Synergistic Effect of a Molecular Cocatalyst and a Heterojunction in a 1 D Semiconductor Photocatalyst for Robust and Highly Efficient Solar Hydrogen Production Daochuan Jiang + , Rana Muhammad Irfan + , Zijun Sun, Dapeng Lu, and Pingwu Du* [a] Introduction Photocatalytic water splitting for the production of hydrogen (H 2 ) is a promising pathway for the direct conversion of solar energy to chemical energy, which could help meet the world’s future energy requirements. [1–4] Three main steps are involved in a typical photocatalytic reaction system: [5, 6] (1) Under light ir- radiation, the semiconductor/photosensitizer absorbs photons with sufficient energy to generate electron–hole pairs as charge carriers; (2) The charge carriers then diffuse from the inner semiconductor to the near-surface region for charge sep- aration; (3) Electrons/holes drive the catalytic redox reactions of the absorbed molecules on the surface active sites. The quantum efficiency in a photocatalytic H 2 production system is generally low because a significant number of the photoexcit- ed charge carriers recombine radiatively or nonradiatively in the second step. Therefore, precise control of the charge trans- port to suppress charge recombination and improve the pho- tocatalytic efficiency for the production of H 2 is highly de- sired. [7–13] In the past few decades, various approaches have been developed for this purpose. One attractive approach is to functionalize the semiconductor/photosensitizer with a cocata- lyst, which provides trapping sites for the photogenerated charge carriers and promotes charge separation. [6, 14–17] The widely studied cocatalysts include both noble-metal-based ma- terials (e.g., Pt, Pd, Ru) [14, 18–20] and noble-metal-free materials (e.g., Ni, Co, Cu). [4, 21–23] Another strategy is to couple one semi- conductor with other semiconductors that have suitable band gaps to form a heterojunction, which can facilitate charge transfer and separation. The typical semiconductor composites used to enhance charge separation include CdS/TiO 2 , [24, 25] CdS/ Cu 2 O, [26] CdS/g-C 3 N 4 , [27, 28] CdS/ZnO, [29, 30] and ZnS/CdS/Cu 2 x S. [31] However, to date, there has been no report of a suitable effi- cient, stable, and low-cost system for photocatalytic produc- tion of H 2 by combining these two strategies, that is, using a molecular cocatalyst and a semiconductor heterojunction. In this present study, we report a novel and efficient photo- catalytic system for the production of H 2 by incorporating core–shell CdS/ZnS nanorod (NR) heterojunctions and a water- soluble nickel–salen complex cocatalyst as the photosensitizer. The more efficient charge separation can boost the activity of the photocatalytic production. Under optimal conditions, this system shows impressive H 2 production activity under visible light irradiation (l > 420 nm) with an average apparent quan- tum yield of 58.3 % at 420 nm. Results and Discussion Design principle The design principle is illustrated in Figure 1. Core–shell CdS/ ZnS heterojunction material was employed as the photosensi- tizer instead of only CdS because the CdS/ZnS heterojunction Photocatalytic production of hydrogen by water splitting is a promising pathway for the conversion of solar energy into chemical energy. However, the photocatalytic conversion effi- ciency is often limited by the sluggish transfer of the photo- generated charge carriers, charge recombination, and subse- quent slow catalytic reactions. Herein, we report a highly active noble-metal-free photocatalytic system for hydrogen production in water. The system contains a water-soluble nickel complex as a molecular cocatalyst and zinc sulfide on 1D cadmium sulfide as the heterojunction photocatalyst. The complex can efficiently transport photogenerated electrons and holes over a heterojunction photocatalyst to hamper charge recombination, leading to highly improved catalytic ef- ficiency and durability of a heterojunction photocatalyst– molecular cocatalyst system. The results show that under opti- mal conditions, the average apparent quantum yield was ap- proximately 58.3 % after 7 h of irradiation with monochromatic 420 nm light. In contrast, the value is only 16.8 % if the molec- ular cocatalyst is absent. Such a remarkable performance in a molecular cocatalyst-based photocatalytic system without any noble metal loading has, to the best of our knowledge, not been reported to date. [a] D. Jiang, + R. M. Irfan, + Z. Sun, D. Lu, Prof.Dr. P. Du Key Laboratory of Materials for Energy Conversion, Chinese Academy of Sciences, Department of Materials Science and Engineering, iChEM (Collab- orative Innovation Center of Chemistry for Energy Materials) University of Science and Technology of China 96 Jinzhai Road 230026 HefeiAnhui Province (P.R. China) E-mail : dupingwu@ustc.edu.cn [ + ] These authors contributed equally to this work. Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/ cssc.201600871. ChemSusChem 2016, 9, 1 – 10  2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 & These are not the final page numbers! ÞÞ These are not the final page numbers! ÞÞ Full Papers DOI: 10.1002/cssc.201600871