I think you mixed something up. Information depth means that the signal for true photoelectrons is damped exponentially. For an air exposed sample in UPS it is often hard to determine the energetics as water, carbon and oxides are submerging the true signal. For delicate samples sputtering is not an option, as most compound semiconductors suffer from preferential sputtering of one of the compounds. With XPS you have for nonmonochromatted sources the sattelites of the Al or Mg radiation disturbing the valenceband edge. In most cases a sophisticated sattelite subtraction may help to get the valence band edge, but the resolution is lower then for UPS, but in most cases sufficient. For the work function in cases of air exposed samples the value obtained might be meaningless as you have at least the dipole layer of the contaminants altering the work function. Try to heat the sample to remove some contaminants. Transition meta Oxides have often d-electrons at the valence band edge which have a quite high cross section compared to s or p levels. This might help.

Dear Pettenkofer,

Thanks for you answer. As for the transition metal oxides having d-electrons at the valence band edge, can you elaborate more what does a high cross section compared to s or p levels and what its significance. I do not have much understanding into it.

Compared to simple semiconductors like Si or GaAs transition metal compounds have a pronounced peak at the valence band edge due to the d-states which are in part not involved directly in the bondformation.This peak facilitates the determination of the band edge. 

Hello Chenin Lu,

I think you do not need to think too much about the equipment condition,for the UPS you can detect valence band more clearly ,since these two methods can also be used to measure the chemical state and composition of the transition metal oxide. And the biggest deference between XPS and UPS is the energy source.These two methods work as one from X-ray source in and electron out, another from Ultra-violet in and electron out. For the elcetron emitts by XPS is primary electron which determine mostly the electronical structure.For the UPS ,which electron emitts out depending on the materials you have measured,since the spectrum you get from the UPS is the relationship between atomic number and  Hartetre-Fock energy, you can see clearly the d-electron,s-electron and f-electron spectrum. So you can know your valence band energy of transition metal Oxide system by using UPS instead of XPS.

This will be further supported by PL measurements. The difference between the Fermi level (Ef) and valance band maximum (EVBM) was evaluated from the XPS spectra shown in figures 5(e) and (f). This was done by extrapolating the leading edge of the XPS spectra to the baseline as shown in the figure 5(f) and produced a value of 2.63 eV (Ef–EVBM)

" One-step synthesis of strongly confined, defect-free and hydroxyterminated ZnO quantum dots "