Robustly photogenerating H2 in water using FeP/CdS catalyst under solar irradiation.

The production of chemical fuels using sunlight is an attractive and sustainable solution to global energy and environmental problems1,2. Since the 1970s, splitting water using solar energy has received much attention as a possible means for converting solar energy to chemical energy by creating clean and renewable hydrogen fuel3,4. Molecular hydrogen (H2) production using semiconductor photocatalysts is one of the most promising strategies for light-driven proton reduction5,6,7. However, most semiconductors cannot produce H2 without a co-catalyst, even in the presence of sacrificial electron donor. This is attributed to the quick recombination of electron and hole pairs while migrating to the surface, and the surface reaction being too slow to efficiently consume these charges3. Generally, to prevent the recombination of electron and hole pairs, co-catalysts (such as metals and especially noble metals) are used to serve as electron sinks and provide effective proton-reduction reaction sites8. Platinum (Pt) is the most widely used co-catalyst for the photocatalytic production of H2 from water because of its high activity and stability under the often harsh operational conditions. However, noble metals like Pt are expensive and scarce. It is therefore useful to develop high efficiency, low-cost, noble-metal-free co-catalysts to further facilitate the development of H2 photogeneration. Several new earth-abundant metal compounds have emerged and can be good candidates for co-catalysts, including MoS29,10,11,12, NiS13,14, NiSx15, CuS16, Cu(OH)217, Co(OH)218, and other related materials19. However, these co-catalysts also have the drawback of instability during the photocatalytic reaction. Very recently, metal phosphides, such as Ni2P20, CoP21,22, CuP23, MoP24, and FeP25,26,27,28 have been found to have the high electrochemical catalysis activity and good stability for the hydrogen evolution reaction (HER) in acid or alkali solutions. Transition metal phosphides, which involve the alloying of metals and phosphorus (P), have demonstrated high activity for the HER and hydrodesulfurization reactions because of their ability to reversibly bind hydrogen. However, the photocatalytic activity of these metal phosphides as the co-catalyst for H2 production has not yet been fully explored. We recently reported that a colloidal metal phosphide (Ni2P or Co2P) catalyst combined with colloidal CdS nanorod photosensitizers displayed good photocatalytic H2 evolution activity in an aqueous lactic acid solution, revealing the co-catalyst potential of metal phosphides29,30. Iron-based alternatives are especially attractive because Fe is the most abundant transition metal and its price is typically at least two orders of magnitude less than that of other highly abundant and catalytically relevant metals, including Ni and Co25. Iron phosphide (FeP) nanoparticles (NPs) as co-catalysts deposited on TiO2 have been shown to be exceptionally active for sustained H2 production in either acidic or neutral-pH aqueous solutions under UV light irradiation25. However, highly active photocatalysts composed of high-quality, iron-based nanoparticulate materials under visible light irradiation are among the most desired because of their low cost, abundance, and ease of processing. Herein we performed a noble-metal-free system of visible-light driven H2 production with the best activity and significant longevity. The system includes the semiconductor CdS and a FeP composite photocatalyst that together exhibit high activity and good photochemical stability under artificial and natural irradiation. The essence of the thermodynamic relationship between CdS and FeP is elucidated, and the mechanism of effective charge separation based on the band alignment in such system is also studied in depth. This information will be useful for providing insight for the design and preparation of efficient semiconductor-based photocatalysts.