All calculations in this study are carried out using the Crystal09 program49 at the DFT level alongside the PBE0 hybrid exchange-correlation functional.50-52 DFT calculations were employed since they are suitable for modeling molecular and crystal systems as well as adsorption processes. DFT has been widely used because it provides both qualitative and quantitative insights into the structures of active surfaces and the surface reactions.53 The standard def-TZVP basis sets are used for iron, nickel, manganese, sulfur, carbon, hydrogen, and oxygen atoms,54 while for zinc, copper, and cobalt atoms, the optimized def-TZVP basis sets are applied [supplementary data in ref 55 and 56]. Spin-paired calculations are used for the adsorption study on pure sulfide and oxide surfaces while spin-unpaired calculations are employed for the doped sulfide surfaces due to the magnetic properties of the dopant atoms.
The density of the k-point is set high enough to ensure convergence. In this work, the adsorption energy (ΔEads) is calculated by ΔEads = EA/S – EA – ES, where EA/S, EA, and ES are the calculated energies of adsorption system, adsorbate, and surface, respectively. The counterpoise method has been utilized to correct the basis set superposition error (BSSE) in the calculated adsorption energies.57
23
3 Adsorption of rubber adsorbate models on sulfide surfaces 3.1 Adsorption on zinc sulfide surfaces
The calculated adsorption energies of various substances onto a zinc atom of a ZnS(110) surface are compiled in Table 3. The adsorption strength of elementary functional group models from the weakest to the strongest are as follows: saturated hydrocarbons < ethene <
hydrogen sulfide. This adsorption pattern is also reflected in larger models, where methyl-substituted ethene exhibits lower adsorption energy than thiol-methyl-substituted ethene. As seen from Table 3, there are only minor quantitative differences between the results obtained using smaller and larger adsorbate models in the respective adsorbate models grouping. In addition, similar adsorption trends are predicted for the different functional groups of rubber adsorbate models. Therefore, throughout our study, the same rubber adsorbate models are used, since the smaller adsorbate models are found to be as effective as the larger models in terms of the adsorption on sulfide and oxide surfaces.
Table 3. The calculated adsorption energies on the zinc and copper sulfide surfaces (in kJ mol‒
1).55
Rubber adsorbate models ZnS(110) Cu2S(111) CuS(001)
Methane –3.0 –0.5 ‒0.4
Ethane ‒4.3 ‒0.8 ‒1.1
Propane ‒3.0 ‒1.0 ‒1.8
Ethene ‒29.8 ‒48.2 ‒36.6
Hydrogen sulfide ‒61.4 ‒39.3 ‒17.4
Methyl-substituted ethene ‒39.8 ‒53.7 ‒37.4
Thiol-substituted ethene ‒50.5 ‒41.5 ‒29.4
The outward relaxation of interacting zinc atom in ethene and hydrogen sulfide cases shown in Figure 9(d) and 9(e) is not visible in the case of saturated hydrocarbons (Figure 9(a)-(c)), indicating a stronger interaction is achieved with ethene and hydrogen sulfide. Saturated hydrocarbons only undergo weak physisorption. On the other hand, ethene interacts through the carbon-carbon double bond and the hydrogen sulfide interaction occurs via the lone pair electrons in sulfur. Notable outward relaxation is also visible in the case of larger models (Figure 9(f) and 9(g)). The way that the models interact relate closely to ethene and hydrogen sulfide, since methyl-substituted ethene adsorbs via a carbon-carbon double bond while the thiol-substituted ethene interaction occurs through the lone pair sulfur electrons instead of a double bond, as hydrogen sulfide interacts stronger than ethene.
Figure 9. Side view of the optimized configurations on the ZnS(110) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.
3.2 Adsorption on copper sulfide surfaces
The obtained adsorption energies are presented in Table 3. Both copper sulfide surfaces show the same adsorption trend, with the rubber adsorbate models having a stronger interaction with the Cu2S(111) than with the CuS(001) surface. Saturated hydrocarbons still show the weakest adsorption, as seen with the ZnS(110) surface, but the adsorption strength for ethene and hydrogen sulfide on both copper sulfide surfaces are reversed when compared to ZnS(110) surface. This phenomenon is further confirmed by the interaction with larger models because of the similar adsorption strengths seen in ethene and hydrogen sulfide is reflected on both copper sulfide surfaces.
