Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
pubs.acs.org/JPCC
Contrasting Roles of Water at Sliding Interfaces between SiliconBased Materials: First-Principles Molecular Dynamics Sliding Simulations Yusuke Ootani,† Jingxiang Xu,† Takahiro Hatano,‡ and Momoji Kubo*,† †
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Earthquake Research Institute, The University of Tokyo, 1-1-1 Yayoi, Bunkyo, Tokyo 113-0032, Japan
‡
S Supporting Information *
ABSTRACT: It is known that the wear of silicon-based materials is due to the tribochemical reaction with water at the sliding interface, but the detailed mechanisms remain under debate. In this study, we used a first-principles molecular dynamics method to investigate the tribochemical wear mechanism. When a small amount of water was present at the sliding interface, the formation of interfacial bridge bonds connecting the two surfaces was observed. These bonds transmitted shear force to the surfaces that induced strain therein. The strained surface Si−O bonds subsequently reacted with water (Si−O−Si + H2O → Si−OH + Si−OH), that is, the hydrolysis reaction occurred. Because the hydrolysis reaction resulted in dissociation of the surface Si−O bonds, water promoted tribochemical wear. However, when a large amount of water was present, it separated the two surfaces. The water thereby suppressed the formation of interfacial bridge bonds and in turn the hydrolysis of Si−O bonds and thus tribochemical wear. Our results indicate that water could either promote or suppress tribochemical wear, depending on how much was present. We suggest that the previously reported humidity dependence of the tribochemical wear of silicon-based materials can be explained in terms of these contrasting roles of water.
1. INTRODUCTION The tribological properties of materials affect the energy efficiency and durability of machines and therefore are of great importance in various fields. In particular, the tribological properties of silicon-based materials have been widely studied. For example, silicon nitride (Si3N4) and silicon carbide (SiC) have low friction coefficients (≤0.01) in water environment1 and are thus expected to be useful as lubricants in situations where contamination by oils is prohibited. The formation of a silicon dioxide (SiO2) layer by means of tribochemical reactions with water has been suggested to result in even lower friction.1 The frictional properties of silicon (Si) are important for microelectromechanical systems.2 Because the ratio of surface area to volume is large at microscales, the effects of friction and wear are larger for microscale devices than for macroscale machines. The wear of microelectromechanical systems is of particular concern, and tribochemical reactions of water reportedly affect the wear behavior in such systems.2 Thus, the development of durable devices requires optimal surface preparation. The frictional properties of SiO2 have been studied by seismologists because the frictional properties of rocks, which typically contain quartz, play an important role in earthquake mechanisms. For example, Toro et al. reported that the frictional force of quartz rock decreases with increasing slip velocity above 1 mm/s.3 These investigators suggested that a thin silica gel layer that formed at the sliding interface by the © XXXX American Chemical Society
tribochemical reaction with water reduces the force of friction and thereby leads to fault lubrication. In all these cases, the tribochemical reactions of water at the sliding interface play a key role by affecting friction and wear. Thus, understanding these reactions is important for a wide range of fields. The microscale tribological properties of silicon-based materials were studied by atomic force microscopy (AFM) with a SiO2 tip and a Si substrate having a native oxide layer (SiO2).4−7 One of the largest concerns in this experiment is tribochemical wear,4−7 which is important in many applications.2,8 The extent of wear of silicon-based materials has been found to increase with increasing humidity up to 50% and to then decrease with increasing humidity at values of >50%.4,6 Chemical reactions with water, rather than mechanical damage, are thought to induce the observed wear because it is not accompanied by subsurface damage.4,6 In addition, because tribochemical wear has been found to decrease when another material, such as diamond, is used for the AFM tip, it has been suggested that interfacial bridge bonds connecting the SiO2 tip and the Si substrate induce tribochemical wear.5 However, the details of the chemical reactions that promote tribochemical wear and interfacial bridge bond formation remain under Received: February 26, 2018 Revised: April 18, 2018
A
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C debate because in situ observations of sliding interfaces are difficult. Moreover, the mechanism of the above-mentioned humidity dependence of the extent of wear remains unclear. Computational simulations have been used to obtain atomistic insights into chemical reactions at sliding interfaces.9−20 For example, Barnette et al. used first-principles calculations to investigate the mechanism of tribochemical wear of SiO2, which mimics the native oxide layer of a Si substrate.9 These investigators proposed that Si−O bonds on the surfaces are cleaved by reaction with water molecules: Si−O−Si + H2O → Si−OH + Si−OH. However, such static calculations do not provide an insight into the dynamic aspects of chemical reactions. Moreover, static calculations neglect the effects of friction, such as normal load, shear force, and collisions among asperities. Recently, sliding simulations based on classical reactive molecular dynamics were performed to investigate chemical reactions at sliding interfaces of silicon-based materials.10−13 These simulations took into account both friction and chemical reaction dynamics. Chemical reactions, such as interfacial bridge bond formation and Si substrate oxidation, were observed. Reactive molecular dynamics methods are important tools for investigating the correlation between tribochemical reactions and tribological properties, but these methods do not provide an insight into electronic structures, which is required for a full understanding of chemical reactions. In addition, the results of reactive molecular dynamics simulations depend strongly on empirical parameters in general. Thus, classical reactive molecular dynamics methods are disadvantageous for chemical reactions with an unclear mechanism, and a more reliable and accurate sliding simulation method that considers electronic structures is required. Firstprinciples molecular dynamics (FPMD) simulation is an appropriate method for investigating chemical reactions whose mechanism is unclear, but few FPMD studies of tribological properties have been published.14−16 In this work, we used FPMD simulations to investigate the roles of water in the tribochemical reactions of silicon-based materials. Specifically, we performed sliding simulations with αquartz, which served as a model for the native oxide layer of silicon-based materials. The details of the tribochemical reactions and the mechanism of tribochemical wear were analyzed.
