J. Phys. Chem. C 2007, 111, 16871-16877
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Evidence for High Spin Transition Metal Ion Induced Infrared Spectral Enhancement James L. Gole,*,† Sharka M. Prokes,‡ Mark G. White,§ Tsang-Hsiu Wang,| Raluca Craciun,| and David A. Dixon| Schools of Physics and Mechanical Engineering, Georgia Institute of Technology, 837 State Street, NW, Atlanta, Georgia 30332-0430, NaVal Research Laboratory, Washington, DC 20375, DaVe C. Swalm School of Chemical Engineering, James Worth Bagley College of Engineering, Box 9595, Mississippi State UniVersity, Mississippi 39762, and Department of Chemistry, UniVersity of Alabama, Tuscaloosa, Alabama 35487-0336 ReceiVed: July 20, 2007
Using cobalt(II) and nickel(II) ion seeding of a nitrogen doped titanium oxynitride, TiO2-xNx, we demonstrate a significant enhancement of the infrared spectrum associated with adventitious water and minor contaminants associated with the oxynitride synthesis from a porous TiO2 nanocolloid sample. It is suggested that this infrared enhancement is associated with the formation of protonated amines near the oxynitride surface. The intensity enhancement may be associated, in part, with the modification of the anatase ionic crystal dipole moment due to the displacement of charged ions by the electric field, which is associated with the introduced high spin CoII and NiII ions. Raman spectra demonstrate the concomitant incorporation of a spinel-like structure into the TiO2 lattice which may account, in part, for the enhancement.
Introduction Recent studies1-4 have suggested the importance of structural porosity, as it influences and enhances the rate and efficiency of the main group nitrogen doping process in porous TiO2 nanocolloids to form improved visible-light-absorbing oxynitride photocatalysts at the nanoscale. In this paper, we demonstrate a unique feature of high spin transition metal ion seeding into porous nanoscale oxynitride structures, establishing the effects of simultaneous metal ion and main group nitrogen doping. This combination provides a mechanism for the significant enhancement of the intensity of the infrared spectra of minor constituents and intermediates associated with the synthesis of the oxynitride structure. Underlying both the physical modification and the optical response of any material is its electronic structure, related to its chemical composition (chemical nature of the bonds between the atoms or ions and the polarizability of those bonds), and physical dimension (for nanosized materials) not only as it affects the confinement of carriers but also as it heightens the importance of the porosity of a given material’s structure.2 For substitutional doping of TiO2, either the metal (titanium) or the nonmetal (oxygen) component can be replaced. For the former, different metals have been employed to tune the electronic structure of TiO2-based materials, either by an ion implantation method or via wet chemical methods.5,6 In this study, we continue our investigations of the influence of nitrogen dopant concentration on the electronic structure and optical properties of porous TiO2-xNx nanoparticles. We find evidence for significant spectral enhancement of those weak features associated with the TiO2-xNx infrared spectrum when * To whom correspondence should be addressed. E-mail: jim.gole@ physics.gatech.edu. † Georgia Institute of Technology. ‡ NRL. § Mississippi State University. | University of Alabama.
CoII and NiII ion seedants are introduced into the oxynitride and attribute these effects, in large part, to ion-induced enhancement of the vibrational intensities. Experimental Section TiO2 and TiO2-xNx were prepared at room temperature at the nanoscale by a highly efficient synthetic procedure previously discussed in detail.2,4 In all cases, the preparation of the porous TiO2 nanocolloid was done under nitrogen using the 2-propanol/Ti[OCH(CH3)2]4/acetic acid/doubly ionized water synthesis combination. All further studies with additional CoII and NiII seeding were carried out on room-temperature-prepared samples. The CoII and NiII were seeded directly into the TiO2 nanocolloid. Subsequently, the oxynitride samples were prepared by the room-temperature treatment with triethylamine previously described in the literature.2,4 To prepare the CoII- and NiII-seeded samples for IR transmission spectroscopy, 250 mg of KBr (large excess) was ground and mixed thoroughly with 10 mg of TiO2/CoCl2, TiO2-xNx/ CoCl2, TiO2/NiCl2, or TiO2-xNx/NiCl2 powder. The above sample (25 mg) was pressed into a pellet and was analyzed for IR transmission. Film thickness was evaluated by requiring a 10% transmittance at 4000 cm-1. A Buck Scientific transmission IR spectrometer was used to obtain the spectrum. The Raman spectra evaluated in this study were obtained with a µ-Raman system, which consisted of a Mitutoyo microscope and a SPEX Triplemate spectrometer equipped with a CCD. The 514.5 nm line of an Ar ion laser was used as the excitation source. The microscope had 10×, 50×, and 100× objectives for focusing the laser light and was coupled to the spectrometer through a fiber optic bundle. In these experiments, the spot size used was of the order of 100 µm, in order to reduce the power density. The light from the microscope was filtered by a 514.5 nm notch filter. The positions of the Raman lines in a given spectrum were calibrated against the 546.0 nm emission line from a fluorescent light source.
