Synthesis, Characterization, and Photodegradation Behavior of Single

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Synthesis, Characterization, and Photodegradation Behavior of Single-Phase Anatase TiO2 Materials Synthesized from Ti-oxychloride Precursors Gerardo Coloń,† Patricia Sampedro,‡ Marcos Fernańdez-Garcıá,*,‡ Haiyan Chen,§ Jonathan C. Hanson,§ and Jose A. Rodriguez*,§ Instituto de Ciencia de Materiales de SeVilla, Centro Mixto CSIC-UniVersidad de SeVilla, C/Ame´rico Vespucio 49, 41092-SeVilla, Spain, Instituto de Cata´lisis y Petroleoquı´mica, CSIC, Campus Cantoblanco, 28049 Madrid, Spain, and Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed May 6, 2008. ReVised Manuscript ReceiVed June 26, 2008 Single-phase anatase-TiO2 nanomaterials with a size of ca. 10 nm and variable quantities of anion impurities were prepared using a novel pathway based on the use of amorphous ammonium Ti-oxychloride precursors synthesized using Ti/Cl initial ratios between 1 and 6. The precursor nature and evolution under thermal treatment were studied using chemical analysis, XRD, XPS, DRIFTS, and mass spectrometry. The nature and concentration of anatase-TiO2 materials anion impurities were analyzed by XPS and DRIFTS. It is shown that negatively charged impurities located in substitutional positions of the anatase network are maximized for a sample synthesized using a Ti/Cl 1:1 atomic ratio and are responsible for the elimination of liquid-phase (phenol) and gas-phase (isopropanol or methylcyclohexane) pollutants under sunlight excitation. A link is established among the initial chemical characterization of the precursors, the final morphological, structural, and chemical composition of the oxide materials, and their photochemical properties.

1. Introduction Titanium dioxide (TiO2) is one of the most prominent oxide materials for performing various kinds of industrial applications related to catalysis among which the selective reduction of NOx in stationary sources1,2 and photocatalysis for pollutant elimination3 or organic synthesis4 are rather important. Additional applications include its use as a white pigment in paint,5 as part of photovoltaic devices6 and sensors,7 as a food additive,8 in cosmetic,s9 and as a potential tool in cancer treatment.10 Experimental synthetic approaches to scale down the TiO2 primary particle size to the nanometer scale are now being actively investigated in order to improve its current applications in sensor and catalysis fields. Anatase, rutile, and brookite are the most common polymorphs of titania (TiO2). Anatase is the dominant outcome of the vast majority of liquid-solid and gas-solid transformation-based preparation methods.11,12 This is a consequence of being the stable polymorph at working temperatures * Corresponding authors. E-mail: [email protected] (M.F.-G.); rodrigez@ bnl.gov (J.A.R.). † Centro Mixto CSIC-Universidad de Sevilla. ‡ Instituto de Cata´lisis y Petroleoquı´mica, CSIC. § Brookhaven National Laboratory.

(1) Bosh, H.; Janssen, F. Catal. Today 1988, 2, 369. (2) Forzatti, P. Catal. Today 2000, 62, 51. (3) (A) Hoffman, M. R.; Martin, S. T.; Choi, W.; Bahneman, D. W. Chem. ReV 1995, 95, 69. (B) Agrios, A. G.; Pichat, P. J. Appl. Electrochem. 2005, 35, 655. (4) Maldoti, A.; Molinari, A.; Amadeni, R. Chem. ReV. 2002, 102, 3811. (5) Johnson, R. W.; Thieles, E. S.; French, R. H. Tappi. J. 1997, 80, 233. (6) Kalyanasendevan, K.; Gratzel, M. In Optoelectronics Properties of Inorganic Compounds;. Roundhill, D. M., Fackler, J. P., Eds.; Plenum: New York, 1999; pp 169-194. (7) Sheveglieri, G., Ed.; Gas Sensors; Kluwer: Dordrecht, The Netherlands, 1992. (8) Phillips, L. G.; Barbeno, D. M. J. Dairy Sci. 1997, 80, 2726. (9) Selhofer, H. Vacuum Thin Films 1999, 15 August. (10) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C: Photochem. ReV. 2000, 1, 1. (11) Synthesis, Properties and Applications of Solid Oxides; Rodrı´guez, J. A., Fernańdez-Garcıá, M. Eds.; John Wiley: New York, 2007; Chapters 4, 5, and 20. (12) Dietbold, U. Surf. Sci. Rep. 2003, 48, 53.