Figure 1. Sliding simulation models: (a) flat surface model and (b) vicinal-type asperity model. The inset in (b) shows the structure of the asperities. The yellow, red, and white balls indicate Si, O, and H atoms, respectively, and the circles indicate the asperities.
topmost Si atoms by applying a harmonic potential in the zdirection. A restraint force in the y-direction was also applied to the topmost Si atoms by using harmonic potential (force constant = 0.01) to restrict the sliding of the upper slab in the y-direction. All the simulations were performed until 15 ps, during which time the upper slab slid through almost one period (14.769 Å) of the super cell along the x-direction (i.e., the a-direction). First, we investigated the frictional behavior and the structure of the sliding interface by using the flat surface model (Figure 1a). Each slab consisted of 36 Si atoms, 90 O atoms, and 36 H atoms. A normal load of 1, 3, 5, or 7 GPa was applied; we used loads that were slightly higher than the typical contact pressure (∼1 GPa) in AFM experiments4−7 so that the role of the normal load would be clear. The sliding simulations were performed after a several-picosecond equilibration in which only the normal load was applied. Next, we performed sliding simulations using the vicinal-type asperity model (Figure 1b) to investigate the effect of collisions between asperities. We introduced the smallest possible asperities on the SiO2 surfaces. The height of each asperity was about 1.7 Å (Figure 1b), and the Si atoms of the asperities had one OH group: the so-called vicinal-type silanol group. Because the vicinal-type silanol group is the typical silanol group on amorphous silica surfaces,25 the phenomena that triggered by the collisions between asperities can occur at the sliding interface of amorphous silica. (We also performed simulations with an asperity model having geminaltype silanol groups, in which the Si atoms in the asperity had two OH groups. The results for the geminal-type asperity model are shown only in the Supporting Information because they were similar to the results for the vicinal-type asperity model.) Each slab of the vicinal-type asperity model consisted of 38 Si atoms, 93 O atoms, and 34 H atoms. The simulation was performed so that the Si atoms in the asperities of the upper and lower slabs collided during the sliding. A normal load of 5 or 7 GPa was applied to enhance the chemical reactions in the simulations. For the asperity model, we performed five sliding simulations, each with different initial atomic coordinates. The initial atomic coordinates of the sliding simulations were sampled from the equilibration process in which a normal load and a restraint force in the y-direction were applied. For both models, we compared two types of simulation environments to investigate the role of water at the sliding interfaces: one environment had no H2O molecules at the interface and the other had 10 H2O molecules, which corresponds to about half of a water monolayer on the αquartz (0001) surface.21
2. COMPUTATIONAL DETAILS The sliding simulations were performed by means of the FPMD method based on density functional theory (DFT), and two sliding simulation models were used: a flat-surface model and a vicinal-type asperity model (Figure 1). Crystalline αquartz served as a model for the native oxide layer of siliconbased materials. To construct the sliding interface, we placed two α-quartz slabs in a super cell. The α-quartz (0001) surface was chosen because it is widely studied both experimentally and theoretically.21−23 The surfaces of the slabs were terminated with hydroxyl (OH) groups to mimic the surface at a sliding interface in the presence of water. The super cell had a hexagonal structure with the following parameters: a = 14.769 Å, b = 14.769 Å, c = 25.000 Å, α = 90.0°, β = 90.0°, and γ = 120.0°. All the parameters except for c were based on experimentally determined lattice constants.24 In the sliding simulations, the positions of the bottom Si atoms in the lower slab were fixed, whereas the topmost Si atoms in the upper slab were allowed to slide in the x-direction at 100 m/s (=0.001 Å/fs). A normal load was applied to the B
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 2. Snapshots of sliding simulations with the flat surface model: (a) no H2O/1 GPa and (b) 10 H2O/1 GPa.
The Car−Parrinello molecular dynamics method was used for the sliding simulations, with a fictitious electron mass of 300.0 atomic units (au) and a time step of 4 au (=0.966 fs). The temperature of the atoms was controlled at 300.0 K with a Nosé−Hoover chain thermostat. The DFT calculations were performed with a plane-wave basis set and a kinetic energy cutoff of 70 Ry under a periodic boundary condition. Brillouin zones were sampled at the Γ-point. We used a generalized gradient approximation-type PBE functional and a Goedecker− Teter−Hutter pseudopotential.26 In the trajectory analysis, chemical bonds were identified on the basis of cutoff lengths of 1.8 and 1.2 Å for Si−O and O−H bonds, respectively. In the bond order analyses, we used a Gaussian-type 6-31G** localized auxiliary basis set to project the wavefunction onto atomic orbitals. All sliding simulations and bond order analyses were performed with the CPMD program package.27 Finally, we analyzed the energy barriers for the chemical reactions observed in the sliding simulations. The energy barrier calculations were performed with a Si4O10H4 cluster model to simplify the transition-state calculations.28 The optimized structures were obtained by DFT calculations with a hybrid-type B3LYP functional. The energy barriers were calculated using the Møller−Plesset second-order perturbation theory with optimized structures obtained by DFT calculations. We used the Def2SVP basis set. The solvent effect of water was considered by means of a conductor-like polarizable continuum model.29 The energy barrier calculations were performed with the Gaussian 09 program package.30 All the molecular structures were visualized with VMD.31
Figure 3. Variation of force of friction with normal load. Both quantities were evaluated by averaging over the entire simulation time. Time variation of the force of friction and normal load is shown in the Supporting Information (Figures S1 and S2). The solid lines indicate best fits.
