The Reaction Thermodynamics during Plating Al on Graphene Process

This research explored a novel chemical reduction of organic aluminum for plating Al on a graphene surface. The thermodynamics of the Al plating reaction process were studied. The Al plating process consisted of two stages: the first was to prepare (C2H5)3Al. In this reaction, the ΔH(enthalpy) was 10.64 kcal/mol, the ΔG(Gibbs free energy) was 19.87 kcal/mol and the ΔS(entropy) was 30.9 cal/(mol·K); this was an endothermic reaction. In the second stage, the (C2H5)3Al decomposed into Al atoms, which were gradually deposited on the surface of the graphene and the Al plating formed. At 298.15 K, the ΔH was −20.21 kcal/mol, the ΔG was −54.822 kcal/mol, the ΔS was 116.08 cal/(mol·K) and the enthalpy change was negative, thus indicating an endothermic reaction.


Introduction
Graphene/aluminum composites have high strength, high conductivity and high toughness. Thus, graphene/aluminum composites have wide application potentiality in the electronics, automotive and aerospace industry [1][2][3][4][5][6][7][8]. However, graphene/aluminum composites are difficult to prepare; because of the poor wettability between Al and graphene, the graphene aggregates easily in the Al matrix, which can decrease the mechanical properties of the composites [9,10]. In order to improve the wettability between graphene and Al, the ideal method is to coating melt on the surface of the graphene, by methods including self-assembly, chemical reduction, electrochemical deposition, redox method and chemical vapor deposition. For instance, Bagheri et al. prepared graphene-gold nanocomposites by self-assembly [11]. Tsai et al. coated the Cu nanoparticles on the graphene surface through coalescence and epitaxial self-assembly and studied molecular dynamics during the process [12]. Muszynski et al. synthesized gold nanoparticles through the chemical reduction of AuCl 4 − (Aldrich) with NaBH 4 and coated the gold nanoparticles on the surface of graphene [13]. Zhao et al. prepared graphene nanoplatelets by reinforcing copper matrix composites with electrochemical deposition and the composites' hardness and conductivity reached 111.2 HV and 89.2% IACS [14]. Kim et al. prepared single-atomic-layer graphene film on the surface of Cu through chemical vapor deposition (CVD), then obtained multi-layer graphene/copper composites with the strength of 1.5 GPa [15]. According to previous investigations, gold, copper, or nickel nanoparticles were usually coated on the graphene. However, these metal nanoparticles may be viewed as impurities in Al alloys, which can affect their properties. Plating Al on graphene is an effective method to improve graphene's wettability and reduce these impurities. Because the Al is active, it is difficult to displace Al atom from conventional Al salt solution [16]. Selective laser melting (SLM), through melting successive layers of metal powder, is a promising metal additive manufacturing method [17,18], it has huge advantages compared to traditional

Experiment
The Al powders and the graphene were employed as raw materials, as shown in Figure 1. During the Al plating reaction process, H 2 gas was pumped into the reaction vessel and the Al powder (1.5 g), aluminum chloride (0.1 g) and iodine (0.1 g) were dried and added into the C 2 H 5 Br (29 mL) at 39 • C. Al reacted with C 2 H 5 Br and the (C 2 H 5 ) 2 AlBr and C 2 H 5 AlBr 2 were obtained as follows [26]: 3C 2 H 5 Br + 2Al → (C 2 H 5 ) 2 AlBr + C 2 H 5 AlBr (1) The Al reacted with C 2 H 5 AlBr 2 to produce (C 2 H 5 ) 2 AlBr, Al and AlBr 3 [26]: 2C 2 H 5 AlBr + Al → (C 2 H 5 ) 2 AlBr + Al + AlBr 3 (2) The (C 2 H 5 ) 2 AlBr and Al further reacted to produce Al, (C 2 H 5 ) 3 Al and AlBr 3 via Equation [26]: 3(C 2 H 5 ) 2 AlBr + Al → 2(C 2 H 5 ) 3 Al + Al + AlBr 3 (3) After reaction, the solution temperature was kept at 0 • C for 1 h. The tetrahydrofuran was added to the solution. The solution was filtered after the reaction and the alkyl aluminum solution was obtained. Then, the graphene (0.05 g) was added to the alkyl aluminum solution. The temperature was kept at 70-100 • C for 1-1.5 h and the (C 2 H 5 ) 3 Al was decomposed into Al, H 2 and C 2 H 4 [26]: The Al atoms were gradually deposited on the surface of the graphene and the Al plating formed. Al atoms absorbed on graphene may form upon (C 2 H 5 ) 3 Al/graphene collisions. This reaction is initiated by ethane elimination from the (C 2 H 5 ) 3 Al molecule, similar to the observations reported for (CH 3 ) 3 Al/graphene [33].