Despite the fact that the order of the adsorption strength of ethene and hydrogen sulfide is opposite of the order than on ZnS(110) surfaces, they behave similarly as on ZnS(110) surface in that they interact through a carbon-carbon double bond and the lone pair electron of sulfur, respectively. The only difference occurs in thiol-substituted ethene, where the carbon-carbon double bond dominates the interaction as opposed to the thiol group, since the adsorption strength of ethene is higher than that of hydrogen sulfide. The optimized structures of the rubber adsorbate models on both Cu2S(111) and CuS(001) surfaces are illustrated in Figure 10 and Figure 11, respectively. For both copper sulfide surfaces, stronger interactions caused by ethene, hydrogen sulfide, and substituted ethenes lead to outward relaxation of the interacting copper atom.
25 Figure 10. Side view of the optimized configurations on the Cu2S(111) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.
Figure 11. Side view of the optimized configurations on the CuS(001) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.
4 The effect of dopant atoms on adsorption of rubber adsorbate models on doped sulfide surfaces
4.1 Adsorption on doped ZnS(110) surfaces
The calculated adsorption energies for the rubber adsorbate models positioned above the dopant atom on doped ZnS(110) surfaces are tabulated in Table 4. The energies are then compared to those on the undoped ZnS(110) surface. Despite having different dopant atoms, the adsorption trend seen on the undoped ZnS(110) surface is followed, but different adsorption strengths are observed. Among the elementary functional group models, saturated hydrocarbons still show the weakest interactions, but there is a notable enhancement in adsorption due to cobalt and manganese doping. For the ethene and hydrogen sulfide cases, only copper dopant atom decreases the adsorption strength, while cobalt doping has the highest positive effect showing notable enhancement in the calculated adsorption energies. The next most effective dopant atom is manganese, while both iron and nickel doping atoms display similar adsorption strengths as the undoped ZnS(110) surface. A similar qualitative observation on the changes in the adsorption strength can also be deduced for methyl- and thiol-substituted ethenes, depending on the inclusion of the corresponding dopant atom.
Table 4. The calculated adsorption energies on the undoped and doped ZnS(110) surfaces (in kJ mol–1)
Rubber adsorbate models Undoped55 Mn doped Fe doped Co doped56 Ni doped Cu doped56
Methane –3.0 –10.2 –2.4 –27.0 –3.1 –2.0
Despite the differences in the adsorption strength, the functional group models interact similarly on all the doped surfaces, which are comparable to those on the undoped surface, with the exception of saturated hydrocarbons on cobalt and manganese doped zinc sulfide.
Optimized adsorption geometries on cobalt doped zinc sulfide are shown as an example in Figure 12. The promotional effect caused by cobalt doping in all rubber adsorbate models, including saturated hydrocarbons, is visible in the optimized adsorption geometries, where the interacting cobalt atom relaxed outward, indicating a stronger interaction. Moreover, the stronger interaction between methane and the dopant atom leads to a larger distortion of bound methane from the free geometry, as listed in Table 5. The dominant functional group that is responsible for the interaction with the ZnS(110) surface is further proved through larger model cases that have two functional groups. For methyl-substituted ethene, the interaction is dominated through the carbon-carbon double bond, while the thiol group is responsible for and dominates the adsorption with thiol-substituted ethene, rather than the carbon-carbon double bond, since the preferential binding is via sulfur containing groups.
27 Figure 12. Side view of the optimized configurations on the cobalt doped ZnS(110) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.
Table 5. Adsorption energies (ΔEads) and geometrical parameters of methane interacting with metal on the undoped and doped ZnS(110) surfaces. Included as a reference are the geometrical parameters of free methane.