these values are on the same order of magnitude as experimentally obtained values.9 We evaluated the distribution of the distances between the Si atoms in the first layer and the O atoms of the silanol groups (−Si−OH) at the sliding interface in the absence of H2O at normal loads of both 1 and 7 GPa (Figure 4a,b). For the 10 H2O case, we also evaluated the distribution of the distances between the first-layer Si atoms and the O atoms of the H2O molecules (Figure 4c,d). (The results for the 3 and 5 GPa cases are shown in Figure S3.) The sharp peak near 1.8 Å that was observed in all the distributions corresponds to the Si−O bonds of the surface −Si−OH groups. The rather broad peak near 3.5−4.0 Å for the O atoms in the same slab (indicated by a purple line) corresponds to the second-nearest-neighbor O atoms. The difference between the peak positions for the O atoms in the other slab (indicated by a green line) corresponds to the difference between the surface−surface distances. In the presence of 10 H2O molecules (Figure 4c,d), the peak for the O atom in the H2O molecules corresponds to the surface− water distance. In both the no H2O and the 10 H2O cases, the surface−surface distance decreased with increasing normal load. The surface−surface distances were 1.0−1.5 Å larger in the presence of water than in its absence. This result shows that H2O molecules separated the two surfaces. We investigated the averaged changes in the numbers of H2O molecules, terminal −Si−OH groups, H3O+ ions, and terminal −Si−O− groups in the presence of 10 molecules of H2O and in the absence of H2O (Figure 5). In each MD step, the differences in the number of the species (H2O, −Si−OH, H3O+, and −Si−O−) from that in the initial structure were evaluated, and then, they were averaged over all the MD steps. In the 10 H2O case, the numbers of H2O molecules and −Si− OH groups decreased, and the numbers of H3O+ ions and
3. RESULTS AND DISCUSSION 3.1. Flat Surface Model. Sliding simulations performed with the flat surface model (Figure 1a) at a normal load of 1 GPa revealed that sliding was smooth both in the absence of H2O and in the presence of 10 molecules of H2O (Figure 2a,b). The formation of an interfacial bridge bond connecting the upper and lower surfaces was not observed. Even at higher normal loads (3−7 GPa), sliding was smooth, and interfacial bridge bonds did not form. That the force of friction increased linearly with increasing normal load in both the no H2O and the 10 H2O cases (Figure 3) indicated that the friction coefficient was independent of normal load. The intercepts of the best-fit lines were nonzero owing to the adhesion force between the two surfaces. These frictional behaviors are consistent with the results of previously reported classical molecular dynamics sliding simulations.32 The coefficients of friction, obtained from the slopes of the best-fit lines, were 0.16 and 0.11 for the no H2O and 10 H2O simulations, respectively; C
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
results for the geminal-type asperity model were similar to those for the vicinal-type asperity model (Figures S6−S8). Unlike the sliding simulation with the flat surface model (Figure 2), the sliding simulations of the vicinal-type asperity model showed the formation of interfacial bridge bonds connecting the upper and lower surfaces in the contact area of the asperities (Figure 6). This is consistent with previous
Figure 6. Snapshots of sliding simulations with the vicinal-type asperity model for the 10 H2O/7 GPa case.
Figure 4. Distributions of distances between first-layer Si atoms and silanol group O atoms and between first-layer Si atoms and the O atoms of H2O at the sliding interface: (a) no H2O/1 GPa, (b) no H2O/7 GPa, (c) 10 H2O/1 GPa, and (d) 10 H2O/7 GPa. Distributions were averaged over the entire simulation. The grid size was 0.1 Å.
simulation results, where the interfacial bridge bonds were formed at the contact areas.11,13 Although the asperities were very small, their collisions dramatically promoted interfacial bridge bond formation. This result indicates that atomic-scale roughness should be the key factor to trigger the formation of interfacial bridge bonds. We investigated the variation of the number of interfacial bridge bonds in sliding simulations shown in Figure 7 [The corresponding graph for a typical trajectory along with the snapshots is shown in the Supporting Information (Figure S4)]. The number of interfacial bridge bonds was defined as the number of O atoms bonded to Si atoms in both the upper and the lower slabs in the initial structure. In the no H2O/5 GPa case, one or two interfacial bridge bonds formed starting at about 5 ps, but the number of interfacial bridge bonds did not increase markedly with time (Figure 7a). In contrast, in the no H2O/7 GPa case, the number of interfacial bridge bonds rapidly increased in several of the five simulations (Figure 7b). That the plots show regions where the number of interfacial bonds was constant indicates that the interfacial bridge bonds were rigid covalent bonds. These results indicate that a high normal load led to the formation of the bonds. However, the number of bonds was lower in the simulations with H2O molecules than in the no H2O case, at both 5 and 7 GPa (Figure 7c,d); note in particular that in the 10 H2O/5 GPa case, no interfacial bridge bonds were detected after about 7 ps. These results indicate that the H2O molecules separated the two surfaces and reduced the contact area between the asperities. To verify that the H2O molecules separated the two surfaces, we performed a sliding simulation with 30 H2O molecules at a normal load of 7 GPa and found that no interfacial bridge bonds formed (Figure S5), a result indicating that the H2O molecules completely separated the two surfaces and prevented bond formation. We next performed simulations with no sliding in the absence of water at a normal load of 7 GPa to investigate the role of the collisions of asperities caused by the sliding (Figure 7b, inset). We found that the number of interfacial bridge bonds in these simulations was much lower than the
Figure 5. Changes in the number of H2O molecules, terminal −Si− OH groups, H3O+ ions, and terminal −Si−O− groups averaged over the entire simulation for the 10 H2O cases. The inset shows the results for the no H2O cases.
