Graphitic carbon nitride (g-C3N4) has been extensively studied as a model of polymeric semiconductor material in photoelectrochemical (PEC) water oxidation reaction; however, its low PEC performance is still a concern of significant importance among researchers. Herein, we did a novel host-guest design of a photoanode device comprising three dimensional (3D) TiO2 nanotree (NT) arrays as an electron transfer layer (ESL) and Ag/g-C3N4 heterojunction as a photo absorber layer. In this design, 3D TiO2 NT arrays synthesized via a hydrothermal growth treatment were deposited with an alcoholic suspension of Ag/g-C3N4 using different coating methods such as drop casting, spin coating, and ultrasonic assisted-spin (U-spin) coating methods. The optimized photoanode prepared via U-spin coating exhibited a remarkable photocurrent density of 1.4 mA cm(-2) at 1.23 V vs. RHE and a negatively shifted onset potential of 0.2 V in 1.0 M KOH when it was illuminated from the backside using a Xenon lamp (150 W) equipped with a 400 nm cut off filter. This hierarchical 3D design photoanode demonstrated constant stability during the water oxidation reaction for at least 8 h.