Tag Archives: Xin-Zheng Li

A closer look at the interface between water and solid surfaces sparks memories of the water/air interface

Researchers at China’s Peking University have developed a technique for taking the closest look possible at the interface  between water and solid surfaces according to a Jan. 10, 2014 news item on Nanowerk,

The interaction of water with the surfaces of solid materials is ubiquitous. Many remarkable physical and chemical properties of water/solid interfaces are governed by H-bonding interaction between water molecules. As a result, the accurate description of H-bonding configuration and directionality is one of the most important fundamental issues in water science. Ideally, attacking this problem requires the access to the internal degrees of freedom of water molecules, i.e. the O-H directionality. However, resolving the internal structure of water has not been possible so far despite massive efforts in the last decades due to the light mass and small size of hydrogen.

Recently, the teams led by Professor Ying Jiang and Professor Enge Wang of International Center for Quantum Materials (ICQM) of Peking University succeeded to achieve submolecular-resolution imaging of individual water monomers and tetramers adsorbed on a Au [gold]-supported NaCl [sodium chloride](001) film at 5 K, using a cryogenic scanning tunneling microscope (STM).

The Jan. 9, 2014 University of Peking news release, which originated the news item, provides more detail,

… They first decoupled electronically the water molecule from the metal substrate by inserting an insulating NaCl layer and then employed the STM tip as a top gate to tune controllably the molecular density of states of water around the Fermi level. These key steps enabled them to image the frontier molecular orbitals which are spatially locked together with the geometric structures of water molecules. Notably, they were able to discriminate in real space the orientation of water monomers and the H-bonding directionality of water tetramers based on the submolecular-resolution orbital images.

This work opens up the possibility of determining the detailed topology of H-bonded networks at water/solid interfaces with atomic precision, which is only possible through theoretical simulations in the past. The ability to resolve the O-H directionality of water provides further opportunities for probing the dynamics of H-bonded networks at atomic scale such as H-atom transfer and bond rearrangement. In addition, the novel orbital-imaging technique developed in this work reveals new understanding of STM experiments and may be applicable to a broad range of molecular systems and materials.

Here’s a link to and a citation for the paper,

Real-space imaging of interfacial water with submolecular resolution by Jing Guo, Xiangzhi Meng, Ji Chen, Jinbo Peng, Jiming Sheng, Xin-Zheng Li, Limei Xu, Jun-Ren Shi, Enge Wang, & Ying Jiang. Nature Materials (2014) doi:10.1038/nmat3848 Published online 05 January 2014

This paper is behind a paywall although you can get a preview via ReadCube Access.

As noted in the headline, this work sparked a memory of research conducted on the water/air interface and the discovery that the boundary between water and air is not as distinct as was believed. (I have a longstanding interest in boundaries, which often have an arbitrary nature.) From the 2011 (?) news item on Softpedia,

The question of where water stops and where air begins is a very old, and difficult-to-answer one. Experts have been trying to do so for years, and now it would appear that they finally have an answer. The layer separating the two is as thin as the distance between two atoms in a hydrogen molecules.

At the topmost layer of water, in an ocean for example, water (H2O) molecules are having a real problem – they cannot really decided whether to exist as gas or liquid. As such, one of the two hydrogen atoms remains in the water, while the second ones pokes out into the air.

Physicists now call this the layer of molecular ambiguity, and say that it has little to no effect on the way water below this level behaves. …

Interestingly, just one molecule beneath this first layer of H2O molecules, the rest of the water behaves as if nothing is going on in the layers above. This discovery is critically important for many fields of research, including for example atmospheric chemistry.

“In some ways this is a negative result. Sometimes a negative result can be very positive,” says Pavel Jungwirth, a scientist at the Academy of Sciences of the Czech Republic, in Prague. …

With the new data, says University of Victoria [located in British Columbia, Canada] physical chemist Dennis Hore, scientists will be able to refine models seeking to explain how water interacts with other chemicals within living cells. …

There’s a link to and a citation for the 2011 paper,

Hydrogen bonding at the water surface revealed by isotopic dilution spectroscopy by Igor V. Stiopkin, Champika Weeraman, Piotr A. Pieniazek, Fadel Y. Shalhout, James L. Skinner, & Alexander V. Benderskii. Nature 474, 192–195 (09 June 2011) doi:10.1038/nature10173 Published online 08 June 2011

This paper is behind a paywall although you can get a preview via ReadCube Access.