New Atomistic Insights on the Chemical Mechanical Polishing of Silica Glass with Ceria Nanoparticles

Abstract

Reactive molecular dynamics simulations have been used to simulate the chemical mechanical polishing (CMP) process of silica glass surfaces with the ceria ( 111) and ( 100) surfaces, which are predominantly found in ceria nanoparticles. Since it is known that an alteration layer is formed at the glass surface as a consequence of the chemical interactions with the slurry solutions used for polishing, we have created several glass surface models with different degrees of hydroxylation and porosity for investigating their morphology and chemistry after the interaction with acidic, neutral, and basic water solutions and the ceria surfaces. Both the chemical and mechanical effects under different pressure and temperature conditions have been studied and clarified. According to the simulation results, we have found that the silica slab with a higher degree of hydroxylation (thicker alteration layer) is more reactive, suggesting that proper chemical treatment is fundamental to augment the polishing efficiency. The reactivity between the silica and ceria (111) surfaces is higher at neutral pH since more OH groups present at the two surfaces increased the Si−O−Ce bonds formed at the interface. Usually, an outermost tetrahedral silicate unit connected to the rest of the silicate network through a single bond was removed during the polishing simulations. We observed that higher pressure and temperature accelerated the removal of more SiO 4 units. However, excessively high pressure was found to be detrimental since the heterogeneous detachment of SiO 4 units led to rougher surfaces and breakage of the Si−O−Si bond, even in the bulk of the glass. Despite the lower concentration of Ce ions at the surface resulting in the lower amount of Si−O−Ce formed, the (100) ceria surface was intrinsically more reactive than (111). The different atomic-scale mechanisms of silica removal at the two ceria surfaces were described and discussed.

Catalysts Prepared from Atomically Dispersed Ce(III) on MgO Rival Bulk Ceria for CO Oxidation

“…18 This comparison holds also when one considers the activity per surface cerium atom (Ce) s calculated on the basis of a density of ∼7 atoms Ce/nm 2 on the surface of bulk ceria (Table S3). 61 Our Ce(III)/MgO 500 is characterized by higher reaction rates per mole of cerium than all the other catalysts listed in Table S3�a result that we associate with the complete exposure of the cerium atoms on the MgO surface. These data show that notwithstanding the low cerium loading of only 1.0 wt % in Ce(III)/MgO 500 , its performance is at least comparable to those of bulk CeO 2 materials and nanomaterials.…”

Impact of Atomic Defects on Ceria Surfaces on Chemical Mechanical Polishing of Silica Glass Surfaces

“…Figure shows the tops of the pristine and defective ceria slab models separated from the silica model after 2.5 ns of polishing. From the smaller models with defects, 8 and 7 silicate units were removed when sliding long x and y directions, respectively, while less silicate units (6 and 5 for x and y directions, respectively) were removed from the pristine surface . Moreover, contrary to the pristine surface, which extracted only isolated units, a three-site chain and a pentamer were removed by the defective surface, indicating their stronger interaction with the silica gel.…”

“…All the MD simulations of the polishing process were performed using the LAMMPS software . To investigate the polishing of silica glass with the defective ceria surfaces, we employed the silica model with an alteration layer of 5 Å thickness, called Gel40%, which was taken from our previous investigation because it is the most reactive . This model had been generated from a bulk silica glass model by truncating the periodic boundary condition along the z -axis, resulting in a slab of 43 Å thickness, which matches the (111) ceria model (see below).…”

“…The mechanism of interaction between silica and ceria at the atomic level has been probed using computer simulations based on Car–Parrinello and reactive molecular dynamics simulations. , Onodera et al investigated the detachment of a silicate unit connected to the α-quartz [0001] surface through one Si–O–Ce bond by small ceria clusters (H 4 Ce 6 O 12 ) sliding on the surface using Car–Parrinello MD simulations, which were performed for a few picoseconds under both dry and wet conditions . They observed the formation of a first Si–O–Ce bond by the interaction of oxygen of the silane group with an undercoordinated Ce atom of the cluster and the formation of a second Si–O–Ce bond through the interaction of oxygen of the cluster with the silicon atom of the silicate unit.…”

“…Recently, we investigated the CMP process between a silica glass and ceria surfaces using more realistic models by reactive MD simulations, which employed our ReaxFF library for the Si/O/H/Ce elements derived from quantum mechanics. 18 We used larger models of silica glass surfaces generated by MD simulations in water solutions under different pH conditions (acidic, neutral, and basic). The effects of the morphologies of the glass surfaces with different porosity and degree of hydroxylation were studied, and the gelation of the silica surface was found to be crucial to form strong bonding between silica and ceria.…”