Microstructure observation was carried out using a scanning electron microscope (SEM) (Zeiss Ultra 55, Carl Zeiss Microscopy, Jena, Germany) equipped with energy dispersive spectroscopy (EDS).
H1C1C2 increased from 109.471° to 111.590°, ∠C1C2Br increased from 109.469° to 111.496° and ∠ BrC2H5 decreased from 109.472° to 103.736°. The bond length of H1-C1 decreased from 1.14 Å to 1.1 Å, the C1-C2 bond was reduced from 1.54 Å to 1.517 Å, the C2-Br bond as increased from 1.91 Å to 2.025 Å and the C2-H5 bond was reduced from 1.14 Å to 1.095 Å. During the structure optimization process, the bond angle and bond length of atoms tended to be stable through the vibration displacement and the total energy was gradually minimized.

Computation Details
During the process of plating Al on the graphene, the thermodynamics of the chemical reduction of organic aluminum were simulated by density functional theory (DFT) methods implemented in the DMol3 package of Materials Studio. The structure of the reaction products was analyzed through the DFT, revealing the thermodynamic properties and reaction types of the chemical reactions. Spin-unrestricted DFT in the generalized gradient approximation (GGA) with the Revised Perdew-Burke-Eruzerhof (RPBE) exchange-correlation functional approach and double numerical plus polarization atomic orbitals was employed as the basis set. The Brillouin zone was sampled using the 4 × 4 × 1 k-point grid thickness, which presented a good approximation of the model below the article. In addition, the energy tolerance accuracy, maximum force and displacement were set as 1 × 10 −5 Ha, 2 × 10 −3 Ha/Å and 5 × 10 −3 Å, respectively, to ensure high accuracy in all calculations.
During the Dmol3 simulation process, the relationship between thermodynamic properties (entropy S, enthalpy H, heat capacity Cp, Gibbs free energy G) and temperature can be calculated from the vibration frequency. The total energy at 0 K was obtained during the simulation. The translational energy, rotational energy and vibration energy were used to calculate the thermodynamic properties at an instantaneous temperature. The instantaneous enthalpy H is: where E vib (T), E rot (T) and E trans (T) are vibration energy, rotational energy and translational energy respectively at temperature T and R is an ideal gas constant. The contribution of vibration to enthalpy is: The contribution of vibration to entropy is: The contribution of vibration to heat capacity at normal pressure is: where k is the Boltzmann constant, h is the Planck constant and v i is the vibration frequency of the ith atom. Each of the chemical bond vibrational frequencies was calculated by DFT at 298.15 K and then assumed to remain constant with Temperature.

Preparation and Reaction Mechanism of Al-Coated Graphene
During the Al plating process, with the increase of reaction time, more Al was deposited on the graphene, as shown in Figure 1a,b. When the chemical reduction reaction was at 1.5 h, abundant Al atoms were deposited on the graphene uniformly, the Al plating was formed and the content of the Al element was 71%, as shown in Figure 1c,d.