−Si−O− groups increased. The increases in the numbers of the ionic species were due to the presence of H2O molecules at the sliding interface, as indicated by the fact that these numbers did not change in the absence of H2O (Figure 5, inset). The H2O molecules stabilized the ionic species, which were formed by friction. The amounts of the ionic species increased with increasing normal load. As we will show later, these stabilized species played an important role in the tribochemical reactions at the sliding interface. Although we used crystal quartz model, these results can be transferable to amorphous silica because the results do not depend on the detailed structure of the surfaces. 3.2. Vicinal-Type Asperity Model. We also performed sliding simulations using the vicinal-type asperity model (Figure 1b) to investigate the effect of collisions between asperities, which occur frequently on the real surface. The simulation D
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 7. Numbers of interfacial bridge bonds connecting the two surfaces in the sliding simulation with the vicinal-type asperity model: (a) no H2O/5 GPa, (b) no H2O/7 GPa, (c) 10 H2O/5 GPa, and (d) 10 H2O/7 GPa. The inset in (b) shows the results for the no H2O/7 GPa case with no sliding. Five simulations were performed for each case. A moving average method was used with a characteristic smoothing time of 97 fs (=1000 MD steps) to smooth the graphs.
number in the sliding simulations. This result shows that the collisions of asperities accelerated interfacial bridge bond formation. In summary, a high normal load induced interfacial bridge bond formation, but the presence of H2O molecules at the sliding interface suppressed bond formation by separating the two surfaces; that is, both a normal load and asperity collisions resulting from sliding were required for interfacial bridge bond formation. We next focused on the chemistry of interfacial bridge bond formation. In the no H2O case, the bond formation processes can be classified into two types: (1) bond alternation processes, in which the interfacial bridge bond is formed by means of direct Si−O bond alternation; and (2) proton-transfer processes, in which proton transfer mediates the bond formation (Figure 8). In the bond alternation process (Figure 8a), the O atoms on the upper surface (O1) and the lower surface (O2) approached a Si atom on the opposite surface (Si2 and Si1, respectively); the Si1−O1 and Si2−O2 bonds then dissociated, and Si1−O2 and Si2−O1 bonds formed (Figure 8a, left snapshot). A plot of the temporal dependences of the bond orders (Figure 8a, bottom panel) showed that bond dissociation and formation occurred simultaneously (∼5 ps)
Figure 8. Snapshots of the typical interfacial bridge bond formation processes observed in the sliding simulation with the vicinal-type asperity model for the no H2O/7 GPa case, along with a plot showing the variation of the bond orders during the processes. (a) Bond alternation process. In the left snapshot, the Si1 and O1 atoms are on the upper surface, whereas the Si2 and O2 atoms are on the lower surface. (b) Proton-transfer process. In the left snapshot, the Si3 and O4 atoms are on the upper surface, whereas the O3, O5, and H1 atoms are on the lower surface.
without the formation of an intermediate structure. As a result, two interfacial bridge bonds formed between the upper and lower surfaces (Figure 8a, right snapshot). We also observed a bond alternation process in which one interfacial bridge bond formed (Figure S8). During the proton-transfer process (Figure 8b), the hydrogen atom in the −Si−OH group on the lower surface (H1) was transferred to the O atom of the neighboring −Si−OH group (O5) as a proton, and the O atom of the −Si− O− group (O3) approached the Si atom on the upper surface (Si3) (Figure 8b, left). At 2−4 ps, an intermediate structure formed, as indicated by the variation of the bond orders (Figure 8b, lower panel). Finally, the Si3−O4 bond dissociated, and a E
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C −Si−O− (−Si−O4) group formed on the upper surface. That is, an interfacial bridge bond, Si3−O3−Si, formed (Figure 8, right). During this process, the −Si−OH groups mediated proton transfer. The proton hopped between the −Si−OH groups as the Grotthuss mechanism where the proton hops between water molecules. In the 10 H2O case, interfacial bridge bonds formed only via the proton-transfer process; the bond alternation process was not observed (Figure 9). Proton transfer was mediated by the
the H2O molecules stabilized the ionic species and thereby facilitated proton transfer. In the no H2O case, the interfacial bridge bonds that formed at the sliding interface dissociated as a result of sliding. The typical process is shown in Figure 10. After formation of the
Figure 10. Snapshots of the typical interfacial bridge bond dissociation process observed in the sliding simulation with the vicinal-type asperity model for the no H2O/7 GPa case, along with a plot showing the variation of the bond orders during the process. In the top left snapshot, the Si6 and O8 atoms are on the upper surface, and Si8−O9− Si7 is the interfacial bridge bond.
