Short communication Improved photo-stability of silicon nanobelt arrays by atomic layer deposition for efficient photocatalytic hydrogen evolution Xiao-Qing Bao, Lifeng Liu * International Iberian Nanotechnology Laboratory (INL), 4715-330 Braga, Portugal highlights  Conformal and pinhole-free protection layer over nanobelt array photocathodes.  Photocurrent loss is drastically suppressed after depositing a layer of titania using ALD.  Si nanobelt arrays with a 3 nm titania coating can operate more than 12 h without marked performance degradation. article info Article history: Received 18 March 2014 Received in revised form 16 June 2014 Accepted 18 June 2014 Available online 25 June 2014 Keywords: Silicon nanobelt Atomic layer deposition Solar hydrogen evolution Photo-stability abstract Silicon nanostructures have recently drawn great interest for use as photocathodes to produce hydrogen through water photoelectrolysis. Despite the high photocurrent observed, nanostructured Si photo- cathodes usually exhibit poor photo-stability in aqueous solution and rapidly deactivate. Herein, we report that by coating a thin titania protection layer using atomic layer deposition (ALD), the photo- stability of silicon nanobelt arrays fabricated by metal assisted chemical etching can be markedly improved. The photocurrent loss of the silicon nanobelt array photoelectrode coated with a 3 nm titania layer is found to be much lower than that of the electrode without a titania coating. We also demonstrate that the 3 nm titania coated Si nanobelt arrays can sustain more than twelve hours without a significant loss in photocurrent under operation conditions before it eventually fails. The possible failure mechanism is preliminarily investigated. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Solar energy has been thought to have potential to provide sufficient carbon-free energy for the sustainable development of our society [1]. However, due to its intermittent nature, the har- vested solar energy must be immediately used or stored in a sec- ondary device for use upon demand. While different battery technologies have presently been employed to store energy in solar power plants, directly converting solar energy into storable chemical fuels such as hydrogen (H 2 ) based on water electrolysis represents a potentially more cost-effective and practical approach to solar energy storage, and has attracted tremendous interest in recent years [2,3]. So far, a number of semiconducting materials such as p-type silicon (p-Si) [4e7], Cu 2 O [8], InP [9e11], GaP [12] and WSe 2 [13], have been investigated as photocathodes for solar-driven photocatalytic H 2 production, among which p-Si is of great interest because Si is earth abundant and has a small band gap allowing for broad absorption of sunlight; moreover, the production and pro- cessing techniques of Si-based materials are well matured and compatible with the current microelectronics. A major limitation for Si being used as an electrode for photocatalytic water splitting is that Si is very easy to oxidize in aqueous solution, even in acidic medium with the presence of a cathodic current, thereby often quickly deactivates upon operation due to the passivation of the formed insulating SiO x layer [14]. It is believed that a chemically inert yet electrically conductive protection layer intimately coated on the Si photocathode surface would effectively prevent Si from photo-oxidization and meanwhile allow the photo-generated electrons to pass through the protection layer to reduce water. Bearing this in mind, some attempts have been made very recently [14e18], and it was reported that the photo-stability of Si photo- cathodes can be remarkably improved with a protective coating layer. For instance, Seger and co-workers used sputtered Ti and TiO 2 as protection layers, and achieved stable photocurrents for many hours [14,15]. With a thin titania layer grown by atomic layer * Corresponding author. Tel.: þ351 253 140112; fax: þ351 253 140119. E-mail addresses: lifeng.liu@inl.int, caslfliu@hotmail.com (L.F. Liu). Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour http://dx.doi.org/10.1016/j.jpowsour.2014.06.098 0378-7753/© 2014 Elsevier B.V. All rights reserved. Journal of Power Sources 268 (2014) 677e682