High UV/visible light activity of mixed phase titania: A generic mechanism Ranjith.G. Nair, Samrat Paul, S.K. Samdarshi n Solar and Energy Materials laboratory, Department of Energy, Tezpur University, Tezpur, Assam 784028, India article info Article history: Received 11 December 2010 Accepted 19 February 2011 Available online 12 March 2011 Keywords: Sol–gel Anatase Rutile Mixed phase Photocatalysis abstract Synergistic effect of the mixed phase in titania photocatalyst on its performance compared to the pristine phases has been investigated in terms of the bulk and interfacial behavior of the phases in contact. The experiments were conducted under both UV and visible light irradiations. The photo- activity variation has been correlated with the changes in the ratio of anatase to rutile phases (A/R ratio), and their unique response to UV and visible radiations. For this, a set of pure (rutile or anatase) and mixed phases (with varying A/R ratio) titania nanoparticles were synthesized. The physico- chemical characterization was done using SEM, XRD, EDAX, UV-DRS, PL and FTIR analyses. The activity of catalysts in UV and visible light was investigated by monitoring the degradation of phenol. The results show that the mixed phase catalysts show enhanced photoactivity compared to pristine phases across the irradiation wavelength range. Further, the catalysts having a narrow range of high A/R ratio ( 41) around 5.0 show high UV activity while those having low A/R ratio ( o1) around 0.5 show high visible light activity. A mechanism is proposed based on the influence of interfacial phenomena under both UV and visible light irradiations. It explains the differences observed in the behavior of the catalyst irradiated with UV and visible light and also the high activity of mixed phase catalysts compared to the pristine phases across the wavelength ranges. & 2011 Elsevier B.V. All rights reserved. 1. Introduction Synergy of the phases in a mixed phase titania on their perfor- mance as photocatalyst has been investigated by many research- ers [1–10]. The investigations were mainly focused on studying and correlating the physico-chemical characteristics of mixed phase photocatalysts with their photoactivity under either UV or visible light irradiation. The enhanced activity of mixed phases compared to its pristine phases has been explained through different models explaining the causality relationships [2, 3,6, 7]. Out of these models, two models that have gained popularity are referred to as ‘‘rutile sink’’ model and ‘‘rutile antenna’’ model [2]. The approach in these models is based on the investigation of the relative position of conduction band edges in two phases in intimate contact and the existence of trap levels in the bulk. In ‘‘rutile sink’’ model proposed by Bickley et al. [3] the anatase phase is considered to be the collector of incident photons and rutile as the sink for photo-excited electrons diffusing into the rutile region. The photo-excited electrons go to the conduction band edge of rutile phase, which is at a lower energy level compared to anatase. It must be noted that ‘‘rutile sink’’ model was based on the experiments conducted under UV light illumination. Consequently, the role of rutile region in contributing charge carriers was apparently ignored because of high level of recombination of photogenerated energetic carriers in the region under UV illumina- tion. This model did not consider activity of mixed phase under visible light. This was followed by a work by Hurum et al. [2], wherein they proposed a model called ‘‘rutile antenna’’ model. The small visible light activity exhibited by mixed phase titania may be explained by considering rutile as a collector of incident photons because of its smaller band gap. Also it needs a lower energy level on the anatase side for enabling the photo- generated electrons on the conduction band edge of the rutile side to cross over to anatase side. Hurum et al. [2], through a series of experimental studies conducted under visible light irradiation, established the existence of a large number of electron trapping sites in the anatase side. This was supported by the Time-resolved Photo-acoustic Spectroscopic (TRPAS) study by Leytner and Hupp [11] which exhibited the existence of electron trapping sites 0.8 eV below the conduction band edge of anatase phase. Here the role of interface becomes very important. The electron will not move to the other region unless it is energetically favored and more probable. It is to be noted that the behavior of the interfacial quantized levels of the anatase and rutile phases under UV and visible light irradiations will be different due to their band gap difference. Anatase shows activity only under UV while rutile may be activated by visible light also. In the present study, titania was synthesized employing sol–gel technique. Thereafter, pristine and mixed crystalline phases Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2011.02.017 n Corresponding author. Fax: þ91 3712 267005/6. E-mail address: drsksamdarshi@rediffmail.com (S.K. Samdarshi). Solar Energy Materials & Solar Cells 95 (2011) 1901–1907