for sizes (e.g., primary particle size) below ca. 15 nm.13,14 However, in the presence of impurities as Cl- ions, there is conclusive evidence of the stabilization of the rutile phase even for such nanometric particle size materials.15,16 The photodegradation of organic pollutants in the presence of TiO2 appears as a viable decontamination process of widespread application, no matter the state (gas or liquid) or chemical nature of the process target. TiO2-anatase is by far the most active single-phase photocatalytic material.17,18 It is also an n-type semiconductor having a wide band gap (3.0-3.4 eV depending on the primary particle size and other properties), which needs UV light to create energy-rich electron-holes pairs upon absorption. All TiO2-based technological applications aiming to use a free, renewable (e.g., solar light) energy source are thus limited and typically would need to optimize the handling of visible-light photons by the oxide material. This appears to be of prime importance and furthermore corresponds to an appealing challenge for the future generation of photocatalytic materials.19-21 The pioneering work of Sato22 and Asahi et al.23 highlights the possibility of doping the TiO2-anatase structure with anions to yield new, high-performance, visible-light-driven photosystems. After this report, the use of N3-, C4-, S4-, or halides (F-, Cl-, Br-, I-) as doping agents has been subjected to intense research and is summarized in several review articles.18,19,21,24,25 (13) Zhang, H.; Bandfield, J. F. J. Mater. Chem. 1998, 8, 2073. (14) Hu, Y.; Tsai, H. L.; Hung, C. L. Mater. Sci. Eng., A 2003, 344, 209. (15) Fernańdez-Garcıá, M.; Martıńez-Arias, A.; Hanson, J. C.; Rodrı´guez, J. A. Chem. ReV. 2004, 104, 4063. (16) Chong, K. C.; Pratsinis, S. E. J. Am. Ceram. Soc. 2001, 84, 92. (17) Serpone, N.; Pelizzetti, E., Eds.; Photocatalysis Fundamental and Applications; Wiley: New York, 1989. (18) 18 Thompson, T. L.; Yates, J. T. Chem. ReV. 2006, 106, 4428. (19) Coloń, G.; Belver, C.; Fernańdez-Garcıá, M. Photocatalysis; In Synthesis, Properties and Application of Oxide Nanoparticles; Fernańdez-Garcıá, M., Rodrı´guez, J. A., Eds.; Wiley: New York, 2007. Chapter. 17. (20) Chartterjee, D.; Dagupta, S. J. Photochem. Photobiol., C 2005, 6, 186. (21) Zhao, J.; Chen, C.; Ma, W. Top. Catal. 2005, 35, 267. (22) Sato, S. Chem. Phys. Lett. 1986, 123, 126. (23) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Ega, Y. Science 2001, 293, 269.

10.1021/la8014018 CCC: $40.75  2008 American Chemical Society Published on Web 09/13/2008

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In this work,we explore the incorporation of anion impurities into the TiO2 lattice using ammonium Ti-oxychlorides as initial solid precursors. To the best of our knowledge, the preparation of ammonium Ti-oxychlorides has not been reported, in spite of the significant number of studies devoted to the homologous Ti-oxyfluoride family and its conversion upon calcination in the technologically valuable TiO2-anatase oxide.26-28 However, several studies have explored the influence of hydrochloric acid (HCl) addition to titania synthesis as a potential source of surface/ bulk anion (halogen) impurities and for phase control (e.g., anatase:rutile ratio).16,29,30 Others used TiOCl2 as the initial solid precursor of the titania materials.31,32 The absence of studies using ammonium Ti-oxychlorides would come from the fact that they are are XRD-amorphous materials, which complicates their characterization, at least with respect to the oxyfluoride family. The method described here for the doping of TiO2 allows a large N concentration in the precursor as well as a variable Ti/Cl atomic ratio and the study of the influence of these two parameters in the final structural and morphological features of the TiO2 phase. The initial Ti/Cl ratio is critical in managing the nature of the impurities observed after calcination, and this, in turn, seems to closely govern the photoactivity of the material.