Reaction Thermodynamics during Plating Al on Graphene Process
The molecular model of each substance was established and its structure optimized during the chemical reduction reaction process. The vibration frequency was calculated and the thermodynamic properties of each substance were analyzed. The thermodynamics of the formation and decomposition of (C 2 H 5 ) 3 Al were calculated according to the laws of thermodynamics.
During the process of plating Al on grapheme, based on the reaction Equations (1) and (3), the structural optimization and thermodynamic calculation of C 2 H 5 Br, (C 2 H 5 ) 2 AlBr, C 2 H 5 AlBr 2 , (C 2 H 5 ) 3 Al and AlBr 3 were carried out through the Al cluster (Al 3 ) molecular model [34].
3.2.1. Structure Optimization and Thermodynamic Properties of C 2 H 5 Br Figure 2 shows the structure of the C 2 H 5 Br molecule. The initial structure of the C 2 H 5 Br molecule built in MS is shown in Figure 2a; the stable molecular structure after structure optimization is shown in Figure 2b. After structure optimization, ∠H 1 C 1 C 2 was reduced from 109.471 • to 108.747 • , ∠H 1 C 1 C 2 increased from 109.471 • to 111.590 • , ∠C 1 C 2 Br increased from 109.469 • to 111.496 • and ∠BrC 2 H 5 decreased from 109.472 • to 103.736 • . The bond length of H 1 -C 1 decreased from 1.14 Å to 1.1 Å, the C 1 -C 2 bond was reduced from 1.54 Å to 1.517 Å, the C 2 -Br bond as increased from 1.91 Å to 2.025 Å and the C 2 -H 5 bond was reduced from 1.14 Å to 1.095 Å. During the structure optimization process, the bond angle and bond length of atoms tended to be stable through the vibration displacement and the total energy was gradually minimized. Figure 3 shows the relationship between the thermodynamic properties (entropy S, enthalpy H, heat capacity Cp, Gibbs free energy G) of the C 2 H 5 Br and temperature was obtained through Equations (5)- (8). In the range of 25-1000 K, the enthalpy of the C 2 H 5 Br molecule had a linear relationship with the temperature and the enthalpy value increased with the increase of temperature. The heat capacity of C 2 H 5 Br gradually increased with the increase of temperature, although the free energy decreased. At 298.15 K, the enthalpy, entropy, free energy and heat capacity of C 2 H 5 Br molecules were 43.533 kcal/mol, 68.433 cal/(mol·K), 15.174 cal/(mol·K) and 23.127 kcal/mol respectively, as shown in Table 1.  Figure 3 shows the relationship between the thermodynamic properties (entropy S, enthalpy H, heat capacity Cp, Gibbs free energy G) of the C2H5Br and temperature was obtained through Equations (5)- (8). In the range of 25-1000 K, the enthalpy of the C2H5Br molecule had a linear relationship with the temperature and the enthalpy value increased with the increase of temperature. The heat capacity of C2H5Br gradually increased with the increase of temperature, although the free energy decreased. At 298.15 K, the enthalpy, entropy, free energy and heat capacity of C2H5Br molecules were 43.533 kcal/mol, 68.433 cal/(mol·K), 15.174 cal/(mol·K) and 23.127 kcal/mol respectively, as shown in Table 1.

Structure Optimization and Thermodynamic
Properties of (C2H5)2AlBr Figure 4 shows the structure of (C2H5)2AlBr. After structure optimization, ∠H1C1H2 decreased from 109.415° to 107.039°, ∠C1C2Al increased by 9.965°, ∠H4C2H5 decreased by 5.254° and ∠ C2AlC3 increased by 2.571°. The length of the Br-Al bond increased from 2.250 Å to 2.331 Å, the length of the Al-C3 bond increased from 1.88 Å to 1.98 Å, the length of the C3-H7 bond decreased by 0.03 Å, the length of the C3-C4 bond increased by 0.009 Å and the length of the C4-H9 bond decreased from 1.140 Å to 1.102 Å. Figure 5 shows the relationship between the thermodynamic properties of (C2H5)2AlBr and temperature. It can be seen that the enthalpy, entropy and heat capacity of (C2H5)2AlBr increased with the increase of temperature in the range of 25-1000 K and the free energy decreased with the increase of temperature. At 298.15 K, the enthalpy, entropy, heat capacity and free energy were 85.548 kcal/mol, 97.648 cal/(mol·K), 33.078 cal/(mol·K) and 56.435 kcal/mol, respectively, as shown in Table 1.

