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Nonlinear Spectro-Electrochemistry – Physical Chemistry – Fritz Haber Institute  
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Research

To fully understand the electrochemical processes such as ion/solvent adsorption, charge transfer at the heterogeneous interface and electrochemical catalytic reactions generally requires the combination of electrochemical and spectroscopic methods. The electrochemical analysis can provide the potential, current and charge information and the spectroscopies can reveal the electronic, magnetic and vibrational properties of the unknown reaction intermediates or products. The combination of these two is known as spectroelectrochemistry. Among the modern spectroelectroscopic techniques, vibrational ones such as Infrared absorption and Raman scattering are less affected by the present of ambient air or liquid and have been widely employed for in situ spectroelectrochemical characterization. While useful, IR and Raman spectroscopies are not surface specific. When the surface and the bulk have similar chemical compositions, i.e. bulk solvents and anions adsorb to the electrode surface, it is impossible to selectively probe the surface species from those of the bulk in order to understand the interaction between electrode and the bulk species. Vibrational sum frequency spectrosocpy, based on the 2nd order nonlinear optical process, is intrinsically surface specific. The energy diagram of the process is given in Fig.1(a). It can be found that the vibrational sum frequency generation process can be regarded as a combination of a infrared absorption and an anti-stokes raman processes. This determines the selection rule of VSFG: for a vibrational mode to be VSFG active, it must be both Raman and IR active. Since the selection rules of Raman and IR for centrosymmetric vibrational modes are exclusively complimentary, VSFG is vanish in any centrosymmetric or isotropic media such as gas, liquid or amorphous bulk but active at the interface where the symmetry is break. This unique selection has made it an excellent tool in surface science to study various surfaces and interfaces. In our group we mainly employ VSFG to study various important electrochemical systems.

 
Experimental Geometry for electrochemical vibrational sum frequency spectroscopy

In order to conduct in situ vibrational sum frequency measurement for electrochemical system, generally two different types of experimental geometries i.e. internal and thin layer geometries can be employed (Fig 1(b)&(c).). In Fig1(b), the thin film working electrode was coated on some optical transparent substrate. The incident and reflected beams are on the back side of the working electrode. In this geometry the shape of the cell is relatively flexible, standard reference electrode can be employed. One can run the electrochemistry as in normal bulk cell and without worrying the mass transport problems. The disadvantage of this geometry is that when metal electrodes are employed most of the two incident electromagnetic fields are reflected strongly by the metal thin film, only very small portion of the incident light can reach the metal solution interface. Even sum frequency signal is generated at the electrode/solution interface, little can transmit through the metal and substrate back to the detector. To obtain measurable signal under this geometry, the working electrode metal thin film must be less than 5nm, which makes the working electrode thin film discontinuous and unstable under electrochemical conditions. On the other hand, the vibrational sum frequency electrochemical measurement can be also done under thin layer geometry (Fig.1(c)), in which a thin electrolyte is sandwiched between the window and electrode. All three beams need pass both the window and the thin electrolyte layer. The absorption of the laser beams, especially the infrared by the electrolyte limits the electrolyte layer thickness. Generally the thickness needs to be less than 25um. Due to this spatial limitation, generally quasi- reference electrode, i.e. coated metal film or metal thin wire were employed. Furthermore, when very thin (<2 um) electrolyte film is employed, there is also mass transport problem especially for the electrochemical reactions with large exchange current density. Nevertheless, by choosing appropriate thickness and with flow cell, these problem can be circumvented. The big advantage of this experimental geometry is that the shape of the working electrode is not limited to coated film, bulk electrode such as single crystal electrode can be studied. Thereby many model reactions can be investigated. The validity of such geometry for spectro-electrochemistry has been well discussed in our previous papers. [J. Chem. Phys., 133, 034704 (2010).]

Tong, Y.; Campen, R. K.; In-Situ Probing of Adsorbates at Electrochemical Interfaces with Vibrational Sum Frequency Spectroscopy. Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, (accepted).
 
Principal research projects

Understanding noble-metal single crystal electrochemistry

Selected publications:

Tong, Y.; Lapointe, F.; Thämer, T.; Wolf, M.; Campen, R. K. ; Angewandte Chemie. 129, 4275 (2017).

Tong, Y.; Lu, L.; Zhang, Y.; Gao, Y.; Yin, G.; Osawa, M.; Ye, S.; J. Phys. Chem. C, 111, 18836-18838 (2007).

Resolving the key reaction intermediates for important electrochemical reactions 

Selected publication:

Tong, Y.; Cai, K.; Wolf, M.; Campen, R. K.; Catalysis Today, 260, 66-71 (2016).

Monitoring the molecular structural changes during the photo-/ electric- switch 

Selected publications:

Garling, T.; Tong, Y.; Darwish, T. A.; Wolf, M.; Campen R. K. Journal of Physics: Condensed Matter. 29, 414002 (2017).

Darwish, T.; Tong, Y.; James, M.; Hanley, T.; Peng, Q.; Ye, S.; Langmuir, 28, 13852-13860 (2012).

Revealing the transient species during ultra-fast heterogeneous charge transfer 

Selected publication:

Lapointe, F.; Wolf, M.; Campen, R. K.; Tong, Y.; in preparation