2. Experimental Section To vary the Ti/Cl ratio of our precursor solids within the 1:1 to 1:6 range, we prepared corresponding Ti-isopropoxide/HCl mixtures in isopropanol. These Ti/Cl complexes were stirred in a water/ isopropanol (1:10) mixture for 4 h and subsequently precipitated with an ammonia solution corresponding to an NH3/Ti ratio of 3. Solids were filtered and washed with distilled water, dried at 383 K, and finally subjected to calcination treatments consisting of a heating ramp of 3 K/min up to 450 °C in a N2 atmosphere, followed by 2 h in 20% O2/N2 at this temperature. A blank is obtained using a similar procedure but without HCl in the preparation and precipitation steps of the precursor solid. The precursor state of the solid was obtained after the drying treatment, and the sample state was obtained at the end of the above-mentioned heat treatment. Solids are described by an initial set of letters that were followed (expect for the blank case) by a number indicating the specific preparation condition of the sample: B for a blank reference and TC11 to TC16 for samples prepared with Ti/Cl ratios of, respectively, 1:1 to 1:6. Table 1 summarizes the nomenclature and synthesis details. For in situ XRD experiments, the precursor solids were loaded into a sapphire capillary (i.d. 0.7 mm) cell specifically designed for time-resolved XRD monitoring. The gas was admitted into the cell through a flow system, and the capillary was heated by a resistive heater wrapped around the capillary. The temperature was monitored with a 0.1 mm type K thermocouple inside the capillary and was program controlled using a temperature controller (CN2011R, Omega). The flow rate was 10 mL min-1 for both nitrogen and air. In the reaction studies, the temperature was ramped to 450 °C from room temperature over 1.5 h in nitrogen and then held at 450 °C for 2 h in air. The time-resolved X-ray diffraction experiments were carried out on beamline X7B (λ ) 0.922 Å) of the National Synchrotron Light Source (NSLS) at Brookhaven National Labora(24) Serpone, N. J. Phys. Chem. B 2006, 110, 24287. (25) Emeline, A. V.; Kuznetsov, V. N.; Rybakutv, U. K.; Serpone, N. Int. J. Photoenergy 2008, art no. 258394. (26) Laptash, N. M.; Maslennikova, I. G.; Kaidalova, T. A. J. Fluorine Chem. 1999, 99, 133. (27) 27 Li, D.; Haneda, H.; Hishita, S.; Osashi, N. Chem. Mater. 2005, 17, 2596. (28) Maeda, K.; Shimodaira, Y.; Lee, B.; Teramura, K.; Lu, D.; Kobayashi, H.; Done, K. J. Phys. Chem. C 2007, 111, 18264. (29) Gopal, M.; Chang, W. J. M.; Delonge, L. C. J. Mater. Sci. 1997, 32, 6001. (30) Li, G.; Gray, K. A. Chem. Mater. 2008, 19, 1143. (31) Li, Y.; Lee, N.-H.; Hwang, D.-S.; Song, J. S.; Lee, E. G.; Kim, S.-J. Langmiur 2004, 20, 10838. (32) Pottier, A. S.; Cassaignon, S.; Chaneac, C.; Villain, F.; Tronc>, M.; Ito, S. J. Mater. Chem. 2003, 13, 877.

Coloń et al. Table 1. Synthesis Details and Cl and N Content of the Materialsa

sample

Ti/(NH4); Ti/Cl preparation atomic ratios

N/Ti (atom %)b

Cl/Ti (atom %) precursor/ sampleb

B TC11 TC12 TC13 TC14 TC15 TC16

3; 3; 1 3; 2 3; 3 3; 4 3; 5 3; 6

0.8/0.2 52.0/0.4 64.1/0.3 85.0/0.5 87.4/0.4 91.0/0.5 81.0/0.4

22.1/bd 45.2/bd 67.3/bd 66.1/bd 66.5/bd 65.2/bd

a Precursor and sample-state compositions were measured with chemical analysis. Additionally, XPS atomic ratios were measured after thermal treatment (in the sample state). b Numbers for precursors correspond to washed specimens. Cl content of samples after heat treatment below the detection limit (bd).

tory. Two-dimensional in situ diffraction patterns were collected with a Mar345 image plate detector during the annealing at ∼2 min intervals. The sample size analysis was performed using the RELEX module of Material Studio 4.0 (Accelrys). The preparation step of the samples was also followed by infrared spectroscopy and mass spectrometry using similar experimental conditions. Diffuse reflectance infrared spectra (DRIFTS) were taken on a Bruker Equinox 55 FTIR spectrometer fitted with an MCT detector. The gas phase was constantly monitored in XRD and DRIFTS experiments by an RGA-100 quadrupole residual gas analyzer (Stanford Research Systems). After the preparation treatment, oxide samples were analyzed by a combination of XPS, XANES, UV-vis, and IR spectroscopy and TEM. XPS experiments were performed in a standard UHV chamber (base pressure 3 × 10-9 Torr) equipped with a 100 mm hemispherical electron analyzer (Scienta, SES 100). Mg KR radiation (hν ) 1253.6 eV) was used to acquire the core-level spectra (O 1s, N 1s, Ti 2p). The powder samples were smeared on carbon tape, and the binding energy is referenced to the C 1s peak at 284.6 eV from the carbon tape. Oxygen K-edge and Ti L-edge XANES spectra were taken at the U7A NIST/DOW end station of the NSLS. The partial electron yield (PEY) signal was collected using a Channeltron electron multiplier with an adjustable entrance grid bias (EGB) of -150 V. The incident photon energy resolution was 0.2 eV. The powder samples were smeared onto Cu tapes and mounted onto a stainless steel sample holder inside a UHV chamber through a sample load lock system. A low-energy electron flood gun was used to compensate for charging effects. UV-visible diffuse reflectance spectroscopy experiments were performed with a Shimadzu UV2100 apparatus with a nominal resolution of ca. 5 nm using BaSO4 as a reference. Selected samples were also studied by transmission electron microscopy (TEM) using a Philips CM200 instrument. The microscope was equipped with a top-entry holder and ion pumping system, operating at an accelerating voltage of 200 kV and giving a nominal structural resolution of 0.21 nm. Samples were prepared by dipping a 3 mm holey carbon grid into an ultrasonic dispersion of the oxide powder in ethanol. Photo-oxidation activity studies were tested using both liquidand gas-phase degradation reactions. Gas-phase photodegradation tests were carried out with two organic pollutants exhibiting differences in polarity (methylcyclohexane and isopropanol). In this way, a wide range of the photo-oxidation application potential will be covered. Runs of liquid-phase phenol oxidation over the different catalysts (1 g/L) were performed in a Pyrex immersion well reactor (450 mL) using a 250 W metal halide lamp (Sylvania, metalarc HIS; cutoff at 390 nm; three main emission lines between 450-550 nm). In the oxidation tests, oxygen flow was used to produce a homogeneous suspension of the catalyst in the solution. Before each photoexperiment, the catalysts were settled in suspension with the reagent mixture for 15 min in the dark. The evolution of the initial phenol concentration (ca. 50 ppm in water) was followed by UV-vis spectroscopy through the evolution of its characteristic 270 nm band using a filtered aliquot of ca. 2 mL of the suspension (Millipore

