Hydrogen (H2) is an ideal green energy source to alleviate the impact of fossil fuel scarcity and environmental risk because it is renewable, energy dense, and environmentally friendly. Splitting water using a photocatalytic reduction reaction on the semiconductor surface utilizing abundant solar energy has been demonstrated as a promising approach for clean, cost-effective production of hydrogen. The key of this technique is the discovery of high efficient visible-light-driven semiconductor photocatalysts, which still faces much challenge and is far from real commercialization. To meet these challenges, our efforts have been put into the design and development of new photocatalyst materials (or composites). In this thesis, a series of semiconductor composites base on a kind of air-stable n-type organic semiconductor, perylene tetracarboxylic diimides, have been fabricated and utilized for photocatalytic water splitting reaction to produce hydrogen. The supramolecular self-assembly concepts and methods of perylene tetracarboxylic diimides have been introduced into this field, providing novel insight into this field. The research covers several main aspects as following: 1. A series of perylene tetracarboxylic diimide (PTCDI) molecules have been designed and synthesized via facile and efficient reaction routes. Based on the side-chain modification, these molecules can be divided into both symmetric PTCDI molecules: N,N'-didodecyl-PTCDI, N,N'-di(4-(dimethylamino)phenyl)-PTCDI and N,N'-di(4-(dimethylamino)benzyl)-PTCDI, and asymmetric molecules: N-dodecyl-N'-(4-(dimethylamino)phenyl)-PTCDI and N-dodecyl-N'-(4-(dimethylamino)benzyl)-PTCDI. Herein, the modification of dodecyl group has been used to improve the solubility of PTCDI molecules which is good for their supramolecular self-assembly in solution phase. The electron-rich moieties, 4-(dimethylamino)phenyl and 4-(dimethylamino)benzyl groups can be in combination with electron-deficient PTCDI cores to form electron donor-acceptor type molecular structures of PTCDI molecules. 2. The self-assembly behavior of PTCDI molecules has been studied through facile bisolvent phase-transfer methods using chloroform as the “good” solvent and methanol as the “poor” solvent. The effect of dodecyl and/or phenylamino group modification on the nanofibil morphology of PTCDIs, intramolecular charge transfer behaviour and the photocatalytic performance of PTCDI-based composites has been discussed. Different form the side-group modification, a series one-dimensional (1D) nanofibers of PTCDIs have been fabricated. These 1D nanofibers possess thermodynamic stable π-π stacking morphologies and can be transferred from the solutions to the surfaces of solid substrates without disrupting their structures. 3. A series of novel nanocomposite structures have been fabricated by in-situ deposition of TiO2 layers and/or a co-catalyst (Pt) on 1D self-assembled nanofibers of PTCDIs, or employing molecular aggregates of PTCDI bearing electron-rich phenylamino or benylamino moiety as a sensitizer to combine with Pt/TiO2 nanoparticles (Degussa P25) or Pt/g-C3N4 via solution processing. These composites thus fabricated exhibit broader visible-light response than TiO2 or g-C3N4, and possess excellent photochemical stability. 4. Under visible-light irradiation ((λ≥ 420 nm), hydrogen production for all composite systems has been explored through photocatalytic water splitting in aqueous solutions with sacrificial electron donor methanol or triethanolamine, proving the applicability of PTCDI organic semiconductors in photocatalytic system. When 25 mg of the composite powders were suspended into aqueous solution, stable hydrogen evolution with the highest activity of ~0.035 μmol h-1 has been achieved for Pt/PTCDI nanofibers, ~0.075 μmol h-1 for PTCDI/Pt/P25, ~0.125 μmol h-1 for Pt/TiO2/PTCDI nanofibers, ~0.0375 μmol h-1 for PTCDI/Pt/g-C3N4 which is about tenfold higher than that of Pt/g-C3N4. In comparison, Pt/PTCDI nanofibers and Pt/TiO2 nanoparticles are nearly ineffective under the same photocatalytic reaction conditions. Compared to the well-defined nanofibil morphology obtained from dodecyl-substituted PTCDI molecules, donor-accepter type PTCDIs attached with electron-rich phenylamino moieties show much improved photocatalytic activity due to efficient inter- and intra-molecular charge transfer. The initial intramolecular charge transfer of PTCDI and its energy level being well matched to TiO2 or g-C3N4, strong visible-light response, excellent photochemical stability, and nanofibril morphology have been found to be the key to the effective visible-light photoactivity of PTCDI assemblies.