Ambient Pressure XPS Studies of NO2 Sensing with Titanyl Phthalocyanines

Chia-Hsuan Lin¹, Chia-Hsin Wang², Yaw-Wen Yang^1,2*

¹Dept. of Chemistry, National Tsing-Hua University, Taiwan
²Scientific Research Division, National Synchrotron Radiation Research Center, Taiwan

* Presenter:Yaw-Wen Yang, email:yang@nsrrc.org.tw

X-ray Photoelectron Spectroscopy (XPS) is a powerful technique providing the electronic structure information of the material surfaces, but needs to be performed in an environment better than high vacuum to eliminate electron-molecule scattering. Surface science studies require an even stringent UHV environment to keep the samples in clean conditions; as a result, the knowledge gained from this type of studies is noted for a weakness stemmed from so-called pressure gap and materials gap. To bridge the gaps, Ambient Pressure XPS (APXPS) technique was conceived and developed during the past one and a half decade. Now, the technique has reached a maturity level such that APXPS can be routinely carried out in a few millibar pressure range, enabling an in-situ and in-operando investigation of gas-solid, liquid-solid, and liquid-gas heterogeneous reactions.

NSRRC completed a new APXPS machine stationed at a soft X-ray beamline (BL24A) at TLS, and the endstation has been open to users since Sept. 2017. In this presentation, I will use our investigation of NO2 interaction with metal phthalocyanines (MPcs) to illustrate the APXPS measurement capability. Titanyl phthalocyanine (TiOPc) film has been used as NO2 gas sensing materials, but its sensing mechanism remains unclear. Our study involves APXPS measurements of NO2 adsorption on TiOPc thin films with NO2 pressure ranging from ultrahigh vacuum to 1 mbar. As the backing pressure of NO2 is increased above 0.025 mbar, an abrupt downward binding energy shift of 0.4 eV for all the relevant core levels of TiOPc is observed, indicating an acceptor doping by NO2 on TiOPc semiconducting films. This onset pressure coincides with the 25 ppm detection limit of TiOPc OFET. Through a careful curve fitting, the presence of chemisorbed NO2, NO2-, and physisorbed NO2 is identified by taking into account of paramagnetic nature of gas phase NO2. Nitrogen K-edge NEXAFS spectroscopy of phthalocyanine film also shows the disruption of orientation ordering of aromatic ring of TiOPc initiated by NO2 absorption. The detailed absorption of NO2 on TiOPc and its effect on Ti crystal field splitting will be discussed.

Keywords: Ambient Pressure X-ray Photoelectron Spectroscopy, nitric oxide sensing, metal phthalocyanine