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Millex25, 0.45 µm membrane filter). Total organic carbon measurements (Shimadzu TOC analyzer, model VCPH) were performed in selected samples to confirm conversion rates measured from spectrophotometric analysis, showing no significant differences between both detection techniques and thus the essential absence of intermediates using our samples. The degradation rates were calculated from the slopes of the conversion plots in the first 15 min of reaction and assuming a linear time dependence. The regression coefficients for such fittings are in all cases higher than 0.998. Activity and selectivity for the gas-phase photo-oxidation of two different organic substrates exhibiting different polarities (methylcyclohexane (MCH) and isopropanol) were separately tested in a continuous flow annular photoreactor containing ca. 30 mg of photocatalyst as a thin layer coating on a Pyrex tube. The corresponding amount of catalyst was suspended in 1 mL of water, painted on a Pyrex tube (cutoff at ca. 290 nm), and dried at room temperature. The reacting mixture (100 mL/min) was prepared by injecting methylcyclohexane or isopropanol (Panreac, spectroscopic grade) into a wet (ca. 75% relative humidity, RH) 20 vol % O2/N2 flow before entering the photoreactor at room temperature, yielding an organic inlet concentration of ca. 800 ppmv for methylcyclohexane and 600 ppmv for isopropanol ppmv runs. Under such conditions, the reaction rate is zeroth order with respect to the total flow and organic pollutant/ oxygen concentrations. Note that the catalyst allows the presence of the keto-enol tautomeric pair in the case of isopropanol as initial products. After the mixture flowed for 1 h (control test) in the dark, the catalyst was irradiated by four fluorescent daylight lamps (6 W, Sylvania F6W/D) with a radiation spectrum simulating sunlight (UV content of 3%; main emission lines at 410, 440, 540, and 580 nm) symmetrically positioned outside the photoreactor. Reaction rates were evaluated (vide supra) under steady-state conditions, typically achieved 3 to 4 h after the start of irradiation. No change in activity was detected for any sample within the next 6 h. The concentration of reactants and products was analyzed using an online gas chromatograph (Agilent GC 6890) equipped with HP-PLOT-Q/HPInnowax columns (0.25 mm i.d. × 30 m) and TCP/FID detectors. CO2 is the only product of the degradation observed in both gasphase reactions.

3. Results and Discussion Figure 1 displays in situ XRD patterns of the TC13 sample obtained during the heat treatment detailed in the Experimental Section. The upper panel of this plot displays the heating evolution of the dried TC13 precursor (Figure 1A) and shows that the Ti-containing phase coexists with NH4Cl originating from the excess ammonia used in the synthesis. All precursors except the blank contain this NH4Cl chemical phase. The intensity of the cubic NH4Cl (Pm3m; JCPDS 73-1491) solid decreases with the temperature increase up to ca. 200 °C, suffering a phase transformation to the beta phase (Pm3m; JCPDS 73-1492) and decomposing soon after this point. From 200 to 450 °C, only an amorphous phase is detected using XRD. When air is introduced at 450 °C, the formation of the anatase oxide phase (I41/AMDZ; JCPDS 84-1286) is almost instantaneous, developing its maximum intensity very quickly. Similar results are observed for the sample washed with hot water (70 °C, 1 h) to eliminate NH4Cl (Figure 1B). The amorphous nature of the Ti-containing precursor becomes evident from Figure 1B and is a general feature of all of the TC samples. The infrared analysis displayed in Figure 2 allows the indentification of the nature of the precursors and gives details of their thermal evolution during the heat ramp in N2. The blank reference (B) shows the presence of HC residues (2975/2936 and 2894/2871 cm-1, sym-asym stretchings of the CH3 and CH2 groups, respectively; 1466/1381 and 1446/1360 cm-1, sym-asym deformation modes of CH3 and CH2 groups,

Figure 1. In situ XRD patterns during the heat treatment of TC13 (A) and washed TC13 (B) precursors. Images were taken about every 2.5 min.