Structure Optimization and Thermodynamic
Properties of (C 2 H 5 ) 2 AlBr Figure 4 shows the structure of (C 2 H 5 ) 2 AlBr. After structure optimization, ∠H 1 C 1 H 2 decreased from 109.415 • to 107.039 • , ∠C 1 C 2 Al increased by 9.965 • , ∠H 4 C 2 H 5 decreased by 5.254 • and ∠C 2 AlC 3 Materials 2019, 12, 330 6 of 14 increased by 2.571 • . The length of the Br-Al bond increased from 2.250 Å to 2.331 Å, the length of the Al-C 3 bond increased from 1.88 Å to 1.98 Å, the length of the C 3 -H 7 bond decreased by 0.03 Å, the length of the C 3 -C 4 bond increased by 0.009 Å and the length of the C 4 -H 9 bond decreased from 1.140 Å to 1.102 Å. Figure 5 shows the relationship between the thermodynamic properties of (C 2 H 5 ) 2 AlBr and temperature. It can be seen that the enthalpy, entropy and heat capacity of (C 2 H 5 ) 2 AlBr increased with the increase of temperature in the range of 25-1000 K and the free energy decreased with the increase of temperature. At 298.15 K, the enthalpy, entropy, heat capacity and free energy were 85.548 kcal/mol, 97.648 cal/(mol·K), 33.078 cal/(mol·K) and 56.435 kcal/mol, respectively, as shown in Table 1.

Structure Optimization and Thermodynamic
Properties of C2H5AlBr2 Figure 6 shows the original and the optimal structure of C2H5AlBr2. After structure optimization, ∠H1C1H2 decreased from 109.511° to 104.953°, ∠H3C2H4 decreased from 109.52° to 107.514°, ∠ C2C1Al increased from 109.239° to 117.439°, ∠C1AlBr1 increased by 0.3° and ∠Br2AlBr1 decreased by 3.685°. The bond length of H2-C1 decreased from 1.14 Å to 1.102 Å and the bond length of C1-C2 increased by 0.007 Å, indicating that the C-C bond was relatively stable. The bond length of C1-Al was increased by 0.085 Å and the bond length of Al-Br1 was increased by 0.45 Å. Figure 7 shows the relationship between the thermodynamic properties of C2H5AlBr2 and temperature. It can be seen that the enthalpy, entropy and heat capacity of C2H5AlBr2 increased with the increase of temperature in the range of 25-1000 K. The free energy decreased with the increase of temperature. At 298. 15

Structure Optimization and Thermodynamic
Properties of C2H5AlBr2 Figure 6 shows the original and the optimal structure of C2H5AlBr2. After structure optimization, ∠H1C1H2 decreased from 109.511° to 104.953°, ∠H3C2H4 decreased from 109.52° to 107.514°, ∠ C2C1Al increased from 109.239° to 117.439°, ∠C1AlBr1 increased by 0.3° and ∠Br2AlBr1 decreased by 3.685°. The bond length of H2-C1 decreased from 1.14 Å to 1.102 Å and the bond length of C1-C2 increased by 0.007 Å, indicating that the C-C bond was relatively stable. The bond length of C1-Al was increased by 0.085 Å and the bond length of Al-Br1 was increased by 0.45 Å. Figure 7 shows the relationship between the thermodynamic properties of C2H5AlBr2 and temperature. It can be seen that the enthalpy, entropy and heat capacity of C2H5AlBr2 increased with the increase of temperature in the range of 25-1000 K. The free energy decreased with the increase of temperature. At 298. 15 Figure 5. The relationship between the thermodynamic properties of (C 2 H 5 ) 2 AlBr and temperature.
