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Advanced Lithium Ion Batteries for Electric Vehicles: Promises and Challenges of Nanostructured Electrode Materials
Published in Thandavarayan Maiyalagan, Perumal Elumalai, Rechargeable Lithium-ion Batteries: Trends and Progress in Electric Vehicles, 2020
Venkata Sai Avvaru, Mewin Vincent, Vinodkumar Etacheri
Another form of carbon investigated to a large extent is amorphous carbon. Amorphous carbon gained more attention for its synthesis at lower temperatures, with superior lithium storage capacities. Main attraction of amorphous carbon is its high surface area which is nearly 8.6 times higher than that of graphite, which is mainly attributed to the mixed morphology of graphitic nature and microporous structures. Graphitic crystallites embedded in the matrix ensure excellent conductivity of the material [35]. On the other hand, the amorphous phase leads the lithium storage function. Defective surfaces and closed voids of the amorphous region have a direct relation to the storage mechanism. An optimized combination of defects and void sites with priority on the former is usually more favourable for better ion transport and capacity retention. Owing to the morphological and electrochemical advantages amorphous carbon is widely used to coat on other anode materials like TiO2 , graphite, Si, Sn etc. to increase conductivity as well as to buffer the volume change during cycling [36–39]. Carbon allotrope which is of more interest in the recent years is the hard carbon or disordered carbon [40]. The main advantage of hard carbon as an anode is the absence of exfoliation induced by intercalation due to available active sites on graphitic structures. This effect leads to a lower sensitivity towards the electrolyte compositions. Hence, it could be used with any electrolyte even without the addition of a film forming agent [41, 42]. In addition, these hard carbons can also exhibit competent specific capacities and stable cycling using LiTFSI electrolytes [41].
Carbon Nanomaterials Are Resolving the Challenges and Issues of Future Lithium Ion Batteries
Published in Figerez Stelbin Peter, Prasanth Raghavan, Graphene and Carbon Nanotubes for Advanced Lithium Ion Batteries, 2018
Figerez Stelbin Peter, Prasanth Raghavan
Carbon is well known to form distinct solid-state allotropes with diverse structures and properties ranging from sp3 hybridized diamond to sp2 hybridized graphite. Historically, chemists have known only two allotropes, or pure forms, of carbon: graphite, a greasy, electrically conducting black substance; and diamond, crystal clear, electrically insulating material that is harder than any other solid. But they have constantly theorized about other possible carbon allotropes. There are eight main allotropes of carbon: (i) diamond, (ii) graphite, (iii) lonsdaleite, (iv) C60 (buckminsterfullerene or buckyball), (v) C540, (vi) C70, (vii) amorphous carbon, and (viii) CNT or buckytube. Until the 1960s, when “new carbon” materials were synthesized, only two allotropic forms of carbon were known, graphite and diamond, including their polymorphous modifications. Recently, “amorphous carbon” has come to be considered as the third allotrope of carbon. Graphite is the most common allotrope of carbon, the most thermodynamically stable form of carbon at room temperature. Therefore, it is used in thermochemistry as the standard state for describing the heat produced in the formation of carbon compounds. Graphite consists of a layered two-dimensional (2D) structure where each layer possesses a hexagonal honeycomb structure of sp2 bonded carbon atoms with a C-C bond length of 1.42 Å. These single atom thick layers (i.e., graphene layers) interact via non-covalent Van der Waals forces with an interlayer spacing of 3.35 Å. The weak interlayer bonding in graphite implies that single graphene layers can be exfoliated via mechanical or chemical methods, as will be outlined in detail here. Graphene is often viewed as the two-dimensional (2D) building block of other sp2 hybridized carbon nanomaterials in that it can be conceptually rolled or distorted to form CNTs and fullerenes. Graphite is an electrical conductor and is applicable in electronics. Graphite conducts electricity due to the delocalization of the π-bond electrons above and below the planes of the carbon atoms. These electrons are free to move and so are capable of conducting electricity. However, the electricity is only conducted along the plane of the layers. In diamond, all four outer electrons of each carbon atom are localized between the atoms in covalent bonding. The movement of electrons is cons trained, and diamond does not conduct an electric current. In graphite, each carbon atom uses only three of its four outer energy level electrons in covalently bonding to three other carbon atoms in a plane. Each carbon atom contributes one electron to a delocalized system of electrons that is also a part of the chemical bonding. The delocalized electrons are free to move throughout the plane. So, graphite conducts electricity along the planes of carbon atoms. Amorphous carbon is the carbon that does not have any crystalline structure. As with all glassy materials, some short-range order can be observed, but there is no long-range pattern of atomic positions. While completely amorphous carbon can be produced, most examples contain microscopic crystals of graphite-like or even diamond-like carbon.