respectively),33 which will be identified using mass spectrometry (see below). These residues appear in all samples but with a drastic diminishing of intensity with respect to the B one (Figure 2). The B sample also display bands associated with presence of water and surface hydroxyls (3260, 1650-1600 cm-1).34 When the temperature rises, a progressive decrease of all bands and, in particular, the elimination of the HC residues from the solid surface above 275 °C are observed. The concomitant dehydroxylation leaves isolated OH groups above ca. 300 °C, giving (33) Hirose, F.; Kurita, M.; Kimura, y.; Niwano, M. Appl. Surf. Sci. 2006, 253, 1912. (34) Fuerte, A.; Hernańdez-Alonso, M. D.; Maira, A. J.; Martıńez-Arias, A.; Fernańdez-Garcıá, M.; Conesa, J. C.; Soria, J.; Munuera, G. J. Catal. 2002, 212, 1.

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Figure 2. In situ DRIFT spectra during the heat treatment of the B (A, B), TC16 (C, D), and washed TC16 (E, F) precursors.

rise to the 3710/3670 cm-1 contributions.35 These groups are characteristic of the oxygen-terminated surface and, in accordance with chemical knowledge, suggest that the blank precursor is a titanium oxo-hydroxide. However, sample TC16 (Figure 2C,D) (35) Yamazoe, S.; Okumura, T.; Hitomi, Y.; Sishido, T.; Tanaka, T. J. Phys. Chem. C 2007, 111, 11077.

displays a different behavior; whereas initially the NH4Cl bands (3155, 2835, 2000, 1760, and 1403 cm-1)36 dominate the spectrum, after its sublimation around 300 °C we detect broad peaks ascribable to NH4+ species (ca. 3150, 3050, 2850-2800, 1450-1400 cm-1) together with OH-interacting species (ca. 3225 (36) See www.webbok.nist.giv/chemistry.


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Figure 3. Mass (m/z) spectrometry signals during the heat treatment of the B (A) and TC16 (B) precursors.

cm-1). The practical absence of the isolated OH bands previously detected in the blank specimen is noticed. The NH4+-related features display frequencies rather similar to those detected in ammonium Ti-oxyfluorides.26 To confirm beyond doubt the presence of ammonium Ti-oxychlorides, the washed TC16 was also subjected to IR analysis (Figure 2E,F). Whereas after the washing the TC samples are free of NH4Cl (Figure 1B), by IR we detect only surface water (ca. 3260/1640 cm-1) and HC residues (2920/2854/2810/1440 cm-1) at low temperature. Above 200 °C, the high frequency bands (3150, 3050, 2810 cm-1) of the NH4+ group can be distinguished, with the lower counterpart (1403 cm-1) detected afterward. The well-defined, narrow nature of these peaks would indicate that the washing treatment enhances the crystallinity of the precursor, still lacking long-range order to be XRD silent. The stability of NH4+ IR bands at temperatures above 450 °C together with the absence of significant quantities of OH surface groups (Figure 2A,C,E) evidence the presence and thermal stability up to the end of the treatment in N2 of ammonium Ti-oxychloride compounds. As mentioned, these chemical species have not been reported to our knowledge, likely because of the impossibility of XRD characterization. Chemical analyses of the precursors (Table 1) indicate that the Ti/(NH4) (inferred from the Ti/N ratio measured) molecular ratio roughly grows from nearly 0.5 to 1 while the Ti/Cl one goes from ca. 0.2 to 0.67. The IR study proves the hygroscopicity of the materials, making an accurate prediction of their molecular structure difficult, but a characteristic feature is the low halogen content, at least with respect to those typical of known ammonium Ti-oxyfluorides.26 A second interesting point is the constancy of the molecular structure for the precursor state in TC13 to TC16 samples. This again reinforces the point concerning the limited maximum halogen content. As can be expected from the amorphous nature of the XRD patterns, these materials would lack long-range order, likely by an effect of the significant number of O and Cl anion local arrangements around the Ti cations. This would mean that the chemical contents of the precursor solids reported in Table 1 are average chemical compositions of solids presenting significant variability on a local level. The mass spectrometry signal detection during thermal evolution provides evidence that only HC residues and water evolve from the blank during thermal evolution and allow the identification of the former as propene fragments coming from the isopropoxy ligands of the Ti salt. The propene residues account for the main m/z 41, 39, 42, and 27 signals observed in Figure 3A.37 In accordance with IR (Figure 2), the quantity of such HC (37) Index of Mass Spectra; Royal Society of Chemistry: Nottingham, U.K., 1983.

