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J. Phys. Chem. C 2008, 112, 17244–17252
Nitrogen-Doped Titanium Dioxide Active in Photocatalytic Reactions with Visible Light: A Multi-Technique Characterization of Differently Prepared Materials S. Livraghi,† M. R. Chierotti,† E. Giamello,*,† G. Magnacca,† M. C. Paganini,† G. Cappelletti,‡ and C. L. Bianchi‡ Dipartimento di Chimica IFM, NIS Centre of Excellence, and CNISM, Via Giuria 7, 10125 Torino, Italy, and Dipartimento di Chimica Fisica ed Elettrochimica, Via Golgi 19, 20133 Milano, Italy ReceiVed: April 30, 2008; ReVised Manuscript ReceiVed: August 26, 2008
Nitrogen-doped TiO2 materials were successfully prepared following three different preparation routes (sol-gel, mechanochemistry, and oxidation of TiN) and characterized by X-ray diffraction, electron microscopy, and various spectroscopic techniques. All samples absorb visible light, and the one obtained via sol-gel, showing the anatase structure, is the most active in the decomposition of organic compounds under visible light. Various nitrogen-containing species have been observed in the materials, whose presence and abundances depends on the preparative route. Ammonium NH4+ ions are residual of the synthesis using ammonium salts (sol-gel, mechanochemistry) and are quite easily eliminated, as shown by the parallel behavior of both NMR and XPS spectra. Cyanide CN- ions form at high temperature in parallel with the phase transition of the solid to rutile. Molecular nitric oxide forms in the case of materials exhibiting close porosity. The already reported bulk radical species, Nb•, is the only paramagnetic center observed in all types of samples, and is responsible for the visible light sensitization of TiO2. A mechanism for the formation of such a species in chemically prepared N-doped TiO2 materials is for the first time proposed based on the reduction of Nitric Oxide (NO) at oxygen vacancies 1. Introduction Titanium dioxide plays a fundamental role in chemical applications using light and, in particular, in photocatalysis for the remediation of pollutants, in the production of hydrogen, and in photoelectrochemical cells for the conversion of solar energy.1-6 This cheap and non-toxic material, however, suffers from an important limitation for photochemical applications as its band gap (3.2 eV for the anatase polymorph) is rather large and the solid can capture only some 3% of the total solar irradiation. In order to overcome this serious drawback, several efforts have been performed to modify the electronic structure of the material based on doping the oxide with both transition metal and nonmetal impurities. Transition metal centers in TiO2, however, may act in some cases as recombination centers for electrons and holes, thus reducing the overall activity of the photocatalyst. More recently, the attention of the researchers has been attracted by the use of p-block non-metals as dopants and, in particular, by nitrogen. The number of papers on N-doped TiO2 (N/TiO2), after the initial reports by S. Sato7 and R. Asahi et al.,8 is undergoing an exponential increase. As often occurs in the case of an explosively growing subject, a certain degree of confusion due to conflicting evidence and interpretations is present in the literature. Many reports indicate that N/TiO2 exhibits modified optical properties with the onset of absorption in the visible region of the spectrum and, in parallel, it shows catalytic activity in various reactions performed under visible light irradiation.9-20 This activity is higher than that observed, in similar conditions, for the bare oxide. However, the reason of this enhancement is not yet clear and a debate is open on the chemical nature of the doping centers, on their role on the * To whom correspondence should be addressed: Tel ++390116707574. Fax: + +390116707855. E-mail:
[email protected]. † Dipartimento di Chimica IFM, NIS Centre of Excellence, and CNISM. ‡ Dipartimento di Chimica Fisica ed Elettrochimica.
band structure modifications of the solid and, consequently, on the mechanism of photoactivation.21,22 The reasons for the confusion still present in the literature concerning N/TiO2 are essentially twofold. The former is due to the variety of synthetic methods adopted to prepare the solid, which span from the chemical synthesis (sol-gel,10,13,15,16 chemical treatment of TiO212,18 or of TiN23) to sophisticated physical methods (ion implantation24-26 and magnetron sputtering27,28). All of these different procedures in fact could lead, in principle, to materials quite different from one another.22 The second reason is due to the identification of the active nitrogen center responsible of the photoactivity, which is seldom based on speculative arguments or on the observation of a given species actually present in the system but whose effective role in photoactivity is not directly proved. In the recent past, our group has contributed to the debate with a series of studies based on the coupling of EPR (Electron Paramagnetic Resonance) with state of the art DFT theoretical calculations. These studies have shown that, for N/TiO2 materials obtained by sol-gel, several paramagnetic species are formed29,30 one of which, labeled Nb•, is indeed selectively excited by visible light with λ ) 437 nm, this wavelength corresponding to the maximum of the optical absorption in the visible.31 This species, on the basis of the comparison of the experimental spin parameters with those calculated by advanced theoretical methods,30,32 is compatible with the model of a single N species in interstitial position in the lattice of TiO2, where N lies close to an oxygen ion. The experimental magnetic parameters of the species, however, are also compatible with those expected for a N ion substituting an oxygen in the lattice.30 No other species (either paramagnetic or diamagnetic) generated by interaction of nitrogen with the titanium dioxide matrix was found to be stable by theoretical modeling.