interfacial bridge bond connecting the two surfaces (Si8−O9− Si7), the Si7−O9 bond dissociated as a result of sliding. Meanwhile, the Si7 atom was terminated by the OH group of the neighboring −Si−OH group (O8H), and the O9 atom formed a bond with the Si6 atom on the upper surface. These processes occurred simultaneously, at ∼14 ps, without the formation of an intermediate structure (Figure 10, lower panel). The dissociation of chemical bonds is always accompanied by the formation of new chemical bonds because the unbonded atoms are unstable. In this case, as soon as the Si7−O9 bond dissociated, Si7 and O9 atoms formed new chemical bonds with the O8 and Si6 atoms on the SiO2 surfaces, respectively. Thereby, dissociation of the Si8−O9−Si7 interfacial bridge bond led to the formation of a Si8−O9−Si6 bond on the upper surface; that is, Si8−O9−Si7 + Si6−O8H → Si8−O9−Si6 + Si7−O8H. As a result, the surface structure changed upon the dissociation of the interfacial bridge bond. On the other hand, the number of the Si−O−Si bonds did not change. In the 10 H2O case, dissociation of the interfacial bridge bond and a change in the surface structure were induced after formation of the interfacial bridge bonds, as in the absence of H2O. In addition, a hydrolysis reaction occurred in the 10 H2O case, whereas no hydrolysis reaction was observed in the no H2O case because no H2O molecules were present at the sliding interface. A typical reaction process is shown in Figure 11. Two H2O molecules approached the surface Si9−O10 bond next to the Si10−O12−Si9 interfacial bridge bond, and a proton (H4) and a hydroxyl ion (O11H) were transferred from the H2O molecules to O10 and Si9, respectively. The surface Si9− O10 bond then dissociated. These processes occurred almost at
Figure 9. Snapshots of the typical interfacial bridge bond formation process observed in the sliding simulation with the vicinal-type asperity model for the 10 H2O/7 GPa case, along with a plot showing the variation of the bond orders during the process. In the top left snapshot, the Si4 and O7 atoms are on the upper surface, whereas the Si5, O6, and H2 atoms are on the lower surface. In the bottom left snapshot, the H3 atom is in a water molecule.
surface −Si−OH groups as well as by H2O molecules, in contrast to the situation in the no H2O case. During the proton-transfer process, the H atom of the −Si−OH group on the lower surface (H2) was transferred to the neighboring H2O molecule as a proton (Figure 9, top left snapshot). This proton transfer was confirmed by the decrease in the O6−H2 bond order (Figure 9, lower panel). Meanwhile, the O atom of the −Si−O− group (O6) approached the Si atom on the upper surface (Si4) (Figure 9, top right snapshot), as indicated by the increase in the Si4−O6 bond order (Figure 9, lower panel). Finally, the H atom of the neighboring H2O molecule (H3) was transferred to the oxygen atom on the upper surface (O7), and the Si4−O7 bond on the upper surface dissociated (Figure 9, bottom left). Then a Si4−O6−Si5 covalent bond formed between the upper and lower surfaces (Figure 9, bottom right snapshot). In the absence of H2O, the surface −Si−OH groups mediated the proton transfer (Figure 8b), whereas in the 10 H2O case, the H2O molecules also participated in the proton-transfer process. Thus, the H2O molecules provided an alternative pathway for interfacial bridge formation. In addition, F
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
respectively. Figure 12 shows the reactant, transition state, and product structures, as well as their relative energies. Both
Figure 11. Snapshots of the hydrolysis reaction process observed in the sliding simulation with the vicinal-type asperity model for the 10 H2O/7 GPa case, along with a plot showing the variation of the bond orders during this process. In the snapshot on the left, the O10 atom is on the lower surface, the Si10−O12−Si9 bond is the interfacial bridge bond, and the O11 and H4 atoms are in H2O molecules.
Figure 12. Reactants, transition states, and product structures for interfacial bridge bond formation: (a) bond alternation processes and (b) proton-transfer process mediated by a H2O molecule. The numbers indicate the relative energies.