residues is much lower in TC samples. However, no hint is detected for the NH4Cl species decomposition (Cl-related m/z signals followed 35, 36, 37, 50, 61, 62, 63, 67, and 70), indicating its likely sublimation and further condensation in cold parts of the experimental system. The isothermal treatment in the presence of oxygen decomposes the ammonium Ti-oxychlorides in a similar way for all TC samples. The example in Figure 3B indicates the formation of H2O and CO/CO2 as the main products. Also, NO2/NO (m/z 30/44) and isopropanol (m/z 45/29) are detected, indicating that the NH4+ groups are oxidized to N-Ocontaining species whereas, again, no Cl-containing species is detected. Cl and N final contents (e.g., solids after calcination) were analyzed by XPS and are reported in Table 1; this confirm the practical absence of Cl in the final solids and a N content similar to that of previous reports concerning N-doping anataseTiO2 materials.20,21,23,27,28,38,39 As already noted in several reviews, only in a few experimental studies does the N content of the samples exceed ca. 2-2.5 atom %,18,20,24,25 with the corresponding solids mainly synthesized using nucleophilic substitution chemistry.40 Some theoretical calculations would indicate a limited thermodynamical stability of N at network positions of the anatase structure and suggest the existence of a maximum for N incorporation into the anatase structure.41,42 Other theoretical studies indicate that the electronic effects on N are strongly dependent on N content and optimized for low contents.43 This will be discussed below. The XRD patterns of the final products of the synthesis are presented in Figure 4 and provide evidence of an anatase-type structure in all of the TC materials. As already reported, the excess ammonia (Ti/NH3 ) 3) used during the synthesis appears to be responsible for the stabilization of the primary particle size (Table 2) and for the formation of this polymorph.15,16,31 We tested this by working with a Ti/NH3 ratio close to 1, and a mixture of anatase and rutile was obtained for Ti/Cl synthesis ratios below 5 and only rutile for the remaining samples (results not shown). Therefore, the basic medium neutralizes the potential Cl influence on titania polymorphism, presumably by neglecting or severely limiting the presence of positively charged Ti(38) Belver, C.; Bellod, R.; Steward, S. J.; Requejo, F. G.; Fernańdez-Garcıá, M. Appl. Catal., B 2006, 65, 309. (39) Steward, S. J.; Fernańdez-Garcıá, M.; Belver, C.; Mun, B. S.; Requejo, F. G. J. Phys. Chem. B 2006, 110, 16482. (40) Chen, X.; Low, Y.; Sami, A. C. S.; Burda, C.; Gole, J. C. AdV. Funct. Mater. 2005, 15, 41. (41) Nambru, A.; Graciani, J.; Rodrı´guez, J. A.; Wu, Q.; Fujita, E.; FernańdezSanz, J. J. Chem. Phys. 2006, 125, 094706. ´ lvarez, L. J.; Rodrı´guez, J. A.; Fernańdez-Sanz, J. J. Phys. (42) Graciani, J.; A Chem. C 2008, 112, 2624. (43) Wang, H.; Lewis, J. P. J. Phys.: Condens. Matter 2006, 18, 421.

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Figure 4. XRD patterns of samples after the heat treatment. Table 2. Main Physicochemical Characteristics of B and TC Samples after Heat Treatments at 450 °C sample

SBET (m2 g-1)

particle size (nm)

B TC11 TC12 TC13 TC14 TC15 TC16

107.5 104.9 102.7 106.4 107.6 108.7 110.0

10.2 9.2 10.5 10.6 11.0 11.2 12.0

containing polyhedra in solution, hindering the nucleating of rutile aggregates in our case. Negatively charged Ti-containing polyhedra are expected to drive the synthesis to the anatase phase whereas neutral ones should give brookite.29,30 The primary particle size and BET areas of the TC materials are summarized in Table 2. Such materials have high surface areas, ca. 100 m2 g-1, and a nanometric nature with particle size in the 9-12 nm interval. TEM images of the B and TC16 samples are displayed in Figure 5. This Figure gives further evidence of the similar morphological properties of the solids. In both samples, the TEM images show a rather homogeneous rounded shape, exhibiting similar particle sizes of about 12-15 nm, in rough accordance with the data shown in Table 2. It is also worth noting that there is a certain slightly higher particle size and marked contrast/ contours that could allow us to deduce a higher crystallinity for TC16 with respect to the B sample, again in accordance with the primary particle trend reported in Table 2. Analysis of the chemical characteristics of the solids using XPS showed, in first place, the absence of Ti3+ species. Moreover, the more valence-sensitive analysis using the Ti L-edge (Figure 6A) confirms the fully oxidized nature of Ti and shows the practical absence of electronic changes in TC samples with respect to the blank and other nanosized TiO2-based materials.39 The O K-edge analysis also describes nearly null changes in the band structure (Figure 6B). However, the UV study of the samples shows absorption features in the visible part of the spectrum (Figure 7). Because the analysis of the band gap considering the TiO2-based samples as indirect semiconductors as well as the Ti L- and O K-edges indicating the absence of electronic changes in the solid valence and conduction bands, such visible features must correspond to localized states, which are located near the valence band. Although early reports suggested potential effects on the solid valence band,23 current experimental and theoretical research essentially confirms the presence of N-impurity-derived

