A Review of Two-Dimensional Materials in Electrocatalysis and their Potential Applications

Two-dimensional materials are crystalline materials consist of a single layer of atoms and sometimes referred to as single layer materials. Electrocatalytic energy conversion using renewable power sources is one of the most promising ways for energy storage and energy utilization in the new century. Over the past years, a great number of two-dimensional (2D) materials have been explored for various electrocatalytic reactions, such as the hydrogen evolution reaction, Carbon (IV) oxide (CO2) reduction reaction and Oxygen (O2) reduction reaction. This research provides an overview on the synthesis techniques of materials including bottom up approaches such as chemical vapor deposition (CVD) and physical vapor deposition (PVD) and top-down approaches like mechanical exfoliation, chemical exfoliation. Then, the characterization techniques of the twodimensional (2D) materials such as Raman spectroscopy, X-ray diffraction, temperature-dependent resistivity and magnetic susceptibility and scanning tunneling microscopy (STM) are reviewed. Finally, potential applications of two-dimensional (2D) materials and conclusion, challenges and future work are discussed.

Two-dimensional materials are generally classified into two namely; two-dimensional allotropes of various elements (Graphene, Borophene and so on) and two-dimensional allotropes of various compound (Graphane, Molybdenite, Aerographite and many more) (made up of two or more covalently bonding elements) [15]. The elemental two-dimensional materials generally carry the (-ene) suffix in their names while the compounds have (-ane or -ide) suffixes. Layered combinations of different twodimensional materials are called Van der Waals heterostructures.
Recently, two-dimensional (2D) materials have been widely reported as promising non-noble material electrocatalysts due to their abundance, low cost, and highly efficient catalytic activity [16]. An Hopely this review will be useful in identifying the best synthesis and characterization technique, factors affecting the performance and the potential applications of two-dimensional for the future. MoO3, [31,32] and hydrated WO3 [33]. The general procedure of mechanical exfoliation using Scotch tape is divided into two parts. The first step is to thin down the bulk materials by putting them onto the Scotch tape and peeling off repeatedly until the thick bulk materials are thinned down to some degree as shown in Figure 2.3. The second step is to transfer the exfoliated flakes on the tape to the surface of a substrate by sticking the tape on the substrate. A few finished samples are shown in Figure  2.3 and 2.4. After the transferring of the flakes, optical inspection is used to identify the suitable flakes for the subsequent material characterization and device fabrication [34].

II. Synthesis of Two-dimensional Materials
As can be seen in Figure 2.3, there are built-in alignment marks in the form of numbers and squares on the substrate, circled in red together with the material flakes. Since the flakes in various shapes are transferred and then distributed on the surface of substrates in a random order, the alignment marks are needed to record the location of the desirable flakes for the subsequent processing. The period of the alignment mark arrays is 76μm and they are repeated over the entire surface of the substrates. The numbers indicate the row and column of the alignment marks respectively. For example, the numbers of 30 and 29 in Figure 2.3 surrounded by four square marks indicate the column number is 30 and row number is 29 [34]. chemistries, as varied as graphene and its oxide [36][37][38][39], metal oxides and hydroxides [40][41][42][43][44], h-BN [45][46], TMDs [46][47][48] and clays [49][50][51][52][53].
For the solvent assistance exfoliation process, selecting an appropriate solvent with a specific surface tension is critical because the energy of exfoliation could be minimized when the surface energies of flake and solvent match well; therefore, an effective exfoliation could be successfully achieved [47]. Generally, solvents with the specific surface tension of ~40 mJ/m2 are suitable for many materials (such as BN, MoS2, and WS2). Chemical modification of the interlayer composition is needed in some conditions where the bonds between the layers are too strong to be broken with the approaches described above [54][55][56]. An example of this approach is the modification of the perovskite structure KCa2Nb3O10 by proton exchanging in 2M HNO3 and then reacting with tetra (n-butyl) ammonium hydroxide (TBA+OH-) to form TBAβH1-βCa2Nb3O10 which is then easily exfoliated [54].
Sublimation of silicon from silicon carbide, SiC, single crystals by heating it under high vacuum (~ 1.3 x10-4 Pa) [57] or under argon results in forming grapheme [58]. It is worth noting that in parallel to Novoselov et al.
[27] early work (the work that got them Nobel Prize in physics 2010) Berger et al. [59] used SiC to produce few-layers graphene, and they explored their electronic properties, but their paper got published about 6 weeks after Novoselov's.
The most widely used method entails, intercalating of a material or compound between the layers that ultimately results in their separation from each other. An example for that approach is intercalating potassium, K, between the graphene layers then exposing the potassium intercalated graphite to water or ethanol [60]. The vigorous reaction between the intercalated K and water results in separating the graphene layers from each other.
Another example is the reaction of graphite with a mixture of acids (nitric acid, and sulfuric acid) with potassium chlorate, which results in oxidizing the graphite. Then by thermal shock (rapid heating to 1050 °C for 30 s) the intercalant decompose with a large volume expansion those results in separating the two-dimensional (2D) graphene oxide layers from each other [61].
Sonication assisted exfoliation can also be used instead of thermal shock to exfoliate the intercalated graphene oxide layers [62]. Exfoliation of TMDs can be carried out by sonicating their powders in different solvents such as N-methyl-pyrrolidone or isopropanol [46]. Similarly, graphene sheets can be exfoliated by sonication in water with additives of surfactants [63].
As shown in Figure 2.5c, using the right solvent is very important to avoid restacking and reagglomeration [46]. The tantalum pentachloride (TaCl5) and sulfur (S) powder were used as precursors. As the carrier gas, a mixture of N2 with 10% of H2 was used.
The reaction was usually carried out at 820 °C. In CVD synthesis, the system pressure, gas flow rate, reported on roll-to-roll production of more than 76 cm (30 inch screen size) graphene films for use as transparent electrodes. In principle, CVD also allows for the fabrication of electronic devices such as transistors [77].   There are several advantages of using Raman spectroscopy for the characterization of low dimensional materials, including 2D TMDs [104,105]. One reason is that the electronic density of states (DOS) has a so-called van Hove singularity [107], which leads to a strong Raman feature when the photon energy is matched to the van Hove singularity of the DOS for each layer.
The second advantage is that more specific selection rules predicted from the group theory applicable to the particular optical transition can be applied to many TMD layered materials with lower symmetry compared to graphene. In this way, differences in symmetry distinguish the spectral features of a particular TMD layer from another. In contrast to 1T-TaS2 and 1T-VSe2, it is also reported that the CDW phase transition temperature can increase in the thinner materials.  They have excellent mechanical properties [146,147], can be compatible with flexible device fabrication, and unlike CNTs [148,149] do not require any sorting process [150]. At the same time, the mobility of 2DMs, when grown over large areas by CVD, can be larger than some of the organic semiconductors [151],