Measurement on work function of polycrystalline zinc oxide covered by organic dye

The contact potential difference between an undyed or dyed zinc oxide powder layer and a gold reference electrode is measured by the Kelvin method. The work function (Phi(ZnO)) of polycrystalline zinc oxide covered by an optical sensitizing dye (rose bengal) grows with increasing adsorbed dye concentration on the zinc oxide surface, reached its maximum value at the dye concentration, 5.0 x 10 (-7) mole/g of ZnO, and then decreased with increasing dye concentration. The value of Phi(ZnO) is constant at concentrations larger than 76.5 10 (-7) mole/g of ZnO. In air, the measured value of Phi(ZnO) for undyed polycrystalline zinc oxide is 4.7 eV, and the maximum value of dyed ZnO is 4.82 eV. From these experimental results, it is suggested that the adsorbed dye on the zinc oxide surface makes but small contribution to the work function of zinc oxide; adsorption is due to dipole forces with the negative pole outermost. Some analysis of the numerical values is made on the dipole moment amplitude of the adsorbed dye and the potential barrier height formed by the adsorption and coverage of the dye on the surface of zinc oxide grains. It is shown that the results of these analyses give reasonable values for understanding the experimental data. A model is proposed to explain these experimental results.