I think it will be difficult to simply connect your difference in photo-activity to a change in work function. your surface species, FQY and FQE, among many other factors such as surface area all play a role in contributing to the observed rate of photo-catalysis. What method are you using to determine photo-activity?

Thanks Nicholas Chadwick, I agree, many factors influence this behavior 

I determine photoactivity through H2 production in S2-/SO32- solution.

But, the change in work function maybe prove the charge transfer from CdS to TiO2? either indirectly?

Thanks Moisés Canle , Intersting point of view..In my work  also combine UPS data (Work Function) with optical band gap (obtained by difusse reflectance) will be posible draw a diagram of energy. In these diagram the diference of energy of fermi level and LUMO its lower for the doped sample..These combine results could be explain better the photocatalysis effect?

I also heard that photoactivity of a metal oxide is related to its work function. For more details please go through the classic book "surface science of metal oxide" by Henrich and cox. 

Titanium dioxide possess high work function (5-6ev) among metal oxides. I think that, by combining with CdS, there occurs some inter particle electron transfer reaction between CBs of TiO2 and CdS. This reduces the electron hole recombination rate, leads to significant charge separation. Work function is a major factor when we compare the photoactivity of individual metal oxides, ie., TiO2 vs ZnO or ZnO vs CdS etc..If we compare the photoactivity of a metal oxide and mixed metal oxide, we must take other factors into account. The basic conditions are not same, one is pure oxide and the other is mixed oxide. The increase in photoactivity of TiO2-CdS is due to the synergic effect of CdS over TiO2 (IPET, Inter Particle electron Transfer Reaction).

http://pubs.acs.org/doi/abs/10.1021/jp0119025

http://pubs.rsc.org/en/Content/ArticleLanding/1988/F1/f19888400261#!divAbstract

http://pubs.acs.org/doi/abs/10.1021/jp305026h