On this planet, aqueous interfaces of oxides are ubiquitous, and are arguably the most important among all solid/liquid interfaces. The weathering process of silicate minerals in water shapes our landscape, and plays a crucial role in the global carbon cycle. In modern industry, these interfaces host many important reactions, such as the oxygen evolution in renewable energy schemes. Knowing the exact structures and reaction pathways, especially at the molecular level, is one of the most sought-after problems in the surface science community. However, it has also been a hard-nut-to-crack.

Oxide in liquid water: "dark side" of the interface

Being nearly impenetrable to charged particles/atoms, and incompatible with low temperature and vacuum systems, the buried interfaces between solid and liquid are notoriously difficult to probe. Over the last few decades, the surface-specific nonlinear optical methods, such as second harmonic generation (SHG) and sum-frequency generation (SFG) have become some of the most powerful in situ surface science techniques. They can be applied to any interface accessible to photons, and have led to significant advances in our understanding of oxide aqueous interfaces. Nonetheless, while the water side has been extensively studied, the solid side remains barely investigated. If the interface is like a puzzle, then one crucial piece is still missing. The difficulty lies in that, even photons – for excitation of oxide surface vibrations – could not reach this interface, as they are strongly absorbed by both the water and oxide. Previously, we have successfully tackled another “dark interface”, the electrochemical metal/water interface, by coupling SFG to the surface plasmon polariton (SPP)¹. Unfortunately, SPP is only applicable to conductors, and we need to explore alternative methods for the insulating oxide surfaces.

Achieving in situ nonlinear optical spectroscopy

A few years ago, we developed a matrix formalism to calculate field distributions across layered structures with nonlinear optical responses at interfaces². Encouragingly, we found that despite the strong infrared attenuation, the total SFG response from the oxide/water interface could be drastically enhanced with appropriate sample geometries. This approach could allow us to shed light on the “dark side” of these buried interfaces, and complete this jigsaw puzzle at the molecular level.

To begin with, we applied this approach to the ubiquitous, classical prototype system: the silicon dioxide/water interface³. It was considered a thoroughly explored interface, and our initial purpose was simply to test the experimental scheme. Surprisingly, unexpected results emerged once the in situ probe is allowed. In our recent paper, “Unconventional structural evolution of an oxide surface in water unveiled by in situ sum frequency spectroscopy”, we found this interface to be much more complex than traditionally regarded. In existing textbooks, the deprotonation of a silanol group (-SiOH) would always lead to a silanolate group (-SiO^-). However, in situ SFG revealed a different scenario: that the silanol and silanolate groups may not convert to each other directly, which was previously unknown to the community.

New tool, new findings

With the help of ab initio molecular dynamics (AIMD) simulation, we unveiled a new reaction pathway upon deprotonation of silanol groups: a surface reconstruction leading to unconventional five-fold silicon species [Si(5c)]. Inclusion of this pathway successfully explained our key experimental observations, including the simultaneous weakening of both silanol and silanolate groups in the intermediate pH range, the multimodal deprotonation process, and the steady redshift of the silanol band toward high pH. Still more importantly, our finding provides insight into previous controversies and mysteries about this system, such as the origin of the multimodal titration behavior, the size dependence of pKa for nanoparticles, and the microscopic spatial inhomogeneity of silica pKa, etc³. We also find it helps understand recent SFG studies on the interfacial water molecules^4,5, both suggesting a previously unknown reaction pathway. Currently, further investigations on oxide/water interfaces are underway, with more peculiar yet exciting phenomena awaiting discovery.

It has never been easy to develop experimental tools to crack hard nuts, yet every step forward is paid off. Nature is always ready to surprise.

References

1. Wei-Tao Liu & Y. Ron Shen, In situ sum-frequency vibrational spectroscopy of electrochemical interfaces with surface plasmon resonance. Proc. Natl. Acad. Sci. USA, 111, 1293 (2014). 
2. Hui Shi, Yu Zhang, Hongqing Wang, and Wei-Tao Liu, Matrix formalism for radiating polarization sheets in multilayer structures of arbitrary composition. Chin. Opt. Lett. 15, 081901 (2017).
3. J. Bañuelos et al., Oxide-and Silicate-Water Interfaces and Their Roles in Technology and the Environment. Chem. Rev. 123, 6413 (2023).
4. B. Rehl et al., Water Structure in the Electrical Double Layer and the Contributions to the Total Interfacial Potential at Different Surface Charge Densities. J. Am. Chem. Soc. 144, 16338 (2022).
5. F. Wei et al., Elucidation of the pH-Dependent Electric Double Layer Structure at the Silica/Water Interface Using Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy. J. Am. Chem. Soc. 145, 8833 (2023).