Free methane Undoped Mn doped Fe doped Co doped Ni doped Cu doped
ΔEads (kJ mol‒1) - ‒3.0 –10.2 –2.4 ‒27.0 –3.1 ‒2.0
Longest C-H bond length (Å) 1.091 1.092 1.098 1.092 1.099 1.094 1.091 Largest H-C-H angle (o) 109.5 110.8 114.4 110.4 114.9 111.9 110.2 Smallest H-C-H angle (o) 109.4 108.3 106.8 108.8 106.6 107.4 108.9
4.2 Adsorption on doped Cu
2S(111) surfaces
The calculated adsorption energies of the rubber adsorbate models interacting with a dopant atom on doped Cu2S(111) surfaces are presented in Table 6 and compared to those of the undoped Cu2S(111) surface. Generally, the adsorption trend on doped Cu2S(111) surfaces also follows the trend seen on the undoped Cu2S(111) surface, with the exception of methyl- and thiol-substituted ethenes on manganese and iron dopants, where the thiol-substituted ethene exhibits a slightly higher adsorption energy than methyl-substituted ethene when compared to the undoped Cu2S(111) surface. Moreover, a remarkable increase in the adsorption strength is achieved through the olefinic group interacting with the dopant atoms. The strongest effect is from the manganese and iron dopant atoms, followed by cobalt and nickel dopant atoms. This enhanced effect also results in obtaining higher adsorption energies for the methyl- and thiol-substituted ethene models.
Table 6. The calculated adsorption energies on the undoped and doped Cu2S(111) surfaces (in kJ mol–1).
Adsorbate Undoped55 Mn doped Fe doped Co doped56 Ni doped
Methane –0.5 0.0 –5.6 –0.9 0.0
Ethane –0.8 –0.1 –5.8 –1.0 –0.3
Propane –1.0 –0.2 –6.0 –1.2 –1.3
Ethene –48.2 –111.3 –104.4 –79.1 –69.0
Hydrogen sulfide –39.3 –34.3 –47.8 –43.8 –40.8
Methyl-substituted ethene –53.7 –106.5 –97.9 –79.7 –71.3
Thiol-substituted ethene –41.5 –110.1 –101.4 –69.7 –59.6
The optimized structures on cobalt doped copper sulfide are illustrated in Figure 13. The results show that rubber adsorbate models on all doped Cu2S(111) surfaces including cobalt dopant have similar interactions to those that occurred on the undoped Cu2S(111) surface.
Stronger interactions lead to the outward relaxation of the dopant atom interacting with the rubber adsorbate models. This phenomenon is clearly visible in all adsorbate models containing a carbon-carbon double bond, which includes ethene and both of the substituted-ethene models.
In addition to the relative evidence shown through the optimized structures that connect this effect to stronger interaction, the larger distortion of the ethene compared to the free ethene also indicates a stronger interaction and higher adsorption energies, as shown in Table 7.
Figure 13. Side view of the optimized configurations on the cobalt doped Cu2S(111) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene
Table 7. Adsorption energies (ΔEads) and geometrical parameters of ethene interacting with metal on undoped and doped Cu2S(111) surfaces. The geometrical parameters of free ethene are included as a reference.
Free ethene Undoped Mn doped Fe doped Co doped Ni doped
ΔEads (kJ mol‒1) - ‒48.2 –111.3 –104.4 ‒79.1 –69.0
C=C bond length (Å) 1.32 1.35 1.42 1.39 1.38 1.37
Bending of CH2 groups (o) 0.0 7.3 26.9 19.5 17.0 14.9
29
4.3 Promotional impact of dopant atoms on sulfide surfaces
The adsorption of rubber adsorbate models on the sulfide surfaces generally show weak interactions via the saturated hydrocarbons while carbon-carbon double bonds and thiol groups of rubbers lead to stronger interactions. However, with the inclusion of different dopant atoms on both sulfide surfaces, the adsorption strength of the rubber adsorbate models can either be enhanced or reduced. Therefore, the use of different transition metals as dopant atoms on ZnS(110) and Cu2S(111) surfaces has been studied to determine their interaction with rubber adsorbate models, and the promotional impacts are presented in Figure 14 and Figure 15, respectively.
Figure 14. The effect of doping on interaction of the rubber adsorbate models and the doped ZnS(110) surfaces. Saturated and unsaturated hydrocarbons present an average effect on the adsorption of methane, ethane, and propane, and ethene and methyl-substituted ethene, respectively.
