Effect of Mo-Incorporation in the TiO 2 Lattice: A Mechanistic Basis for Photocatalytic Dye Degradation Kaustava Bhattacharyya,* ,† Jerina Majeed, † Krishna Kishore Dey, ‡ Pushan Ayyub, § A. K. Tyagi, † and S. R. Bharadwaj † † Chemistry Division, Bhabha Atomic Research Centre, Mumbai 40085, India ‡ Center of Biomedical Magnetic Resonance, Lucknow 226014, India § Tata Institute of Fundamental Research, Mumbai 400005, India * S Supporting Information ABSTRACT: Photocatalytic activity of TiO 2 (anatase) is appreciably enhanced by substitutional doping of Mo in anatase lattice, in conjunction with the incorporation of nanostructured MoO 3 within the parent anatase lattice. The photocatalyst material was characterized in detail using X-ray diffraction, Raman spectroscopy, diffuse reflectance (DR-UV− Vis spectroscopy), X-ray photoelectron spectroscopy, and electron microscopy. Photocatalysis experiments were con- ducted using a model rhodamine-B (Rh−B) dye reaction using both UV and visible irradiation sources. The observed trends in the case of visible irradiative source can be summarized as follows: Mo-1 < Mo-2 < Mo-5 ≫ Mo-10. Attempts were made to isolate the structural factors that control photochemical behavior of these Mo−TiO 2 photocatalysts and to correlate photocatalytic activity with different structural aspects like oxidation state, band gap, surface species, etc. Mechanistic insights were acquired from ex situ 1 H NMR studies showing different intermediates and different probable routes for the Rh−B dye degradation with UV and visible radiations. The stable intermediates were formed by a direct oxidative fragmentation route, without any evidence of the initial deethylation route. The intermediates found were benzoic acid, different amines, diols, and certain acids (mostly formic and acetic acid). The adsorption of the Rh−B dye on the catalytic surface via the N-charge centers of the Rh−B was also observed. 1. INTRODUCTION Anatase, one of the many polymorphs of the TiO 2 family, is regarded as one of the most efficient photocatalysts for the redox reactions in both the gas, solid, and liquid−solid interphases. The detoxification of organically polluted waste- water primarily involves liquid−solid interphase reactions. Since anatase TiO 2 possesses an indirect band gap of ∼3.2 eV, it acts as a photocatalyst which primarily absorbs in the UV region. One of the common routes employed to shift its absorbance toward the visible range is to dope the cationic site with different transition metal ions. Doping not only affects the band gap of the material but also leads to change in the oxidation state as well as structural parameters. It induces alterations in the redox potential, which plays a primary role in the photocatalytic activity of these doped materials. 1−3 An effective increase in lifetime of the e − /h + may augment the rate kinetics of the photochemical reactions. The dopants assist this strategy either by quenching one of the e − /h + pair, thus effectively increasing the lifetime of the other or by creating requisite defect sites which effectively facilitates the photo- chemical activity. 4 On the other hand, dopants can lead to the formation of surface defect sites which trigger certain adsorption/reaction centers effective in catalysis. 5−7 Mo can assume multiple oxidation states, and the ionic radius of Mo 6+ (0.62 Å) allows it to substitute for Ti 4+ in TiO 2 (0.65 Å). The Mo and W usually impart stability to the anatase TiO 2 phase. 8 Earlier reports of the Mo-doped TiO 2 system include (a) structure−activity link of anatase V, Mo, Nb, and W-TiO 2 mixed oxides for toluene photooxidation under sunlight-type excitation, 9 (b) probable role of Mo catalyst in quenching Rh− B, excited state leading to extended degradation of the dye, through a secondary photochemical process beyond simple deethylation, 10 (c) Mo 6+ incorporation in the anatase structure produces a red shift in the absorption edge and lowers the interfacial charge transfer dynamics, 11 (d) simultaneous cationic and anionic doping in TiO 2 with Mo and N respectively, 12 (e) Mo-doped anatase prepared by thermal hydrolysis of the peroxo−titanium complex showing the presence of Mo 5+ /Mo 6+ Received: June 3, 2014 Revised: June 26, 2014 Published: June 26, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 15946 dx.doi.org/10.1021/jp5054666 | J. Phys. Chem. C 2014, 118, 15946−15962