localized electronic levels.24,39,41,42,44,45 Nevertheless, the influence of N on the valence band is still under debate.46 It is widely accepted that not only the absolute amount of nitrogen species but also their chemical nature and incorporation into the surface/bulk would play a critical role in the photoinduced process.18-25,38-53 Currently, there is a wide debate as to ascertaining (i) the chemical nature of the N-containing species and (ii) their positions (interstitial vs substitutional, surface vs bulk) at the anatase network. Although XPS is commonly utilized to answer such questions, recent reports highlight important technique limitations18,25,50 that still hold after the theoretical analysis of XPS peak assignments.48 In Figure 8, we show the N 1s XPS region, where a broad peak at ca. 400 eV displayed a tail to lower energy with an approximate center around 398 eV. Whereas the second, tailing contribution appears to be maximized for TC11 and decreased as the Ti/Cl ratio increases, the first shows more or less plain behavior throughout the series. According to the literature, a N 1s XPS peak at 396.0-397.0 eV shows the presence of substitutional N3- ions.25,40,48,50 Peaks located above 397.0 but below 399.0 eV received several assignments, among which interstitial NHx (x ) 0-2) species18,47,50 or oxidized (NxOy) species at substitutional positions of the anatase25,50 appear as those broadly accepted; their presence/ abundance is dependent on the synthesis procedure. Around or above 400 eV, a significant number of chemical species contributions may appear, but the predominance of N-N, N-C, and N-O bonds forming part of surface “molecular entities” containing highly oxidized nitrogen is customarily claimed.25,50-53 These surface species may have limited influence on activity.20,21,24,25 As a result of the broad, ill-defined nature of the XPS peaks presented in Figure 8 as well as the inherently difficult assignment, here a combined XPS/DRIFTS approach was used to help in establishing the nature of the N-containing species present in the TC materials. Figure 9 provides evidence of a major NO+-type species giving rise to the 2050 cm-1 band38,54,55 and a second contribution at ca. 2150 cm-1 ascribed to Nn- or (CN)n-/(CNO)nspecies.38,55,56 The presence of NHx species is not observed in the samples, in spite of their (obvious) detection in the precursor state (Figure 2). Although IR does not likely detect all N-containing species present, the combined XPS/IR analysis suggests that NO+ is in fact responsible for the ca. 400 eV XPS peak as a theoretical study indicates that interstitial NO-type species would give an XPS peak at this energy.48 We stress that the broad nature of the XPS peak likely indicates the presence of several species. As mentioned, surface species typically contribute to this XPS feature, but here no hint was obtained using IR for the presence of N-H, N-C, or NOx at surfaces. This is likely due to the final calcination step in the presence of oxygen (2 h; 723 K), as recent results of Chen et al. showed that (44) Finazzi, E.; Di Valentin, C.; Selloni, A.; Pacchioni, G. J. Phys. Chem. C 2007, 111, 9275. (45) Chen, D.; Jiang, Z.; Gang, J.; Wang, Q.; Yang, D. Ind. Eng. Chem. Res. 2007, 46, 2741. (46) Clouser, S.; Sami, A. C. S.; Navok, E.; Alred, J.; Burda, C. Topics Catal. 2008, 47, 42. (47) Diwald, O; Thomson, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D.; Yates, J. T. J. Phys. Chem. B 2004, 108, 6004. (48) Asahi, R.; Morikawa, T. Chem. Phys. 2007, 339, 57. (49) Beraneck, R.; Kish, H. Photochem. Photobiol. Sci. 2008, 7, 40. (50) Chen, C.; Bai, H.; Chang, C. J. Phys. Chem. C 2007, 111, 15228. (51) Li, D.; Huang, H.; Chen, X.; Chen, Z.; Li, W.; Ye, D.; Fu, X. J. Solid State Chem. 2007, 180, 2630. (52) Chen, C.; Bai, H.; Chuang, C. J. Phys. Chem. C 2007, 111, 15228. (53) Zhao, Y.; Qiu, X.; Burda, C. Chem. Mater. 2008, 20, 2629. (54) Hadjiivanov, K. Catal. ReV. 2000, 47, 71. (55) Li, C.; Chen, L.; Dimitrijevic, N. M.; Gray, K. A. Chem. Phys. Lett. 2008, 451, 75. (56) Wang, Z.; Helmersoon, U.; Hall, P. O. Thin Solid Films 2002, 42, 71.


Langmuir, Vol. 24, No. 19, 2008 11117

Figure 5. TEM images of B (A) and TC16 (B) samples after the heat treatment.

Figure 6. Ti L- and O K-edges of the samples after heat treatment.

Figure 7. UV-vis spectra of the samples after heat treatment.

calcination above 673 K drastically reduces the presence of such species.57 Unfortunately, there are no available (to our knowledge) theoretical calculations for (CN)n-/(CNO)n- species at anatase positions, but we can tentatively suggest its association with the ca. 398 eV XPS peak, which is typically ascribed to middle oxidized nitrogen species. (57) Chen, X.; Wang, X.; Hou, Y.; Huang, J.; Wu, L.; Fu, X. J. Catal. 2008, 255, 59.