10.1021/jp075712r CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2007
16872 J. Phys. Chem. C, Vol. 111, No. 45, 2007
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Figure 2. IR spectra of titanium oxynitride: (a) vacuum-dried sample; (b) after subsequent introduction of a small quantity of deionized water. Figure 1. IR spectra of titanium-containing solids. See text for discussion.
Results Metal Ion Seeding of TiO2 and TiO2-xNx Nanocolloids. Porous TiO2 and nitrogen-doped TiO2-xNx nanocolloids were simultaneously treated with cobalt (CoII) ions and nickel (NiII) ions introduced as the dichloride. The prepared solutions/slurries were subsequently dried in vacuum (P e 10-2 Torr). The CoCl2 and NiCl2 seedants used in this study form hexahydrate coordination complexes. These complexes are, respectively, pink and green.7 However, after being dried under vacuum on TiO2 (e3 h), the resulting powdered materials are deep blue and yellow, respectively, the colors of their corresponding anhydrous chlorides, and remain this color for several months. In contrast, under similar conditions, the cobalt and nickel complexes remain deep purple (dihydrate) and light green, respectively, after being dried under vacuum for much longer (12-24 h) periods. Clearly, through their trapping on the TiO2-based nanocolloids, we have introduced a significant degree of hydrophobicity into the normally hydrophilic cobalt and nickel complexes. The corresponding cobalt and nickel chloride systems are, respectively, light green and aquamarine powders after their respective oxynitrides are dried under vacuum. We suggest that the efficiency of these processes can be related to taking advantage of the porous nature of the TiO2 nanocolloid.5 The weight percent of CoII and NiII introduced into these systems exceeds those reported by previous workers8 and, for the infrared spectral studies described below, corresponds to ∼4-8 wt %. Infrared Spectroscopy of TiO2 and TiO2-xNx Nanocolloids Seeded with CoII, NiII Ions. Whereas Raman spectroscopy reveals the surprising room-temperature conversion of anatase to rutile TiO2 (TiO2-xNx) 9 in these systems, significant changes in the intensity of the infrared (IR) spectra observed for the oxynitride may be useful to understand the effects of intermediates and small impurities resulting from sample preparation. Further, the IR spectroscopy demonstrates an important transformation inherent to the formation of an oxynitride surface, which suggests the introduction of an ion-seeded displacement polarization activity not present in the TiO2 nanocolloid alone. A basis for these studies is the significant degree of hydrophobicity introduced to the normally hydrated hydrophilic CoCl2 and NiCl2 complexes, as they are introduced into the TiO2 or TiO2-xNx surfaces. The IR spectra of two samples, vacuumdried CoCl2‚xH2O (pink hexahydrate to purple partially hydrated CoCl2) and vacuum-dried TiO2/CoCl2 (4.2 wt % Co, deep blue), compared to a KBr background plate are depicted in Figure 1.