3.2.3. Structure Optimization and Thermodynamic Properties of C 2 H 5 AlBr 2 Figure 6 shows the original and the optimal structure of C 2 H 5 AlBr 2 . After structure optimization, ∠H 1 C 1 H 2 decreased from 109.511 • to 104.953 • , ∠H 3 C 2 H 4 decreased from 109.52 • to 107.514 • , ∠C 2 C 1 Al increased from 109.239 • to 117.439 • , ∠C 1 AlBr 1 increased by 0.3 • and ∠Br 2 AlBr 1 decreased by 3.685 • . The bond length of H 2 -C 1 decreased from 1.14 Å to 1.102 Å and the bond length of C 1 -C 2 increased by 0.007 Å, indicating that the C-C bond was relatively stable. The bond length of C 1 -Al was increased by 0.085 Å and the bond length of Al-Br 1 was increased by 0.45 Å. Figure 7 shows the relationship between the thermodynamic properties of C 2 H 5 AlBr 2 and temperature. It can be seen that the enthalpy, entropy and heat capacity of C 2 H 5 AlBr 2 increased with the increase of temperature in the range of 25-1000 K. The free energy decreased with the increase of temperature. At 298.15 K, the enthalpy, entropy, heat capacity and free energy were 46.425 kcal/mol, 94.579 cal/(mol·K), 26.606 cal/(mol·K) and 18.226 kcal/mol respectively (Table 1). 3.2.4. Structure Optimization and Thermodynamic Properties of (C2H5)3Al Figure 8 shows the structure of (C2H5)3Al. After structure optimization, ∠C2AlC3 was reduced from 119.992° to 118.949°, ∠C2AlC5 decreased from 119.805° to 119.593°, ∠C3AlC5 increased from 119.891° to 121.407°, ∠AlC5C6 increased from 108.858° to 117.775°, ∠H11C5H12 decreased from 109.536° to 103.956° and ∠H13C6H14 decreased from 109.444° to 107.13°. The H1-C1 bond length was reduced from 1.14 Å to 1.105 Å, the C1-C2 bond length increased from 1.54 Å to 1.551 Å, the C2-Al bond length increased from 1.879 Å to 1.997 Å and the C2-H4 bond length was reduced from 1.14 Å to 1.112 Å. During the optimization process, the C-Al bond rotated, the bond angle had large variation, the bond length changed little and the initial structure was significantly different from the optimized structure. Figure 9 shows the thermodynamic properties of (C2H5)3Al. In the range of 25-1000 K, the enthalpy, entropy and heat capacity of (C2H5)3Al increased with the increase of temperature. The free energy decreased with the increase of temperature. At 298. 15

Structure Optimization and Thermodynamic
Properties of (C2H5)3Al Figure 8 shows the structure of (C2H5)3Al. After structure optimization, ∠C2AlC3 was reduced from 119.992° to 118.949°, ∠C2AlC5 decreased from 119.805° to 119.593°, ∠C3AlC5 increased from 119.891° to 121.407°, ∠AlC5C6 increased from 108.858° to 117.775°, ∠H11C5H12 decreased from 109.536° to 103.956° and ∠H13C6H14 decreased from 109.444° to 107.13°. The H1-C1 bond length was reduced from 1.14 Å to 1.105 Å, the C1-C2 bond length increased from 1.54 Å to 1.551 Å, the C2-Al bond length increased from 1.879 Å to 1.997 Å and the C2-H4 bond length was reduced from 1.14 Å to 1.112 Å. During the optimization process, the C-Al bond rotated, the bond angle had large variation, the bond length changed little and the initial structure was significantly different from the optimized structure. Figure 9 shows the thermodynamic properties of (C2H5)3Al. In the range of 25-1000 K, the enthalpy, entropy and heat capacity of (C2H5)3Al increased with the increase of temperature. The free energy decreased with the increase of temperature. At 298. 15

