Materials Today Communications 4 (2015) 63–68 Contents lists available at ScienceDirect Materials Today Communications j ourna l ho me pa ge: www.elsevier.com/locate/mtcomm Photocatalytic hydrogen generation from water–methanol mixtures using “black” anatase obtained by annealing of titanate nanotubes Marco Serra, Anish Khan, Abdullah M. Asihi, Samia A. Kosa, Hermenegildo Garcia ∗ Instituto Universitario de Tecnología Química CSIC-UPV, Univ. Politécnica de Valencia, Av. De los Naranjos s/n, 46022 Valencia, Spain a r t i c l e i n f o Article history: Received 12 April 2015 Received in revised form 25 April 2015 Accepted 27 April 2015 Available online 3 June 2015 Keywords: Photocatalyst Black titania Hydrogen generation Titanate nanotubes a b s t r a c t Digestion for an extended period of a commercial nanoparticulated TiO 2 sample under strong basic conditions (10 M NaOH) leads to the formation of titanate nanotubes with long aspect ratio. The most remarkable observation was the appearance of light-brown colour in the titanate sample that is reflected in the presence for this titanate nanotube sample of a continuous absorption in the visible region decreasing in intensity towards the red. Titanate nanotubes were annealed under hydrogen atmosphere at 350 and 500 ◦ C. The last temperature leads to the reconstitution of anatase phase and an increase in the intensity of the black colour. The photocatalytic activity of the set of titanium samples was tested for hydrogen generation from water–methanol mixtures under monochromatic visible light (532 nm) or UV light (355 nm) or under simulated sunlight. It was determined that the most active sample in all condi- tions was the one that after NaOH digestion was submitted to hydrogen annealing at 500 ◦ C. This sample exhibits even higher activity than that observed for “black” titania prepared as previously reported by direct treatment of nanoparticulated TiO 2 with H 2 without NaOH digestion. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction In the context of photocatalytic hydrogen generation from water–methanol mixtures there is a continuous interest in the development of more efficient photocatalysts [1–8] TiO 2 -based materials have been the most widely used photocatalysts for this reaction [1–3,7,8]. It had been found that certain parameters including the crystal phase [9–11], the presence of dopant elements [12–17], the morphology [18,19] and structure [19] of the material exert a strong influence of the resulting photocatalytic activity. Related to our work are several reports in where titanium dioxide had been submitted to annealing under hydrogen atmosphere to obtain “black” TiO 2 nanoparticles [20]. It was found that black titania nanoparticles (loaded with 0.6 wt% Pt) exhibit enhanced photocatalytic activity for hydrogen generation from water [20]. It has been proposed that the black TiO 2 sample exhibits an enhanced photocatalytic activity due to the increased light absorption in the visible region. Characterisation of black titania suggests that thermal treatment with hydrogen produces the appearance of a black solid due to the partial amorphization of the outermost layers of the TiO 2 nanoparticle [20]. Formation of “black” TiO 2 ∗ Corresponding author. Tel.: +34 963877887. E-mail address: hgarcia@qim.upv.es (H. Garcia). nanotubes displaying visible-light ( > 450 nm) photocatalytic activity for the oxidation of acetaldehyde has also been reported [21]. In this precedent, it was found that the black colour of titania nanotubes arises from chromium contamination from the stain steel reactor in which the thermal annealing was performed [21]. Titania nanotubes annealed at various temperatures and exhibiting visible light absorption have also been used as photocatalyst for the degradation of dyes [22]. Continuing with this research aimed at developing more efficient titania photocatalysts by submitting TiO 2 to hydro- gen annealing, in the present manuscript we report that titania nanoparticles formed by thermal treatment of titania nanotubes under hydrogen exhibits an enhanced photocatalytic activity for hydrogen generation from water–methanol mixture that is even higher than the activity of an analogous sample of “black” tita- nia obtained from the direct treatment of TiO 2 nanoparticles by hydrogen. 2. Materials and methods 2.1. Sample preparation Five titanium samples (M0–M4) were used in the present study (see Scheme 1) the TiO 2 precursor (M0) used for nanotube produc- tion was a commercial TiO 2 powder (Aldrich, Anatase) consisting http://dx.doi.org/10.1016/j.mtcomm.2015.04.006 2352-4928/© 2015 Elsevier Ltd. All rights reserved.