Diamond and carbon nanostructures for biomedical applications
Published in Functional Diamond, 2022
Yuxiang Xue, Xue Feng, Samuel C. Roberts, Xianfeng Chen
Amorphous carbon belongs to an allotrope of carbon that has no crystalline structure [131]. Compared with diamond and graphite, the atomic-scale structures of amorphous carbon are more complex. Amorphous carbon has sp2 (graphite-like) and sp3 (diamond-like) hybrid bonds. Unlike the three dimensionally cross-linked structure composed of sp3-bonded carbon atoms found in diamond, and the benzene-like ring structure composed of sp2-bonded carbon atoms found in graphite, various structures can be found in amorphous carbon [132, 133]. Hence, amorphous carbons with different structures usually have different properties. Amorphous carbon with high sp3 content is referred to as diamond-like carbon (DLC). Conversely, higher sp2 content yields materials with densities closer to that of graphite. By varying the sp2/sp3 ratio, Bhattarai et al prepared models of amorphous carbon with densities spanning from 0.95 g/cm3 to 3.5 g/cm3 [132]. In amorphous carbon study, the properties of materials are primarily characterised by extrapolating the ratio of sp3 bonded carbon and sp2 bonded carbon atoms using spectroscopic methods [2]. With the development of modern amorphous carbon materials, different amorphous carbon nanocomposites have been synthesised, including amorphous carbon dots, amorphous carbon nanofoam, amorphous carbon nanofibers, and amorphous carbon films [134–138]. These amorphous carbon nanocomposites have offered many opportunities in biomedicine. Table 2 summarised some physical poperties of diamond, graphene DLC and graphite-like carbon (GLC).
Controlled synthesis of Bi2O3/TiO2 catalysts with mixed alcohols for the photocatalytic oxidation of HCHO
Published in Environmental Technology, 2019
Qiong Huang, Qiu Wang, Tao Tao, Yunxia Zhao, Peng Wang, Zhenyao Ding, Mindong Chen
To further investigate the nano-crystallinity of Bi2O3/TiO2, the samples were characterized by TEM, as shown in Figure 7. The Bi2O3/TiO2 sample Ti/E-1/14 contained a large number of primary particles (25–30 nm) owing to the use of a high molar ratio of Ti/ethanol. Meanwhile, spherical particles could also be found on the surface, which is consistent with the SEM results. Upon increasing the amount of ethanol, the particles became better dispersed, and their size decreased from 25–30 nm to 12–15 nm. The structure of Bi2O3/TiO2 was further investigated by TEM (Figure 7(B, C)). The results revealed that the Ti/E-1/50 sample exhibited a low crystallinity, while the Ti/G-1/50 sample was highly crystalline and had a larger particle size. This result demonstrated that glycerol promoted the growth of the crystals due to its high hydroxyl content. As seen in Figure 7(D, E), the Ti/E/G-1/50/50 sample exhibited a flower-like structure with a mass of flake petals due to the spontaneous aggregation of the nanoparticles. Different lattice fringes were also observed, which allowed for the identification of the crystallographic spacings of TiO2, Bi2O3 (Φ12–15 nm) and Bi4(TiO4)3 (Φ2 nm). The fringes of d = 0.352 nm and d = 0.169 nm match the (101) crystallographic plane of the anatase phase and the (221) crystallographic plane of the rutile phase, respectively, and the fringes of d = 0.325 nm and d = 0.234 nm match the (0014) crystallographic plane of Bi2O3 and the crystallographic plane of Bi4(TiO4)3, respectively. The TEM analysis further confirmed that Bi4(TiO4)3 and TiO2 joined together to form a heterojunction structure in the samples, which was beneficial for transferring electrons and decreasing the recombination of photogenerated electrons and holes. Nanoparticles (Figure 7(F)) with a diameter of approximately 2 nm were homogeneously dispersed on the surfaces of the TiO2 petals and identified as Bi4(TiO4)3. Meanwhile, lattice defects, which can increase the catalytic activity, were detected by TEM. Moreover, no significant crystalline planes of carbon could be found, which showed that carbon was present in the form of amorphous carbon.