REACTIONS OF HYDROGEN AT THE INTERFACE OF PALLADIUM TITANIUM-DIOXIDE SCHOTTKY DIODES AS HYDROGEN SENSORS, STUDIED BY WORKFUNCTION AND ELECTRICAL CHARACTERISTIC MEASUREMENTS

Reactions of hydrogen at the interface of palladium (Pd)/n-type titanium dioxide (n-TiO2) (001) and (100) Schottky diodes, which are used as hydrogen sensors, have been investigated by measurements of the current-voltage (I-V) and capacitance-voltage (C-V) characteristics and changes in the workfunction. The workfunction of the Pd surface decreases monotonically with an increase in the concentration of hydrogen introduced to the air because of the substitution of hydrogen atoms adsorbed on Pd for adsorbed oxygen atoms. The energy barrier at the Pd/n-TiO2 interface is also lowered with an increase in the hydrogen concentration. The magnitudes of the barrier height lowering and its dependence on the hydrogen concentration are quite different for the Pd/ n-TiO2(001) and (100) diodes, indicating that the chemical structure of the interface depends strongly on the crystal orientation. The plot of the barrier height lowering for the (001) diodes versus the decrease in the Pd workfunction is parabolic-like throughout the hydrogen concentration region investigated (0-3000 ppm), whereas for the (100) diode, it is parabolic-like only in the low hydrogen concentration region below approximately 100 ppm, and becomes linear in the higher concentration region. Analysis of the plots has led to the following model. In the case of the Pd/TiO2(001) diode, the main reaction at the interface to lower the energy barrier is the reduction of interfacially adsorbed oxygen atoms to water by hydrogen atoms coming from the surface through the Pd layer. In the case of the Pd/TiO2(100) diode, on the other hand, water formation is the dominant reaction at the interface only in the low hydrogen concentration region, and hydrogen adsorption on bare Pd atoms becomes dominant in the higher hydrogen concentration region. The free energy of adsorption of oxygen atoms at the Pd/TiO2(001) interface is estimated to be 0.14 eV in absolute value larger than that at the Pd/TiO2(100) interface. It is found that a larger amount of oxygen atoms are adsorbed at the Pd/ TiO2(001) interface, in accordance with the higher free energy of adsorption. By vacuum evacuation and the introduction of pure oxygen, the I-V and C-V curves for the Pd/ TiO2(001) and (100) diodes behave differently. These results are explained by the presence (or absence) of crystallographic channels perpendicular to the Pd/n-TiO2 interface for the (001) (or (100)) diode, through which hydrogen and oxygen atoms diffuse into the TiO2 bulk.