the same time, as indicated by the variation of the bond orders (Figure 11, lower panel). In this case, the hydrolysis reaction was accompanied by the relay of a proton via the two H2O molecules like the Grotthuss mechanism. Thus, the proton transfer mediated not only the formation of the interfacial bridge bond (Figures 8b and 9) but also the hydrolysis reaction at the sliding interface. The hydrolysis reaction occurred on the surface Si9−O10 bond next to the Si10−O12−Si9 interfacial bridge bond, suggesting that the Si10−O12−Si9 interfacial bridge bond induced the hydrolysis. The shear force resulting from the sliding was transmitted to the lower surface through the Si10− O12−Si9 interfacial bridge bond and induced strain in the surface Si9−O10 bond. The strained bond reacted readily with the H2O molecules because it was energetically unstable. In other words, the strain was used to overcome the energy barrier to the hydrolysis reaction. This idea is supported by the fact that the hydrolysis reaction was not observed when the interfacial bridge bond was absent. In the no H2O case (Figure 10), a structural change was observed, whereas in the 10 H2O case, the hydrolysis reaction changed the chemical composition of the SiO2 surfaces because they reacted with the H2O molecules. The hydrolysis reaction dissociated one Si−O bond on the surface and formed two OH groups: Si−O−Si + H2O → Si−OH + Si−OH. Thus, a decrease in the number of the surface Si−O−Si bonds was accompanied by the consumption of an H2O molecule. Finally, we evaluated the energy barrier for interfacial bridge bond formation to investigate the feasibility of H2O-mediated bond formation. For this purpose, we used a Si4O10H4 cluster model instead of the slab model that we used in the sliding simulations, to simplify the transition-state calculations. Two types of interfacial bridge bond processes were considered: the bond alternation process (alt), which is the typical process in the absence of H2O, and the H2O-mediated proton-transfer process (prot), which is the typical process in the 10 H2O case. The alt and prot processes correspond to the interfacial bridge bond formation processes shown in Figures 8a and 9,
processes were exothermic. The alt process had one transition state with a reaction barrier of 25.6 kcal/mol, whereas the prot process had two transition states separated by an intermediate structure. The rate-determining step of the proton-transfer process was the first transition from the reactant to the intermediate structure. This transition had an energy barrier of 7.1 kcal/mol, which is much lower than the barrier for the alt process. This result indicates that H2O-mediated interfacial bridge bond formation (prot) was more feasible than the bond alternation process (alt). On the basis of our FPMD simulation results, we reached the following conclusions regarding the tribochemical wear behavior of silicon-based materials. When water is not present at the sliding interface, interfacial bridge bonds form (Figures 7a,b and 8), and then these bonds transmit shear force to the surfaces. Eventually, the interfacial bridge bonds dissociate as a result of sliding, and the Si and O atoms from the dissociated bonds form new chemical bonds with other atoms on the SiO2 surfaces (Figure 10): Sia−Oa−Sib + Sic−ObH → Sia−Oa−Sic + Sib−ObH. Thus, the number of Si−O−Si bonds does not change. This fact indicates that tribochemical wear does not occur, even though the surface structure changes. When water is present at the sliding interface, interfacial bridge bonds form, as in the no water case (Figures 7c,d and 9). The bonds then transmit shear force to the surfaces, which destabilizes the surface structure. As a result, the surface Si−O bonds are hydrolyzed (Figure 11). That is, the interfacial bridge bonds induce hydrolysis when water is present at the sliding interface. The hydrolysis dissociates the surface Si−O bonds, consuming water and forming two OH groups: Si−O−Si + H2O → Si− OH + Si−OH. Thus, the number of surface Si−O−Si bonds decreases; that is, the hydrolysis reaction damages the SiO2 surface. If hydrolysis reactions dissociated all of the Si−O bonds connecting a given Si atom to the surface, that Si atom would be removed from the surface. Therefore, water at the sliding interface promotes tribochemical wear via hydrolysis. G
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
force to the surfaces. Because the hydrolysis dissociated the surface Si−O bonds and reduced the number of Si−O−Si bonds in SiO2, water at the sliding interface promoted the tribochemical wear of the surface. However, when a large amount of water was present, it suppressed the formation of interfacial bridge bonds (and thus the hydrolysis reaction) by separating the two surfaces. As a result, water suppressed tribochemical wear. Therefore, water at the sliding interface either promoted or suppressed tribochemical wear, depending on how much was present at the sliding interface. The contrasting roles of water elucidated in this study provide a reasonable explanation of the wear behavior of silicon-based materials previously observed in AFM experiments.