Structure Optimization and Thermodynamic
Properties of (C 2 H 5 ) 3 Al Figure 8 shows the structure of (C 2 H 5 ) 3  bond length was reduced from 1.14 Å to 1.105 Å, the C 1 -C 2 bond length increased from 1.54 Å to 1.551 Å, the C 2 -Al bond length increased from 1.879 Å to 1.997 Å and the C 2 -H 4 bond length was reduced from 1.14 Å to 1.112 Å. During the optimization process, the C-Al bond rotated, the bond angle had large variation, the bond length changed little and the initial structure was significantly different from the optimized structure. Figure 9 shows the thermodynamic properties of (C 2 H 5 ) 3 Al. In the range of 25-1000 K, the enthalpy, entropy and heat capacity of (C 2 H 5 ) 3 Al increased with the increase of temperature. The free energy decreased with the increase of temperature. At 298.15 K, the enthalpy, entropy and heat capacity were 125.294 kcal/mol, 102.836 cal/(mol·K), 41.264 cal/(mol·K) and 94.634 kcal/mol, respectively.  Figure 10 shows the structure of AlBr3. It can be seen that after optimization of the AlBr3 structure, the bond angle of AlBr3 increased from equal 120° to 120.687°, 120.173° and 119.14°. The Br1-Al bond length increased from 2.25 Å to 2.264 Å, the Br2-Al bond length increased from 2.254 Å to 2.264 Å and the Br3-Al bond length increased from 2.25 Å to 2.267 Å. Figure 11 shows the thermodynamic properties of AlBr3. It can be seen that in the range of 25-1000 K, the enthalpy and entropy of the AlBr3 molecule increased with the increase of temperature, the heat capacity tended to be stable with the increase of temperature and the free energy decreased with the increase of temperature. The free energy was 0 kcal/mol at 25 K, which gradually decreased to a negative value with the increase of temperature. At 298.15 K, the enthalpy, entropy, heat capacity and free energy were 6.478 kcal/mol, 88.04 cal/(mol·K), 18.250 cal/(mol·K) and −19.771 kcal/mol, respectively.  Figure 10 shows the structure of AlBr3. It can be seen that after optimization of the AlBr3 structure, the bond angle of AlBr3 increased from equal 120° to 120.687°, 120.173° and 119.14°. The Br1-Al bond length increased from 2.25 Å to 2.264 Å, the Br2-Al bond length increased from 2.254 Å to 2.264 Å and the Br3-Al bond length increased from 2.25 Å to 2.267 Å. Figure 11 shows the thermodynamic properties of AlBr3. It can be seen that in the range of 25-1000 K, the enthalpy and entropy of the AlBr3 molecule increased with the increase of temperature, the heat capacity tended to be stable with the increase of temperature and the free energy decreased with the increase of temperature. The free energy was 0 kcal/mol at 25 K, which gradually decreased to a negative value with the increase of temperature. At 298.15 K, the enthalpy, entropy, heat capacity and free energy were 6.478 kcal/mol, 88.04 cal/(mol·K), 18.250 cal/(mol·K) and −19.771 kcal/mol, respectively.  Figure 9. The relationship between the thermodynamic properties of (C 2 H 5 ) 3 Al and temperature.