Figure 15. The effect of doping on the interaction of the rubber adsorbate models and the doped Cu2S(111) surfaces. Saturated and unsaturated hydrocarbons present an average effect on the adsorption of methane, ethane, and propane, and ethene and methyl-substituted ethene, respectively.
In the case of the ZnS(110) surface (Figure 14), the copper dopant notably weakens the adsorption strength of all the functional groups of rubber. On the other hand, the iron and nickel dopants on the ZnS(110) surface display little or no effect on the adhesion. Manganese and cobalt doping not only enhance the adsorption strength of the carbon-carbon double bonds and thiol groups of the rubber adsorbate but also trigger the saturated hydrocarbon groups, leading to higher adsorption energies. These positive effects are more significant in the case of cobalt doping than when manganese is used for doping.
The enhancement of the adsorption strength of all the functional groups of rubber by cobalt and manganese doping on the ZnS(110) surface is not shown on the doped Cu2S(111) surfaces.
Only the adsorption involving the carbon-carbon double bonds of rubber is initiated on all the doped Cu2S(111) surfaces. Even so, the effect is significantly stronger, as shown in the case of manganese and iron doping when compared to the doped ZnS(110) surfaces (Figure 15). While the highest promotional effect is shown through cobalt doping on the ZnS(110) surface, this is not in the case on the Cu2S(111) surface, since the manganese and iron doping on the Cu2S(111) surfaces have a higher promotional effect than the cobalt doping. On the other hand, while the nickel dopant does not show a promotional effect on the ZnS(110) surface, the adsorption strength of rubber to Cu2S(111) surface is also increased through carbon-carbon double bond when the nickel dopant is used.
31
5 Adsorption of rubber adsorbate models on zinc oxide surfaces
The calculated adsorption energies on both the ZnO(110) and ZnO(001) surfaces are tabulated in Table 8. Even though the most stable polymorph of ZnO and ZnS are different, the results obtained from the adsorption of rubber adsorbate models on the ZnS(110) surface is being used as a reference. The adsorption trend between functional group models and the ZnO(110) surface follow the same trend seen on the ZnS(110) surface, in which saturated hydrocarbons exhibit the weakest interaction, followed by ethene while hydrogen sulfide had a slightly stronger interaction than ethene. Even so, the adsorption energies are slightly enhanced, by approximately 4-7 kJ mol-1 through the saturated hydrocarbons and ethene but are reduced by 20 kJ mol-1 when interacting with hydrogen sulfide. The adsorption strength for larger adsorbate models is also shown to be similar to that on the ZnS(110) surface, where thiol-substituted ethene shows a stronger interaction than methyl-thiol-substituted ethene, but the adsorption energies are lower compare to ZnS(110) surface.
On the other hand, the adsorption is noticeably stronger on the ZnO(110) surface than on the ZnO(001) surface. The adsorption strength for ethene and hydrogen sulfide on the ZnO(110) surface is approximately 30 kJ mol-1 higher than on the ZnO(001) surface. A similar observation can also applied to larger models, where the ZnO(110) surface exhibits higher adsorption strength than the ZnO(001) surface.
Table 8 The calculated adsorption energies on the ZnO surfaces. Calculated adsorption energies on the ZnS(110) surface are included as a reference (in kJ mol–1).
Adsorbate ZnS(110)55 ZnO(110) ZnO(001)
Methane –3.0 –7.6 –1.1
Ethane –4.3 –8.6 –0.4
Propane –3.0 –5.2 –1.4
Ethene –29.8 –37.2 –2.9
Hydrogen sulfide –61.4 –41.7 –9.8
Methyl-substituted ethene –39.8 –26.3 –5.0
Thiol-substituted ethene –50.5 –39.3 –13.1
As on the ZnS(110) surface, saturated hydrocarbons only exhibit weak physisorption on the ZnO(110) surface, while ethene interacts via the carbon-carbon double bond and hydrogen sulfide through the donation of the lone pair electrons of sulfur. On the other hand, for the adsorption on the ZnO(001) surface, all the functional group models exhibit weak physisorption as they display low adsorption strength. Even so, as predicted via adsorption on the ZnS(110) surface, the preferred binding mechanism is through the sulfur containing models, since hydrogen sulfide exhibits a stronger interaction than ethene. The optimized structures of the rubber adsorbate models on the ZnO(110) and ZnO(001) surfaces are presented in Figure 16 and Figure 17, respectively. The outward relaxation of the interacting zinc atom on the zinc oxide surfaces caused by rubber adsorbate models having carbon-carbon double bond and sulfur containing models is smaller than in zinc and copper sulfide surfaces cases. Similar
relative observations can be seen in the adsorption of larger adsorbate models on both ZnO surfaces.