The KBr background plate is known to contain adventitious water as the only IR active species. This is evidenced by a small peak at 1570cm-1, a larger broad peak near 3410 cm-1, and a sharp peak near 3746 cm-1. These are the bending and stretching modes of molecular water sorbed to the KBr surface. For comparison, the frequencies of free H2O are 3756 cm-1 (asymmetric stretch), 3657 cm-1 (symmetric stretch), and 1595 cm-1 (bend).10 The multiple positive and negative sharp peaks near 2345 and 2370 cm-1 correspond to the CO2 doublet of room air that was not completely subtracted by a background scan. We also examined the IR spectrum of the vacuum-dried partially hydrated cobalt chloride hydrate. This sample demonstrates the presence of water as evidenced by the peaks at 1570 and 3410 cm-1, corresponding to the bending and stretching modes of water chemisorbed on the CoCl2 solid. The water peaks in this purple sample are much more intense than those in the KBr background, indicating that the sample contains a significant amount of water. This sample also shows peaks at 3180, 2300, and 2187 cm-1. The peak at 3180 cm-1 most likely results from a surface OH group, while the peak at ∼2300 cm-1 probably results from uncompensated CO2. The TiO2/CoCl2 (deep blue) sample shows several peaks in the vicinity of the water bending mode between 1500 and 1600 cm-1. These correspond to distinct water binding sites on the TiO2 lattice. In addition, a very broad 3400 cm-1 feature is apparent in cobalt chloride, consistent with O-H stretches from water clusters or condensed water. The TiO2 nanocolloid entraps water from the seeding complex. Figure 2 depicts the corresponding IR spectra for the titanium oxynitride obtained after (a) being vacuum-dried and (b) subsequent exposure to a small quantity of doubly deionized water. The water-binding sites near 1570 cm-1 and the broad 3400 cm-1 feature are apparent, but they are greatly muted relative to the corresponding features for the TiO2 nanocolloid. Further, the introduction of water apparently increases the density of water sites on the nitrogen-doped TiO2-xNx nanocolloid lattice, thus decreasing the definition of features in the 1570 cm-1 region. Also, apparent in the spectrum are several minor peaks especially in the range from 1800 to 3200 cm-1. These peaks are of importance as an indication of intermediates or low leVels of impurities that are formed inherently during the sample synthesis. Figure 3 depicts the IR spectrum observed when the titanium oxynitride is seeded with CoCl2 (∼7.7 wt % Co) and, for further comparison, the IR spectrum obtained for TiO2/NiCl2 (∼8 wt % Ni). The nickel chloride treated TiO2 nanocolloid displays a
Ion Induced Infrared Spectral Enhancement
Figure 3. IR spectra of (a) TiO2-xNx nanocolloid decorated with CoII and (b) TiO2 nanocolloid decorated with NiII.
spectrum very similar to that obtained for the CoCl2-seeded sample (Figure 1). The important result obtained here is that the seeding of CoCl2 into the TiO2-xNx lattice signals a pronounced enhancement of the water bending mode region and an even more significant increase in the signal associated with those extremely weak features observed in the undecorated oxynitride system. The one-to-one correspondence between these weak features and the enhanced spectrum is apparent in Figure 4. The comparison shows that the IR intensities for those weak spectral features associated with the synthesis of the oxynitride through the introduction of a CoII magnetic ion are enhanced on seeding. Similar, if not identical, results are obtained after NiII seeding. The corresponding enhanced spectra which can represent intermediates formed in the process of synthesis, minor impurities, or weak emitters, are the subject of further study and modeling. The enhanced sharp peaks observed in the CoII- and NiIIseeded titanium oxynitride spectra arise, in large part, from common components: the nanosized titania that was prepared by sol-gel techniques, and the amines used to treat the titania. Accordingly, we expect vibrations to persist in the solid (at its surfaces) that describe the functional groups present and originating from the TiO2 colloidal solution and/or species developed during the solution process and subsequent gelation (metal alkoxy) after treatment. Moreover, since the nanocolloid was treated with alkyl amines, we also expect the vibrations arising from the amines to be present. We find little or no evidence for spectral features associated with the precursors acetic acid, titanium isopropoxide, or 2-propanol used in the original TiO2 preparation. The peaks appearing between 2700 and 3000 cm-1 are characteristic of the C-H stretching in alkyl groups. The peaks at 1162 and 1025 cm-1 could be ROH moieties. For example, the CH3 rocks in CH3OH occur at 1165 and 1060 cm-1 and the C-O stretch appears at 1033 cm-1.10 The peaks at 838 and 792 cm-1 might correspond to the deformation modes of RNH. For example, the NH2 wag (the inverse motion at N) in CH3NH2 is observed at 780 cm-1.10 The most interesting peaks, however, are those between 2900 and 1540 cm-1, especially those between 2700 and 2000 cm-1. This is not a region typical of common absorptions, as the only peaks that occur in this region in normal molecules are Si-H stretches (near 2200 cm-1 for SiH4)10 or B-H stretches (2300-2600 cm-1 depending on the species).11 These species are not likely to be present, so alternate, more complex, assignments must be considered as discussed below.