The interfacial bridge bonds eventually dissociate as a result of sliding, as in the no water case. However, when a large amount of water is present at the sliding interface, the water suppresses the formation of interfacial bridge bonds (Figure S5) because it separates the two surfaces. Because the hydrolysis reaction is induced by the interfacial bridge bond, this result means that when a large amount of water is present at the sliding interface, water suppresses hydrolysis of the surface Si−O bond, that is, suppresses tribochemical wear. Thus, water at the sliding interface can either promote or suppress tribochemical wear of the surface, depending on how much is present. If the amount of water is not sufficient to separate the two surfaces, it promotes tribochemical wear by means of hydrolysis. In contrast, a large amount of water suppresses tribochemical wear by separating the two surfaces. As stated in the Introduction, AFM experiments have shown that the wear of silicon-based materials increases with increasing humidity up to 50% and then decreases with increasing humidity at humidity values of >50%.4,6 This wear behavior can be explained on the basis of the contrasting roles of water elucidated in the current study. When the humidity is 50%, the amount of water at the sliding interface is sufficient to separate the two surfaces, and tribochemical wear of the surfaces is suppressed.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b01953. Time variation of the force of friction and normal load; distributions of interatomic distances between Si and O atoms for the sliding simulations of the flat surface model at normal loads of 3 and 5 GPa; numbers of interfacial bridge bonds connecting the two surfaces in the sliding simulation with the vicinal-type asperity model in the 10 H2O/7 GPa case, along with a snapshot; numbers of interfacial bridge bonds connecting the two surfaces in the sliding simulation with the vicinal-type asperity model for the 30 H2O/7 GPa case; and sliding simulation results for the geminal-type asperity model (PDF)
4. CONCLUSIONS In this study, we used the FPMD method to investigate in detail the tribochemistry of water at the sliding interfaces of silicon-based materials. In the absence of water, interfacial bridge bonds connecting the two surfaces formed via bond alternation and proton transfer mediated by surface −Si−OH groups. Subsequently, the sliding induced dissociation of the interfacial bridge bonds, and the Si and O atoms from the dissociated bonds formed new chemical bonds with other atoms on the SiO2 surfaces, which resulted in a structural change of the surfaces. When water was present at the sliding interface, interfacial bridge bonds formed only via proton transfer, which was mediated both by the surface −Si−OH groups and by water, in contrast to the situation in the no H2O case. That is, water provided a feasible alternative pathway for interfacial bridge bond formation by mediating proton transfer. After the formation of interfacial bridge bonds, they transmitted shear force to the surfaces, which destabilized the surface structure. As a result, the surface Si−O bonds underwent hydrolysis: Si−O−Si + H2O → Si−OH + Si−OH. Proton transfer also mediated the hydrolysis reaction, thus playing an important role both in interfacial bridge bond formation and in the hydrolysis reaction. The interfacial bridge bonds were eventually dissociated as a result of sliding, as was the case in the absence of water. This new information about the tribochemistry at the sliding interface allowed us to reach the following conclusions about the mechanism of tribochemical wear of the silicon-based materials. When water was not present at the sliding interface, dissociation of interfacial bridge bonds that formed at the sliding interface was accompanied by a change in the surface structure. However, the number of Si−O−Si bonds in SiO2 did not change; therefore, tribochemical wear did not occur. When water was present, the interfacial bridge bonds induced hydrolysis of the surface Si−O bonds by transmitting shear
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jingxiang Xu: 0000-0002-1484-9692 Momoji Kubo: 0000-0002-3310-1858 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was supported by JST CREST, JSPS Grant-in-Aid for Scientific Research (A) (no. 26249011), JSPS Grant-in-Aid for Young Scientists (B) (no. 17K14430), MEXT “Exploratory Challenge on Post-K Computer” (Challenge of Basic ScienceExploring Extremes through Multi-Physics and Multi-Scale Simulations), and Cross-Ministerial Strategic Innovation Promotion Program (SIP) “Innovative Combustion Technology” (Funding agency: JST).
■
REFERENCES
(1) Chen, M.; Kato, K.; Adachi, K. The Difference in Running-in Period and Friction Coefficient between Self-Mated Si3N4 and SiC under Water Lubrication. Tribol. Lett. 2001, 11, 23−28. (2) Wang, X. D.; Yu, J. X.; Chen, L.; Qian, L. M.; Zhou, Z. R. Effects of Water and Oxygen on the Tribochemical Wear of Monocrystalline Si(100) against SiO2 Sphere by Simulating the Contact Conditions in MEMS. Wear 2011, 271, 1681−1688. (3) Toro, G. D.; Goldsby, D. L.; Tullis, T. E. Friction Falls Towards Zero in Quartz Rock as Slip Velocity Approaches Seismic Rates. Nature 2004, 427, 436−439. H