10.1021/jp803806s CCC: $40.75 2008 American Chemical Society Published on Web 10/09/2008
N-Doped TiO2 Active in Photocatalysis In this work, we present a thorough multitechnique investigation of three types of differently prepared N/TiO2 systems obtained by sol-gel synthesis, mechanochemical treatment and TiN oxidation, respectively. Various spectroscopic techniques (XPS, IR spectroscopy, NMR, and EPR) were used in parallel with those basic techniques (XRD, Electron Microscopy, UV-vis spectroscopy) usually employed for a basic characterization of N doped TiO2 materials. Our investigation moved with a twofold aim. First of all, we intended to shade some light on the variety of N containing chemical species present in the solid some of which only, in principle, are expected to play a role in photoactivity. A second purpose was to explore the role of the preparation method in determining the final state of the material and the presence of photoactive species. For this reason, three different preparation methods were adopted which, though based on relatively simple chemical approaches, are sufficiently different from one another to potentially allow an insight into these problems. As will be shown in the content of this article, the photoactive paramagnetic Nb• species is always present in the doped materials thus confirming its role in the sensitization of TiO2. The chemical nature of the species and a reliable mechanism for its formation in materials prepared by chemical syntheses will be proposed. 2. Experimental Section 2.1. Sample Preparation. All reactants employed in this work were purchased from Aldrich and used without any further purification treatment. Three different methods were adopted to prepare N/TiO2 namely sol-gel synthesis, mechanochemical synthesis (by ball milling), and oxidation of titanium nitride (TiN). The sol-gel approach was also used to prepare both bare and N-doped TiO2. The former preparation started from the reaction of a solution of titanium (IV) isopropoxide in isopropylic alcohol (molar ratio 1: 4) with water (molar ratio between water and alcohol 1:10) performed under constant stirring at room temperature. The gel so obtained was aged overnight at room temperature to ensure the conclusion of the hydrolysis and was subsequently dried at 343 K. The dried material was heated at 773 K in air for 1 h. The same procedure was adopted to prepare nitrogen-doped TiO2, the only difference consisting in the addition of a nitrogen source in the water employed for the hydrolysis. Several nitrogen sources like NH4Cl, NH3, NH4NO3, and N2H4 have been tested to dope the solid. Various rates of calcinations of the dried material (in the range from 5 K/min to 200 K/min) were performed to individuate the most convenient method for an efficient doping. Since there are not significant qualitative differences among the materials obtained using different nitrogen sources, in the present work results obtained using NH4Cl (which gives the highest doping level) will be reported.15N containing N/TiO2 samples were also prepared using 70% 15N enriched NH4Cl and following the same procedure described above. The mechanochemical preparation was based on ball milling of bare TiO2 (prepared via sol-gel as described above) in the presence of crystals of NH4Cl. The treatment was performed in a corundum jar set with a 5-mm ball for 2 h and was followed, like in the previous case, by calcinations in air at 773 K for 1 h. In both of these preparation methods, the amount of added NH4Cl was such to obtain a nitrogen nominal contents of 5% by weight (w/w) in N/TiO2 samples. The third preparation method consisted in the oxidation of a commercial TiN powder (purity 99.8%) in air by overnight calcinations in a furnace at various temperatures in the
J. Phys. Chem. C, Vol. 112, No. 44, 2008 17245 573-1173 K range. Also in this case, for sake of brevity, the results related to the sample obtained at 873 K only will be reported. N/TiO2 samples will be labeled indicating both the crystallographic phase (A) anatase, R) rutile) and the preparation procedure (SG ) Sol-Gel, BM ) Ball Milling, ON ) Oxidized Nitride). SG-A indicates, for example, an anatase sample, prepared via sol-gel. TiO2 will indicate bare oxide prepared via sol-gel. 2.2. Sample Characterization. X-Ray Diffraction (XRD) was performed by a Philips 1830 diffractometer using a KR(Co) source and a X’Pert High-Score software for data handling and crystallites size evalulation. Scanning Electron Microscopy (SEM) was performed on a Leica Stereoscan 420 instrument. Diffuse Reflectance UV-visible spectra (DR UV-vis) were recorded by a Varian Cary 5 spectrometer using a Cary winUV/scan software. Electron Paramagnetic Resonance (EPR) spectra were run on a X-band CW-EPR Bruker EMX spectrometer equipped with a cylindrical cavity operating at 100 KHz field modulation and with a temperature controller unit. 15N-CP-MAS NMR (Cross Polarization-Magic Angle Spinning-Nuclear Magnetic Resonance) spectra were recorded on a JEOL GSE 270 spectrometer equipped with a Doty probe operating at 27.25 MHz for 15N. Powdered samples were spun at 4-5 kHz. A contact time of 4 ms, a repetition time of 10 s and a spectral width of 35 kHz were used for accumulations of 3000-4000 transients. 15N chemical shifts were referenced via the resonance of solid (NH4)2SO4 (-355.8 ppm with respect to CH3NO2). This approach was selected for its advantages (sensitivity, short H relaxation time, ubiquitous presence of H at wet solid surfaces) in comparison with the approach of high power decoupling, which requires extremely long accumulations and is conditioned by some uncertainty about the 15N relaxation time. The only potential disadvantage of CP approach is the risk of exalting the role of species having N directly bound to H. X-Ray Photoelectron Spectroscopy (XPS) was performed by a M-Probe Instrument (SSI) equipped with a monochromatic Al KR source (1486.6 eV) with a spot size of 200 × 750 µm and a pass energy of 25 eV, providing a resolution for 0.74 eV. With a monochromatic source, an electron flood gun was used to compensate for the build-up of positive charge on the insulator samples during the analysis. FT-IR (Fourier Transform Infra-Red) spectra were obtained at 4 cm-1 resolution by a Bruker IFS 88 spectrophotometer equipped with a MCT cryodetector. The samples were inspected in the form of thin-layer deposited on a Si wafer of 20 mg/cm2 in a FT-IR cell connected to a conventional high-vacuum apparatus (residual pressure