J. Phys. Chem. C, Vol. 111, No. 45, 2007 16873 Raman Spectroscopy. The Raman spectrum obtained for the CoCl2 treated sol-gel generated porous TiO2 nanocolloid at room temperature is depicted in Figure 5. The Raman signal obtained for the CoCl2 treated TiO2 nanocolloid is to be associated with the formation of the rutile phase of TiO2. The Raman signal intensity decreases as a function of increasing CoII content, and the features are somewhat broad and overlapped with bands at ∼240-250, 303, 430, 606, and 690 cm-1, those at 250, 430, and 606 cm-1 indicating the rutile phase. The results obtained for the CoCl2 doping of the nitridated TiO2-xNx nanocolloid are depicted in Figure 6. The Raman signal strength increases with increased CoII concentration. The Raman bands observed for the CoCl2-TiO2-xNx system occur at 240-244, 380, 440-442, 606-608, and 688-690 cm-1 and appear somewhat sharper than those for the CoCl2-TiO2 system. In addition to the characteristic rutile Raman lines evident in the CoII-TiO2 and TiO2-xNx systems, there are several additional Raman lines that are not characteristic of the rutile or the anatase phase. These can be seen in Figures 5 and 6. The bands at 380 and 690 cm-1 do not correspond to a structural phase of TiO2. However, it has been reported previously that the vibrations of spinel (Co3O4) which are associated with tetrahedral CoII sites result in a 383 cm-1 band,12 while the A1g phonon mode of spinel has been reported at 691 cm-1.13 It is apparent that these phonon modes result from CoO sites in the TiO2 lattice, similar to those sites in Co3O4, most likely formed during the solution treatment of the TiO2-based nanocolloid. Computational Studies. In order to assign the spectral features depicted in Figures 3 and 4, especially those between 2000 and 3000 cm-1, we have used density functional theory14 (DFT) to predict the interaction of various species with (TiO2)x clusters. The calculations were done with the B3LYP exchangecorrelation functional15,16 and the DFT optimized basis set17 DZVP2. All calculations were carried out with the Gaussian 03 program package18 on the 144 processor Cray XD-1 computer at the Alabama Supercomputing Center and with a 260 processor Xeon-based Dell Cluster at the University of Alabama. Geometries were optimized and second derivatives were calculated to ensure that minima were reached and to obtain harmonic vibrational frequencies. We used the following cluster models for the interaction with a TiO2 surface: Ti(OH)3, (TiO2)2, (TiO2)3, and a cage tetramer model of (TiO2)4 terminated with a proton and an OH group. We studied the interaction of CH3NH2, CH3NH3+, (CH3)2NH2+, (CH3)3NH+, NO, and NO+ with these clusters. The structures of the clusters are shown in Figures 7 (dimer) and 8 (trimer and tetramer) with the protonated amine bonded to them. We calculated the structures for H2O and CH3NH2 and their harmonic vibrational frequencies to compare with their experimental values. We apply a scale factor of 0.95 to our calculations in order to provide a good means to estimate the experimental anharmonic values (see Supporting Information). For CH3NH2, the experimentally determined N-H stretches are at 3361 and 3427 cm-1 and the C-H stretches are at 2985, 2961, and 2820 cm-1. We calculated the same stretches for CH3NH3+, and the N-H scaled N-H stretches are slightly red-shifted to 3345(e) and 3264 cm-1 whereas the C-H stretches are blue-shifted to 3065(e) and 2960 cm-1. The maximum intensity for these stretching modes is predicted to be 124 km/mol for an N-H stretch, which is comparable to the intensity of the most intense C-H stretch in CH3NH2 (101 km/mol.). The addition of CH3NH2 to a cluster leads to a Lewis acid adduct between the lone pair of electrons on the N and the Ti, as would be expected. The C-H and N-H stretching modes do not show large shifts,
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Figure 4. Comparison of (a) titanium oxynitride and (b) CoII decorated oxynitride IR spectra. See text for discussion.
Figure 5. Raman spectra of TiO2 nanocolloid (
