FULL PAPER © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com first report of the electrochemical water- splitting on a TiO 2 electrode by Fujishima and Honda, [3] semiconductor photocatal- ysis has become an intriguing approach for the economical and eco-friendly production of hydrogen by using solar energy. This process involves generation of electron–hole pairs in a semiconductor material upon light irradiation and suc- cessful separation and transportation of these charge carriers to the surface active sites, where they can participate in chem- ical reactions. Over the last few years, numerous efforts have been devoted to the development of highly active catalysts for the photocatalytic splitting of water by the hydrogen evolution reaction (HER). [4] However, most of these catalysts are com- posed of wide bandgap semiconductors (like SrTiO 3 , ZnO, K 4 Nb 6 O 17 , and Ta 2 O 5 ) that take advantage of UV light, which constitutes only 4% of the solar spectrum. This is a limitation that restricts their practical application for solar hydrogen production. [4b] Recently, metal chalcogenides have garnered special attention as electro- or photocatalysts for water splitting owing to their remarkable optical and electronic properties. To this end, a variety of tran- sition-metal sulfides (MoS 2 , EMoS x (E = Fe, Co), Cu x Zn 1–x S, etc.) have been rapidly investigated, [5] and among all, CdS is the most extensively used for the photocatalytic reduction of water to hydrogen. [6] The interest in CdS stems from its narrow bandgap (E g ≈ 2.4 eV), which enables the absorption of visible light, high electron mobility (>350 cm 2 V −1 s −1 ) and a favorable conduction band (CB) edge position well above the thermody- namic threshold for water reduction reaction (−0.41 V vs NHE at pH = 7). [6a] However, its hydrogen evolution activity is often plagued by the slow transfer of surface-reaching holes to elec- trolytes and poor electron–hole separation yield. Therefore, the main challenge in designing effective CdS-based photocata- lysts is to eliminate the competitive process of charge carrier recombination. [7] Previous efforts to increase the lifetime of photogenerated carriers in CdS materials mainly focused on the deposition of metal nanoparticles, especially noble metals such as Au, Pt, Rd, and Ag as co-catalysts. [8] These metal nano- particles have been considered as effective electron acceptors, Size Effects of Platinum Nanoparticles in the Photocatalytic Hydrogen Production Over 3D Mesoporous Networks of CdS and Pt Nanojunctions Ioannis Vamvasakis, Bin Liu, and Gerasimos S. Armatas* Catalysts for the photogeneration of hydrogen from water are key for realizing solar energy conversion. Despite tremendous efforts, developing hydrogen evolution catalysts with high activity and long-term stability remains a daunting challenge. Herein, the design and fabrication of mesoporous Pt-decorated CdS nanocrystal assemblies (NCAs) are reported, and their excellent performance for the photocatalytic hydrogen production is demon- strated. These materials comprise varying particle size of Pt (ranging from 1.8 to 3.3 nm) and exhibit 3D nanoscale pore structure within the assembled network. Photocatalytic measurements coupled with UV–vis/NIR optical absorption, photoluminescence, and electrochemical impedance spectros- copy studies suggest that the performance enhancement of these catalytic systems arises from the efficient hole transport at the CdS/electrolyte interface and interparticle Pt/CdS electron-transfer process as a result of the deposition of Pt. It is found that the Pt-CdS NCAs catalyst at 5 wt% Pt loading content exerts a 1.2 mmol h −1 H 2 -evolution rate under visible-light irradiation (λ ≥ 420 nm) with an apparent quantum yield of over 70% at wavelength λ = 420 nm in alkaline solution (5 M NaOH), using ethanol (10% v/v) as sacri- ficial agent. This activity far exceeds those of the single CdS and binary noble metal/CdS systems, demonstrating the potential for practical photocatalytic hydrogen production. DOI: 10.1002/adfm.201603292 I. Vamvasakis, Prof. G. S. Armatas Department of Materials Science and Technology University of Crete Vassilika Vouton, Heraklion 71003, Greece E-mail: garmatas@materials.uoc.gr Prof. B. Liu School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459, Singapore 1. Introduction Hydrogen, an alternative and environmentally-friendly energy carrier, has attracted broad attention in recent years as a poten- tial solution to the global energy demands and environmental pollution. [1] At present, hydrogen gas is predominantly pro- duced by reforming fossil fuels such as petroleum and natural gas or using high-energy consumption processes—all of which are environmentally unfriendly and cost-expensive. [2] Since the Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201603292 www.afm-journal.de www.MaterialsViews.com