In-situ plasma hydrogenated TiO 2 thin ﬁlms for enhanced photoelectrochemical properties Aadesh P. Singh a , Nisha Kodan a , Bodh R. Mehta a, *, Avishek Dey b , Satheesh Krishnamurthy b a Thin Film Laboratory, Department of Physics, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India b Materials Engineering, The Open University, Milton Keynes, MK7 6AA, United Kingdom A R T I C L E I N F O Article history: Received 31 July 2015 Received in revised form 23 November 2015 Accepted 14 December 2015 Available online 29 December 2015 Keywords: Thin ﬁlms Oxides Sputtering Catalytic properties Optical properties A B S T R A C T In this paper, we report the effect of in-situ plasma hydrogenation of TiO 2 (iH:TiO 2 ) thin ﬁlms by the incorporation of known amount of hydrogen in the Ar plasma during rf-sputter deposition of TiO 2 ﬁlms. As compared to pristine TiO 2 ﬁlms (0.43 mA/cm2 at 0.23 V vs Ag/AgCl), hydrogenated TiO 2 showed enhanced photoelectrochemical activity in terms of improved photocurrent density of 1.08 mA/cm2 (at 0.23 V vs Ag/AgCl). These results are explained in terms of reduction in band gap energy, shift in valence band maximum away from the Fermi level, improved donor density and more negative ﬂat band potential in iH:TiO 2 sample. The presence of Ti 2+ states in iH:TiO 2 ﬁlms in addition to Ti 3+ states in pristine TiO 2 act as additional electronic states in the TiO 2 band gap and increases the optical absorption in the visible region. This method of in-situ hydrogenation can be used as a general method for improving the properties of metal oxide thin ﬁlms for photoelectrochemical and photocatalytic applications. ã 2015 Elsevier Ltd. All rights reserved. 1. Introduction The direct synthesis of chemical fuels like hydrogen by photoconversion processes can solve the practical challenge of producing clean and sustainable energy. However, for efﬁcient solar energy conversion to chemical fuel, a unique combination of material properties has to be satisﬁed. The most important properties are: (i) suitable band gap and high absorption coefﬁcient for sunlight, (ii) alignment of conduction and valence band edges with respect to water redox potentials, (iii) long life- time performance and (iv) earth abundance and low production costs [1–3]. Compared to conventional semiconductor systems, photocatalysts made of most stable metal oxide semiconductors offer relatively higher stabilities and allow processing method compatible with industrial production technologies as well as easy and scalable synthesis. Titanium dioxide (TiO 2 ), a n-type transition metal oxide, has attracted considerable attention in the ﬁeld of photoelectrochem- ical (PEC) water splitting due to its stability against photo—and chemical-corrosion and a suitable position of valence and conduction band edges with respect to the redox potential of water [4–7]. However, its large band gap (3.2 eV) limits the overall efﬁciency in PEC cell. In order to reduce the band gap of TiO 2 , a variety of material modiﬁcation techniques such as metal ion doping [8,9], anion doping [10,11], noble metal loading [12,13], addition of electron donors [14], metal ion-implantation [15], and self-doping that produces Ti 3+ species [16] etc have been used. However, the improvement in optical absorption is normally reported to be accompanied by an increased charge carrier recombination which decreases the overall efﬁciency of PEC device [17]. In some studies, nitrogen doping in TiO 2 has shown improved optical absorption due to electronic transition from the dopant N2p level to Ti3d energy band [10,18], but the overall conversion efﬁciency in visible light region is still far from satisfactory. Recent reports on improved photocatalytic activity of TiO 2 nanocrystals treated in molecular/atomic hydrogen atmosphere at certain temperature has opened up new possibility for tuning long wavelength optical absorption [19,20]. Hydrogen treatment of oxide thin ﬁlms has several other applications such as in photocatalysis [21–23], photoelectrochemical sensor [24], lithi- um-ion battery [25,26], supercapacitor [27], fuel cell [28], ﬁeld emission [29], and microwave absorption [30]. Prior to this study Hydrogen treatment of oxide thin ﬁlms has been carried out by exposing as-deposited ﬁlms to post-deposited electrochemical hydrogenation [31,32], hydrogen plasma [33,34], and heating of nanoparticles in hydrogen atmosphere [35] etc. * Corresponding author. Fax: +91 11 26581114. E-mail address: brmehta@physics.iitd.ac.in (B.R. Mehta). http://dx.doi.org/10.1016/j.materresbull.2015.12.015 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved. Materials Research Bulletin 76 (2016) 284–291 Contents lists available at ScienceDirect Materials Research Bulletin journa l homepage: www.elsevier.com/locate/matresbu