Structure Optimization and Thermodynamic Properties of AlBr3
3.2.5. Structure Optimization and Thermodynamic Properties of AlBr 3 Figure 10 shows the structure of AlBr 3 . It can be seen that after optimization of the AlBr 3 structure, the bond angle of AlBr 3 increased from equal 120 • to 120.687 • , 120.173 • and 119.14 • . The Br 1 -Al bond length increased from 2.25 Å to 2.264 Å, the Br 2 -Al bond length increased from 2.254 Å to 2.264 Å and the Br 3 -Al bond length increased from 2.25 Å to 2.267 Å. Figure 11 shows the thermodynamic properties of AlBr 3 . It can be seen that in the range of 25-1000 K, the enthalpy and entropy of the AlBr 3 molecule increased with the increase of temperature, the heat capacity tended to be stable with the increase of temperature and the free energy decreased with the increase of temperature. The free energy was 0 kcal/mol at 25 K, which gradually decreased to a negative value with the increase of temperature. At 298.15 K, the enthalpy, entropy, heat capacity and free energy were 6.478 kcal/mol, 88.04 cal/(mol·K), 18.250 cal/(mol·K) and −19.771 kcal/mol, respectively.    Table 2 shows the thermodynamic properties during Al reacting with C2H5Br to produce (C2H5)2AlBr and C2H5AlBr2. It can be seen that when the reaction temperature was 298.15 K, the ΔH      Table 2 shows the thermodynamic properties during Al reacting with C 2 H 5 Br to produce (C 2 H 5 ) 2 AlBr and C 2 H 5 AlBr 2 . It can be seen that when the reaction temperature was 298.15 K, the ∆H was −160.77 kcal/mol, ∆G was −139.83 kcal/mol and ∆S was −70.2 cal/(mol·K); it was thus an exothermic reaction.  Table 3 shows the total energy and thermodynamic properties of each component during (C 2 H 5 ) 3 Al preparation at 298.15 K. Table 4 shows the thermodynamic properties during the (C 2 H 5 ) 3 Al preparation process (reaction Equation (3)). At 298.15 K, the ∆H was 10.64 kcal/mol, the ∆G was 19.87 kcal/mol, the ∆S was 30.9 cal/(mol·K) and the enthalpy change was greater than 0; indicating this was an endothermic reaction.  Figure 12 shows the structure of C 2 H 4 . After structural optimization, ∠H 2 C 2 H 4 was reduced from 120.001 • to 116.504 • . The bond length of C-H decreased from 1.14 Å to 1.094 Å and the bond length of C=C was reduced from 1.54 Å to 1.342 Å. Figure 13 shows the thermodynamic properties of C 2 H 4 . It can be seen that the enthalpy, entropy and heat capacity of C 2 H 4 increased with the increase of temperature in the range of 25-1000 K. The free energy decreased with the increase of temperature. At 298.15 K, the enthalpy, entropy, heat capacity and free energy were 33.759 kcal/mol, 55.228 cal/(mol·K), 10.372 cal/(mol·K) and 17.293 kcal/mol respectively (Table 5). was −160.77 kcal/mol, ΔG was −139.83 kcal/mol and ΔS was −70.2 cal/(mol·K); it was thus an exothermic reaction.  Table 3 shows the total energy and thermodynamic properties of each component during (C2H5)3Al preparation at 298.15 K. Table 4 shows the thermodynamic properties during the (C2H5)3Al preparation process (reaction Equation (3)). At 298.15 K, the ΔH was 10.64 kcal/mol, the ΔG was 19.87 kcal/mol, the ΔS was 30.9 cal/(mol·K) and the enthalpy change was greater than 0; indicating this was an endothermic reaction.