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C (4) Wang, X.; Kim, S. H.; Chen, C.; Chen, L.; He, H.; Qian, L. Humidity Dependence of Tribochemical Wear of Monocrystalline Silicon. ACS Appl. Mater. Interfaces 2015, 7, 14785−14792. (5) Yu, J.; Kim, S. H.; Yu, B.; Qian, L.; Zhou, Z. Role of Tribochemistry in Nanowear of Single-Crystalline Silicon. ACS Appl. Mater. Interfaces 2012, 4, 1585−1593. (6) Chen, L.; He, H.; Wang, X.; Kim, S. H.; Qian, L. Tribology of Si/ SiO2 in Humid Air: Transition from Severe Chemical Wear to Wearless Behavior at Nanoscale. Langmuir 2015, 31, 149−156. (7) Chen, C.; Xiao, C.; Wang, X.; Zhang, P.; Chen, L.; Qi, Y.; Qian, L. Role of Water in the Tribochemical Removal of Bare Silicon. Appl. Surf. Sci. 2016, 390, 696−702. (8) Zhou, F.; Wang, X.; Kato, K.; Dai, Z. Friction and Wear Property of a-CNx Coatings Sliding Against Si3N4 Balls in Water. Wear 2007, 263, 1253−1258. (9) Barnette, A. L.; Asay, D. B.; Kim, D.; Guyer, B. D.; Lim, H.; Janik, M. J.; Kim, S. H. Experimental and Density Functional Theory Study of the Tribochemical Wear Behavior of SiO2 in Humid and Alcohol Vapor Environments. Langmuir 2009, 25, 13052−13061. (10) Li, A.; Liu, Y.; Szlufarska, I. Effects of Interfacial Bonding on Friction and Wear at Silica/Silica Interfaces. Tribol. Lett. 2014, 56, 481−490. (11) Yue, D.-C.; Ma, T.-B.; Hu, Y.-Z.; Yeon, J.; van Duin, A. C. T.; Wang, H.; Luo, J. Tribochemical Mechanism of Amorphous Silica Asperities in Aqueous Environment: A Reactive Molecular Dynamics Study. Langmuir 2015, 31, 1429−1436. (12) Yeon, J.; van Duin, A. C. T.; Kim, S. H. Effects of Water on Tribochemical Wear of Silicon Oxide Interface: Molecular Dynamics (MD) Study with Reactive Force Field (ReaxFF). Langmuir 2016, 32, 1018−1026. (13) Wen, J.; Ma, T.; Zhang, W.; Psofogiannakis, G.; van Duin, A. C. T.; Chen, L.; Qian, L.; Hu, Y.; Lu, X. Atomic Insight into Tribochemical Wear Mechanism of Silicon at the Si/SiO2 Interface in Aqueous Environment: Molecular Dynamics Simulations using ReaxFF Reactive Force Field. Appl. Surf. Sci. 2016, 390, 216−223. (14) Haw, S. M.; Mosey, N. J. Tribochemistry of Aldehydes Sheared between (0001) Surface of α-Alumina from First-Principles Molecular Dynamics. J. Phys. Chem. C 2012, 116, 2132−2145. (15) Kajita, S.; Righi, M. C. A Fundamental Mechanism for CarbonFilm Lubricity Identified by Means of Ab Initio Molecular Dynamics. Carbon 2016, 103, 193−199. (16) Levita, G.; Righi, M. C. Effects of Water Intercalation and Tribochemistry on MoS2 Lubricity: An Ab Initio Molecular Dynamics Investigation. ChemPhysChem 2017, 18, 1−7. (17) Hayashi, K.; Tezuka, K.; Ozawa, N.; Shimazaki, T.; Adachi, K.; Kubo, M. Tribochemical Reaction Dynamics Simulation of Hydrogen on a Diamond-Like Carbon Surface Based on Tight-Binding Quantum Chemical Molecular Dynamics. J. Phys. Chem. C 2011, 115, 22981− 22986. (18) Hayashi, K.; Sato, S.; Bai, S.; Higuchi, Y.; Ozawa, N.; Shimazaki, T.; Adachi, K.; Martin, J.-M.; Kubo, M. Fate of Methanol Molecule Sandwiched between Hydrogen-Terminated Diamond-Like Carbon Films by Tribochemical Reactions: Tight-Binding Quantum Chemical Molecular Dynamics Study. Faraday Discuss. 2012, 156, 137−146. (19) Bai, S.; Murabayashi, H.; Kobayashi, Y.; Higuchi, Y.; Ozawa, N.; Adachi, K.; Martin, J. M.; Kubo, M. Tight-Binding Quantum Chemical Molecular Dynamics Simulations of the Low Friction Mechanism of Fluorine-Terminated Diamond-Like Carbon Films. RSC Adv. 2014, 4, 33739−33748. (20) De Barros Bouchet, M. I.; Martin, J. M.; Avila, J.; Kano, M.; Yoshida, K.; Tsuruda, T.; Bai, S.; Higuchi, Y.; Ozawa, N.; Kubo, M.; Asensio, M. C. Diamond-Like Carbon Coating under Oleic Acid Lubrication: Evidence for Graphene Oxide Formation in Superlow Friction. Sci. Rep. 2017, 7, 46394. (21) Chen, Y.-W.; Chu, I.-H.; Wang, Y.; Cheng, H.-P. Water Thin Film-Silica Interaction on α-quartz (0001) Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 155444. (22) Steurer, W.; Apfolter, A.; Koch, M.; Sarlat, T.; Søndergård, E.; Ernst, W. E.; Holst, B. The Structure of the α-quartz (0001) Surface
Investigated using Helium Atom Scattering and Atomic Force Microscopy. Surf. Sci. 2007, 601, 4407−4411. (23) Chen, Y.-W.; Cao, C.; Cheng, H.-P. Finding Stable α-quartz (0001) Surface Structures via Simulations. Appl. Phys. Lett. 2008, 93, 181911. (24) Ikuta, D.; Kawame, N.; Banno, S.; Hirajima, T.; Ito, K.; Rakovan, J. F.; Downs, R. T.; Tamada, O. First In Situ X-Ray Identification of Coesite and Retrograde Quartz on a Glass Thin Section of an Ultrahigh-Pressure Metamorphic Rock and Their Crystal Structure Details. Am. Mineral. 2007, 92, 57−63. (25) Zhuravlev, L. T. The Surface Chemistry of Amorphous Silica. Zhuravlev Model. Colloids Surf., A 2000, 173, 1−38. (26) Krack, M. Pseudopotentials for H to Kr Optimized for GradientCorrected Exchange-Correlation Functionals. Theor. Chem. Acc. 2005, 114, 145−152. (27) CPMD, Version 4.1, Copyright IBM Corp 1990-2015, Copyright MPI für Festkö rperforschung Stuttgart 1997−2001. http://www.cpmd.org/. (28) Jelfs, K. E.; Flikkema, E.; Bromley, S. T. Hydroxylation of Silica Nanoclusters (SiO2)M(H2O)N, M = 4, 8, 16, 24: Stability and Structural Trends. Phys. Chem. Chem. Phys. 2013, 15, 20438−20443. (29) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies, Structures, and Electronic Properties of Molecules in Solution with the C-PCM Solvation Model. J. Comput. Chem. 2003, 24, 669−681. (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision E.01; Gaussian, Inc.: Wallingford CT, 2009. (31) Humphrey, W.; Dalke, A.; Schulten, K. VMD: Visual Molecular Dynamics. J. Mol. Graphics 1996, 14, 33−38. (32) Ewen, J. P.; Restrepo, S. E.; Morgan, N.; Dini, D. Nonequilibrium Molecular Dynamics Simulations of Stearic Acid Adsorbed on Iron Surfaces with Nanoscale Roughness. Tribol. Int. 2017, 107, 264−273.
I
DOI: 10.1021/acs.jpcc.8b01953 J. Phys. Chem. C XXXX, XXX, XXX−XXX