The two types of N-containing species detected here lead to visible-light localized absorption features (Figure 7), in accordance with previous experimental and theoretical results.24,25,38-44 The broad nature of the absorption peaks disables any detailed analysis of the electronic effects of N doping but, as detailed below, dramatically enhances the photochemical performance of the solids under sunlight excitation. We tested the photodegradation potential in two gas-phase (isopropanol and MCH) and one liquid-phase (phenol) degradation reaction under solar-light simulation. It is important to stress the fact that these tests are not run using visible light but sunlight; a significant number of solids show important activity enhancements over well-known reference systems (as nanostructured TiO2 or Degussa P25) upon visible-light excitation, which, however, easily disappear under sunlight by an effect of the (minor) UV contribution.19,25,58 Figure 10 summarizes the degradation reaction rates for blank and TC samples, showing a maximum for the TC11 solid. Figure 1S (Supporting Information) shows the corresponding plots of the photodegradation conversion versus irradiation time for selected examples. Although a detailed analysis of the photoactivity requires the application of in situ (IR and EPR) techniques and will be reported in future contributions, the three tests shown here clearly indicate that all solids display activity under sunlight-type excitation for both liquid and gas degradation reactions, covering a wide range of (58) Wang, Z.; Can, W.; Hong, X.; Zhao, X.; Xu, F.; Can, C. Appl. Catal. B: EnViron. 2005, 57, 223.

11118 Langmuir, Vol. 24, No. 19, 2008

Coloń et al.

Figure 10. Photocatalytic activity of calcined B reference and TC samples. Note that the isopropanol rate is 1 order of magnitude higher than the others.

TC11 sample would suggest that the presence of negatively charged N-containing species in substitutional positions favors activity upon visible-light absorption. Other factors such as the slightly higher crystallinity of the TC16 sample and the corresponding potential improvement on charge handling upon light excitation3,18,19 seem much less important in the three reactions measured.

4. Summary and Conclusions

Figure 8. N 1s XPS spectra of the samples after heat treatment.

Figure 9. DRIFT spectra of the samples after heat treatment.

photodegradation applications. In the liquid phase, the TC samples slightly improve the blank performance but show the absence of any dependence on the initial Ti/Cl ratio. On the contrary, a strong dependence is observed in the gas-phase reactions whereas only a modest increase can be detected with respect to the blank sample. Taking into account the specific experimental conditions and extrapolating these to reported photoactivities for phenol degradation,59 our best results clearly overcome the corresponding value for commercial Degussa P25 (ca. 60% improvement). Note also that the above-mentioned differences observed between N-doped TiO2-based materials and TiO2 references upon visible or sunlight excitation have been unequivocally detected in the case of phenol degradation,58 highlighting once again the merit of having N-containing samples with improved activity with respect to TiO2 reference systems. The overall maximum for the (59) Coloń, G.; Hidalgo, M. C.; Navıó, J. A.; Pulido Meliań, E.; Gonza´lez Dyá´z, O.; Donã, J. M. Appl. Catal. B: EnViron. 2008, 78, 176.

This work illustrates the use of ammonium Ti-oxychlorides as precursor materials of nanometric, single-phase anatase-TiO2 oxides. The Ti-containing precursors were described for the first time here with varying Ti/Cl initial ratios. The average chemical composition of the precursors varies with the Ti/Cl synthesis ratio from 1 to 3, being essentially constant for higher synthesis ratios. All solid precursors are XRD-amorphous and appear to have maximum Ti/NH4 and Ti/Cl atomic ratios of 1:1 and 3:2, respectively. When they are subjected to thermal treatment under oxygen, they all evolve into anion-doped anatase-TiO2 nanomaterials with significant photochemical activity in liquid-phase (phenol) and gas-phase (isopropanol and MCH) pollutant elimination under sunlight excitation. The maximization of photoactivity is obtained for systems having negatively charged, N-containing, (CN)n--type species located in substitutional positions of the anatase network. The optimization of the content of such impurity species is reached after calcining the ammonium Ti-oxychloride precursor obtained using an initial synthesis Ti/ Cl atomic ratio of 1:1. Acknowledgment. Financial support by the projects CTQ20045734-CO2-2, CTQ-2007-0480/BQU, and P06-FQM-1406 is fully acknowledged. P.S. thanks the Spanish Ministerio de Educacioń y Ciencia for a predoctoral FPI fellowship. The research carried out at the Chemistry Department of Brookhaven National Laboratory was funded by the U.S. Department of Energy, Division of Chemical Sciences (contract no. DE-AC028CH10086). We are grateful to A. Nambu for his help during the recording of the XPS spectra. Supporting Information Available: Graphs showing conversion values versus irradiation time for the photocatalytic reactions rates shown in Figure 10. This material is available free of charge via the Internet at http://pubs.acs.org. LA8014018

Synthesis, Characterization, and Photodegradation Behavior of Single

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