Noble metal-modified TiO 2 thin film photocatalyst on porous steel fiber support Hongfan Guo *, Marianna Kemell, Mikko Heikkila ¨ , Markku Leskela ¨ Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, Helsinki FI-00014, Finland 1. Introduction TiO 2 is a very promising photocatalyst due to its strong oxidation capacity, high photochemical and biological stability, non-toxicity and low cost. Both powdery and thin film TiO 2 can be used as photocatalyst, but the later is often preferred because there are several problems with the powdery TiO 2 [1–4]: (i) difficulty and high cost in the separation and recovery of the catalyst from suspension; (ii) easy aggregation of the suspended particles; (iii) difficulty in application to continuous flow systems. These problems can be easily solved by immobilizing TiO 2 onto inert support [5]. Moreover, TiO 2 film is very convenient for many special practical applications, such as in hospitals and hotels to kill viruses and bacteria [6–8]. Hence, more efforts are still needed for fixing TiO 2 onto proper supports with suitable immobilization technology. Several materials have been extensively studied as the effective support of TiO 2 , e.g., glass, silica, quartz, clay, ceramics, zeolites. Porous metal fiber material made of metal skeleton and pores/ voids [9] (see Fig. 1a and b) is a type of structural and functional material. Due to possessing many properties of both metal and porous material, it can be used in separation, filtration, gas distribution, heat exchangers, electromagnetic-wave shield, etc [10,11]. Therefore, and because of its large specific surface area and high self-support strength, it deserves attention and study as the structural and functional support of TiO 2 film photocatalyst. There are several optional ordinary methods for immobilizing TiO 2 onto different supports, such as sol–gel method, physical vapor deposition, chemical vapor deposition, liquid phase deposi- tion and doctor blade, however, they all have some limitations [12– 15] in preparing conformal and uniform TiO 2 film on above- mentioned porous metal fiber material to maintain the porosity of the support. Atomic layer deposition (ALD) is a special layer-by- layer chemical gas phase deposition technique capable of coating extremely complex shapes with a conformal material layer of high quality [16–20]. Its special self-limiting growth mechanism, i.e., deposited material is produced by the precursors saturatively chemisorbed on support surface as monolayer, ensures that material is deposited using all the support surface as the template and grows by one layer per cycle. Thus, ALD is an ideal method for preparing TiO 2 coating on such kind of porous material. We have developed an ALD reactor configuration [13] for rapid and uniform coating of through-porous materials. In this configuration, the precursors are forced to flow through the substrate to fast disperse to all the surface of the substrate. In this paper, a process for preparing TiO 2 film photocatalyst is described. In this flow, TiO 2 is first coated on porous steel fiber support using the above-mentioned ALD reactor configuration. Due to the uniform and conformal deposition of TiO 2 only on the surface of the metal skeleton, many properties of porous steel fiber support, such as porosity and gas permeability, are maintained. It is well-known that the photocatalytic activity of TiO 2 originates from the highly reactive electron–hole pairs generated by absorbing incident photons, but these photogenerated electrons and positively charged holes often suffer from fast recombination resulting in decrease of the quantum yield [21–25]. Loading noble Applied Catalysis B: Environmental 95 (2010) 358–364 ARTICLE INFO Article history: Received 1 September 2009 Received in revised form 14 January 2010 Accepted 18 January 2010 Available online 25 January 2010 Keywords: Porous steel fiber Atomic layer deposition Sputtering TiO 2 Photocatalysis ABSTRACT Through-porous steel fiber matrix with high specific surface area and self-support strength was examined as the support of TiO 2 film photocatalyst. TiO 2 was uniformly and conformally deposited onto the surface of the metal skeleton of the porous steel fiber matrix via atomic layer deposition method so that the porosity as well as gas permeability of the matrix is maintained. For further improving the photocatalytic activity, noble metals (Au and Pt/Pd alloy) were sputtered onto the surface of TiO 2 photocatalyst. The photodegradation of methyl orange was used for evaluating the photocatalytic properties. The results show that compared with TiO 2 films deposited on flat Si wafers, TiO 2 films on the porous support display higher photocatalytic activities, owing to their higher specific surface areas. The photocatalytic activity of TiO 2 on porous support was not enhanced by Au, while it was remarkably improved by Pt/Pd. The TiO 2 photocatalysts were also analyzed by scanning electron microscopy and X- ray photoelectron spectroscopy. ß 2010 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +358 919150229; fax: +358 919150198. E-mail address: Hongfan.Guo@helsinki.fi (H. Guo). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.01.014