Photoelectrochemical (PEC) water splitting promises a direct route toward solar-to-chemical energy conversion.  To address the challenges associated with the slow kinetics of complex reactions such as water oxidation, catalysts have been applied onto photoelectrodes.  While PEC performance improvement is often reported, the detailed mechanisms underpinning the improvements are poorly understood.  In the case of heterogeneous oxide-based catalysts such as Co-phosphate, NiFeOx and IrOx, various electrochemical and spectroscopic studies have pointed to the slowdown of surface recombination as a key reason for the improvement.  Due to the complex chemistries involved by these heterogeneous catalysts, however, little is known about whether fast charge transfer plays a role.  Here we report a study that corrects this deficiency.  Our strategy is to directly compare two catalysts systems, heterogenized Ir-complex water oxidation catalysts (het-WOC) and heterogeneous IrOx catalysts, on hematite photoanode.  Using a combination of intensity modulated photocurrent spectroscopy (IMPS), photoelectrochemical impedance spectroscopy (PEIS) and kinetic isotope effect (KIE), we present compelling evidence that points the different functional mechanisms exhibited by the two different types of catalysts.  The het-WOC improves the performance of hematite by speeding up hole transfer for water oxidation.  The surface recombination rates remain unchanged.  The IrOx catalysts, on the other hand, slows down surface recombination.  New understandings of the photoelectrode|water interface are estabilished.