3.2.6. Structure Optimization and Thermodynamic Properties of C2H4 Figure 12 shows the structure of C2H4. After structural optimization, ∠H2C2H4 was reduced from 120.001° to 116.504°. The bond length of C-H decreased from 1.14 Å to 1.094 Å and the bond length of C=C was reduced from 1.54 Å to 1.342 Å. Figure 13 shows the thermodynamic properties of C2H4. It can be seen that the enthalpy, entropy and heat capacity of C2H4 increased with the increase of temperature in the range of 25-1000 K. The free energy decreased with the increase of temperature. At 298.15 K, the enthalpy, entropy, heat capacity and free energy were 33.759 kcal/mol, 55.228 cal/(mol·K), 10.372 cal/(mol·K) and 17.293 kcal/mol respectively (Table 5). 3.2.7. Structure Optimization and Thermodynamic Properties of H2 Figure 14 shows the structure of H2. It can be seen that after structural optimization, the bond length of H-H increased from 0.74 Å to 0.747 Å. Figure 15 shows the relationship between the thermodynamic properties of H2 and temperature. The enthalpy, entropy and heat capacity of H2 increased with the increase of temperature and the free energy decreased with the increase of   Figure 14 shows the structure of H 2 . It can be seen that after structural optimization, the bond length of H-H increased from 0.74 Å to 0.747 Å. Figure 15 shows the relationship between the thermodynamic properties of H 2 and temperature. The enthalpy, entropy and heat capacity of H 2 increased with the increase of temperature and the free energy decreased with the increase of temperature. At 298.15 k, the enthalpy, entropy, heat capacity and free energy were respectively 8.367 cal/mol, 32.531 cal/(mol·K), 6.955 cal/(mol·K) and −1.332 kcal/mol (Table 5).   Figure 13. The relationship between the thermodynamic properties of C2H4 and temperature.
3.2.7. Structure Optimization and Thermodynamic Properties of H2 Figure 14 shows the structure of H2. It can be seen that after structural optimization, the bond length of H-H increased from 0.74 Å to 0.747 Å. Figure 15 shows the relationship between the thermodynamic properties of H2 and temperature. The enthalpy, entropy and heat capacity of H2 increased with the increase of temperature and the free energy decreased with the increase of temperature. At 298.15 k, the enthalpy, entropy, heat capacity and free energy were respectively 8.367 cal/mol, 32.531 cal/(mol•K), 6.955 cal/(mol•K) and −1.332 kcal/mol (Table 5).  3.2.7. Structure Optimization and Thermodynamic Properties of H2 Figure 14 shows the structure of H2. It can be seen that after structural optimization, the bond length of H-H increased from 0.74 Å to 0.747 Å. Figure 15 shows the relationship between the thermodynamic properties of H2 and temperature. The enthalpy, entropy and heat capacity of H2 increased with the increase of temperature and the free energy decreased with the increase of temperature. At 298.15 k, the enthalpy, entropy, heat capacity and free energy were respectively 8.367 cal/mol, 32.531 cal/(mol•K), 6.955 cal/(mol•K) and −1.332 kcal/mol (Table 5).   Table 5 shows the thermodynamic properties during the (C 2 H 5 ) 3 Al decomposition process (reaction Equation (4)). At 298.15 K, the ∆H was −20.21 kcal/mol, the ∆G was −54.822 kcal/mol, the ∆S was 116.08cal/(mol·K) ( Table 6) and the enthalpy change was less than 0, this was an endothermic reaction.

Conclusions
We explored a novel chemical reduction of organic aluminum for plating Al on a graphene surface. The thermodynamics of the Al plating reaction process were studied. The Al plating process consisted of two stages: the first was to prepare (C 2 H 5 ) 3 Al; the ∆H was 10.64 kcal/mol, the ∆G was 19.87 kcal/mol, the ∆S was 30.9 cal/(mol·K); this was an endothermic reaction. In the second stage, the (C 2 H 5 ) 3 Al decomposed into Al atoms, which were gradually deposited on the surface of the graphene and the Al plating formed. At 298.15 K, the ∆H was −20.21 kcal/mol, the ∆G was −54.822 kcal/mol, the ∆S was 116.08 cal/(mol·K) and the enthalpy change was negative, thus indicating an endothermic reaction. The results show that the reaction efficiency can be improved significantly by increasing the reaction temperature and reaction time appropriately.