We report on the first use of nanostructured d-FeOOH as a prom-ising photocatalyst for hydrogen production. The high surface area, interparticle mesoporosity, small particle size and band gap energy in the visible region make nanostructured d-FeOOH a suitable candidate for use as a photocatalyst. Photocatalytic splitting of water is a technique for a clean, low-cost and environmentally friendly production of hydrogen using (amply available) sunlight. Since its discovery by Fujishima and Honda, 1 more than 150 new photocatalysts including TiO 2 , 1,2 metal oxides, 3 metal sulphides, 4 oxysulfides 5 and oxynitrides 6 have been synthesized. However, the rapid recombination of photogenerated electrons and holes and the fast backward reaction of H 2 and O 2 limit the efficiency of most of these photocatalysts for hydrogen production. 7 Further-more, several of the materials currently used for water splitting are difficult to synthesize and consequently expensive to produce, have large band gaps and low stabilities in aqueous solution. Iron oxides, on the other hand, are good alternatives for direct water splitting under sunlight because of their small band gap, stability in solution, non-toxicity, cheap production and ample material availability, iron being one of the most abundant elements on Earth. Here we present a first report on the use of nanostructured d-FeOOH as a promising photocatalyst for hydrogen production. d-FeOOH is a polymorph of several known iron oxyhydroxides with a structure that is based on a hexagonal closed-packed oxygen lattice similar to that of hematite (a-Fe 2 O 3) with iron occupying half of the available octahedral interstices. 8,9 d-FeOOH presents many advan-tages for splitting water because (i) its band gap (E g ¼ 2.2 eV) is small enough to absorb visible light and large enough to promote water splitting, (ii) the high surface area (BET z 400 m 2 g À1) results in good contact between catalyst and water, thus increasing the efficiency of hydrogen production, (iii) its nanoparticles can be easily synthesized, (iv) the synthesis is cheap, (v) the nanoparticles can be highly dispersed in water, thereby increasing the photocatalytic efficiency, (vi) the fast electron/hole recombination probability is decreased due to the small particle size of our material. The synthesis of d-FeOOH was carried out by precipitation of an Fe 2+ solution with NaOH. H 2 O 2 was added immediately after precipitation to form d-FeOOH. The precipitate was washed with distilled water, dried and characterized by TEM, X-ray powder diffraction (XRD), 57 Fe M€ ossbauer spectroscopy at 20 K, diffuse reflectance and N 2 adsorption–desorption measurements. Theoretical calculations were performed using the ADF method. The release of H 2 and O 2 was monitored by chronoamperometric measurements under a 15 W l < 300 nm UV lamp and sunlight. More details are given in the ESI. Fig. 1 shows the morphology, crystalline structure and particle size distribution of our d-FeOOH sample. In Fig. 1A, the formation of Fig. 1 (A) TEM image, (B) HRTEM image and (C) histogram of size distribution of d-FeOOH nanoparticles.