Figure 16. Side view of the optimized configurations on the ZnO(110) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.
Figure 17. Side view of the optimized configurations on the ZnO(001) surface interacting with (a) methane, (b) ethane, (c) propane, (d) ethene, (e) hydrogen sulfide, (f) methyl-substituted ethene, and (g) thiol-substituted ethene.
33
6 Discussion
The role of the functional groups of the rubber structure in rubber–brass adhesion was explored on copper and zinc sulfide and on zinc oxide surfaces at the atomic level, finding distinct differences in the preferential binding between the rubber adsorbate models and the surfaces involved. In general, carbon-carbon double bonds and the thiol groups of rubber are responsible for the interaction with the surface metal atoms. However, zinc and copper sulfide surfaces display contradicting binding preferences. Zinc sulfide surface prefer to bond with sulfur containing functional groups, while copper sulfide preferentially bonds with the carbon-carbon double bond. The preferential binding of the sulfur containing functional groups also shown on the zinc oxide surfaces, but the adsorption strengths are lower compared to zinc sulfide surface.
The bonding via the carbon-carbon double bond occurs in the adsorption mode, where a π-donation from the adsorbate containing the carbon-carbon double bond into the empty orbitals of the metal occurs together with a back-donation from the metal into the empty π*-orbital of the adsorbate. This is the Dewar-Chatt-Duncanson adsorption model.58 On the other hand, the bonding that occurs through the sulfur-containing adsorbate is due to lone pair electron donation from the sulfur in the sulfur-containing adsorbate to the metal atom of the surface and occurs together with the back-donation of the surface electrons to the sulfur-containing adsorbates.59
Additives play an important role in rubber–brass adhesion, as they enhance the interaction.
Hence, the promotional effect of additives towards the rubber–brass adhesion has also been studied. Cobalt is widely used as additive especially in tire manufacturing since it assists in the formation of an essential amount of the copper sulfide layer and affects the durability of rubber–metal bonds.35 From our findings, only the adsorption involving carbon-carbon double bonds is enhanced by the cobalt doping on the copper sulfide surface. However, a notable promotional effect is achieved on all the functional groups of rubber, including saturated hydrocarbons, on the cobalt doped zinc sulfide surface. The dissociation of methane is known to occur on transition metal surfaces, and for a cobalt surface the activation barrier is 120 kJ mol–1.60,61 This could be the reason why a stronger interaction is observed, as the cobalt dopant on the zinc sulfide moderately activates the carbon-hydrogen bond of the saturated hydrocarbons.
Despite its advantages, the limitations of cobalt usage as an additive, which include the cost, have increased the demand for finding an alternative to substitute for cobalt as an additive.
Therefore, we substituted cobalt with manganese, iron, and nickel. The results were compared with those of cobalt doped substances. In the case of zinc sulfide, the promotional effect from cobalt is still the strongest. On the other hand, manganese and iron doping on the coper sulfide surface show a greater promotional effect than cobalt due to the enhancement in the interaction via the carbon-carbon double bond. This finding is important for the interaction between rubber and brass at the adhesive interlayer, since copper sulfide is speculated to be responsible for the interfacial adhesion.12
7 Conclusions
The rubber–brass adhesion process has been investigated using DFT involving the adsorption of rubber adsorbate models on copper and zinc sulfide and on zinc oxide surfaces at the adhesive interlayer. It was found that the functional groups of the rubber structure that are
The rubber–brass adhesion process has been investigated using DFT involving the adsorption of rubber adsorbate models on copper and zinc sulfide and on zinc oxide surfaces at the adhesive interlayer. It was found that the functional groups of the rubber structure that are