiency of ionic and electronic conductivity [105, 106]. Interestingly, there are several strategies that are being applied currently to the synthesis of iron oxide– based nanotube arrays as hematite α-Fe₂O₃and magnetite Fe₃O₄, such as the use of sacrificial templates of ZnO nanowire that can be dissolved by acid, which is produced from the precursor of Fe+3_{hydrolysis. The dissolution of the ZnO template}

can accelerate the hydrolysis of Fe+3_{in order to form the nanotubes structure, as}

shown in Figure 2.20(a):

Figure ‎2.20: (a) Schematic diagram of formation process of iron oxide nanotube arrays, (b)XRD patterns show the stages of formation of α-Fe₂O₃ nanotube arrays; (A) ZnO nanowires pattern after immersion for 30 min in Fe+3 solution; (B) formation of Fe(OH)_𝟑 nanotube array; (C)formation of α-Fe₂O₃ nanotube arrays; (D) formation of Fe3O4 nanotube array [102].

The synthetic strategy for formation of iron oxide –based nanotubes with higher surface area and higher reversible energy storage in LIBs can be observed

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schematically in Fig. 2.20(a) [102]. Moreover, the stage of formation of α-Fe2O3

can be demonstrated as follows from the XRD patterns [Fig.2.20(b)]:

1) The crystallinity and orientation of the nanowire particles that belong to the alloy substrate of zinc oxide are decreased dramatically after 30 min of ZnO nanowires immersion in the Fe+3 solution, as shown in (XRD) patterns A in Figure 2.20(b). The star symbols on some peaks come from the alloy foil [107].

2) During the immersion of ZnO nanowires for a long time in solution the peaks of ZnO nanowires decrease gradually, and finally, they disappear, while at the same time, the formation of an amorphous Fe(OH)3 nanotube arrays

occurs with a high characteristic peak, as shown in pattern B in Figure 2.20(b).

3) After the calcinations at 450 °C under atmospheric pressure, the Fe(OH)₃ nanotube arrays will be transformed to arrays of α-Fe₂O₃ nanotubes as shown in pattern C in Figure 2.20(b) [102].

Furthermore, the morphology of the α-Fe2O3 nanotube arrays during fabrication

on alloy substrate can be demonstrated by many methods, such as scanning electron microscopy (SEM) and transmission electron microscopy(TEM), as shown in Figure 2.21(b-d). Figure 2.21(a) presents the Raman spectrum of a pure phase α-Fe2O3 nanotube array. It can be seen from the SEM images in

Figure 2.21(b,c) that the nanotubes consist of many columns with closed tips that are agglomerated from a huge number of nanoparticles with about 200 nm to 300 nm diameter on the outer shell. Also, it can be clearly seen from the inset image in Figure 2.21(b) that the arrays are homogenously distributed over the high surface area of about 30 cm2_{of alloy substrate with strong adhesion. In addition,}

it is clear the tubes have random walls, as shown in Figure 2.21(d), while from the selected area electron diffraction (SAED) pattern, it can be determined that the α-Fe₂O₃ nanotubes have polycrystalline structure [102].

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Figure ‎2.21:(a) Raman spectrum of α-Fe₂O₃ nanotube array;( b) SEM image at low magnification and optical image(inset) of the array on a large area alloy substrate; (c) SEM image of the array at high magnification; (d) TEM image and corresponding

SAED pattern (inset) of several alpha iron oxide nanotubes [102].

The good cycling performance can be studied and compared with that of C/α−Fe₂O₃ based composite electrode as anode in lithium ion batteries, as shown in Figure2.22 (a). In this regards, it can be clearly seen that during the initial discharge, the C/α- Fe₂O₃ nanotube has two reduction peaks at 1.5 and 0.71 V, which are clearer in the differential capacity vs. voltage. During the charge process, the anodic peak appears at 1.74 V, which corresponds to the oxidation reaction of Fe0to Fe+3. The presence of carbon, which has a very low capacity of 372 mAh/g leads to reduced capacity in the C/α-Fe2O3 nanotube arrays. In Figure 2.22(a), it the different cycling

performances of the C/α-Fe₂O₃ nanotube and the α-Fe₂O₃ up to 150 cycles can be clearly observed. It is also demonstrated that C/α-Fe₂O₃ nanotube array has good cyclability. Although the capacity decreases dramatically over the first 20 cycles, it is stable at 60 cycles with capacity of 700 mAh/g, but then, after 150 cycles, there is a high reversible capacity of 659 mAh/g with a low fading rate of 0.586 mAh/g for each cycle during the last 70 cycles. Nevertheless, the capacity of the α-Fe2O3

nanotube array is reduced dramatically, and just 384 mAh/g can be retained in the final cycle. The reversible capacities of the α-Fe2O3 nanotube and C/α-Fe2O3

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nanotube array electrodes are presented in Figure 2.22(a-b).Moreover, it can be observed from Figure 2.22(b) that after the 50 and 150th cycles at C-rate of 1.5 C, the C/α-Fe2O3 nanotube array retains a reversible capacity of 523 mAh/g and 463

mAh/g, respectively. There are also capacities of 457 mAh/g and 351 mAh/g after 50 and 150 cycles, respectively, at the high rate of 3 C, which is considered better than the capacity of commercial graphite anode. The α-Fe2O3 nanotube array has

capacities of 327 mAh/g and 193 mAh/g at the high C- rate of 1.5 C after 50 and 150 cycles, respectively. Interestingly, this provides a better understanding of the importance of the nanotubes array structure in the lithium battery for good energy storage [102]. Moreover, it can be concluded that the good structure of the α- Fe₂O₃nanotubes with high porosity of the tube walls can play a vital role in achieving higher cycling stability than other nanostructured materials, and a lower percentage of capacity fading than the initial values at 40 cycles of around 50% [80, 108, 109]. This occurs for many reasons a) high lithium ion diffusion, b) larger contact area between electrode and electrolyte, c) high porosity that can provide better accommodation for large volume change during insertion and extraction of lithium ions, and d) higher alignment of nanotubes on the surface of the conductive substrate, which is conductive to easy pathways for transportation of electrons [86, 110, 111]. There are many limitations in the performance of α-Fe2O3 nanotubes as

anode in LIBs, however, such as poor adherence to the substrate during cycling, particularly at very high rates, so that it is too difficult to maintain their integrity and electrical continuity. These problems can be overcome, however, through adding carbon to make a composite electrode as mentioned above [102].

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Figure ‎2.22: a) Cycling stability of α-Fe₂O₃ and C/α-Fe₂O₃ nanotube arrays; with the inset showing the differential capacity vs.voltage; (b) reversible capacity vs. current density during different cycles for α-Fe₂O₃ nanotube and C/α-Fe₂O₃ nanotube array electrodes [102].