Comparison of Size-Dependent Structural and Electronic Properties of

Mar 26, 2009 - DFT Study on Anatase TiO2 Nanowires: Structure and Electronic Properties As Functions of Size, Surface Termination, and Morphology. Ami...
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J. Phys. Chem. C 2009, 113, 6367–6380

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Comparison of Size-Dependent Structural and Electronic Properties of Anatase and Rutile Nanoparticles Vittorio Luca* Australian Nuclear Science and Technology Organisation, Institute of Materials Engineering, New Illawarra Road, Lucas Heights, NSW 2234, Australia ReceiVed: September 19, 2008; ReVised Manuscript ReceiVed: December 21, 2008

Size-dependent variations in the electronic and structural properties of anatase and rutile nanoparticles have been compared. The anatase nanoparticles of the present study were prepared by hydrothermal ripening of an anatase sol and had diameters in the range 2-130 nm whereas the rutile nanoparticles were prepared by calcination of sol-gel derived rutile and had diameters in the range 3.6-60 nm. The hydrothermally ripened anatase nanoparticles had similar surface structures as deduced from the XANES as previously reported sol-gel anatase materials prepared through calcination (Luca et al., J. Phys. Chem. B 1998, 102, 10650). The optical band gap (Eg) of the anatase nanoparticles as deduced from their electronic absorption spectra showed some variation with size but Eg was not a smooth function of crystallite size, as would be dictated by the effective mass model for both types of anatase nanoparticles. In distinct contrast to the anatase nanoparticles, rutile nanoparticles showed a smooth size dependent variation in optical band gap in line with the dictates of the effective mass model. However, the XANES of the rutile nanoparticles was not dependent on size as it was for both the calcined and hydrothermally ripened anatase materials where the pre-edge XANES and EXAFS revealed a high concentration of distorted surface atoms with reduced coordination. The results suggest that sol-gel anatase nanoparticles consist of a core-shell structure in which the core is bulk-like and the shell interphase is less ordered with a high degree of Ti under-saturation. On the other hand, if such an interphase region was present at all in rutile nanoparticles, it was so thin as to avoid detection by XANES. The unique surface structure of anatase nanoparticles derived from sol-gel preparation methods is probably responsible for the lack of a clear quantum confinement effect. 1. Introduction Titanium oxide is one of the most technologically significant oxide semiconductors of the 21st century. It is certainly one of the most widely studied and used photocatalysts for the mineralization of organic compounds and the splitting of water as well as being in almost exclusive use as the nanoporous semiconductor electrode in dye-sensitized solar cells.2 There are essentially four polymorphs of TiO2 including anatase (I41/amd), rutile (P42/mnm), brookite (Pbca), and a high pressure phase. Although in terms of highly crystalline materials rutile is generally regarded to be thermodynamically the most stable polymorph, even this fundamental aspect has recently been questioned.3 Irrespective of phase stability, anatase has attracted by far the most interest, and this probably stems from the ease of preparation of stable colloidal nanoparticle suspensions that can easily be deposited on various substrates to form porous nanoparticulate films. The ease and reproducibility with which such high activity nanoparticulate anatase films can be prepared using simple solution-based techniques such as sol-gel processing has led to the widespread use of the anatase polymorph and the belief that it has a greater reactivity than rutile. For instance, Yan et al.4 state that “It is crystalline TiO2, anatase that has been confirmed to have quite high photocatalytic activity in the photo degradation of most pollutants in water and in air, while the photocatalytic activity of rutile is still indistinct”. Some evidence certainly exists to support this view. However, accurate and valid comparisons between the performance of these two * To whom correspondence should be addressed. Fax: 61-2-9543 7179. Tel: 61-2-9717 3087. E-mail: [email protected].

major polymorphs are relatively scarce. It took almost one decade after the initial discovery of the dye-sensitized sol-gel anatase thin film solar cell by Gratzel and co-workers2 to show that the photovoltaic efficiencies of these two polymorphs are in fact not substantially dissimilar.5 The frequent assertion that anatase has significantly greater photocatalytic activity than rutile is often based on scant pre-1995 literature.6,7 More recent comparisons of polymorphs with comparable specific surface areas have however suggested that the differences are not great as has been suggested. For instance, while many researchers indicate higher activity of anatase over rutile in certain reactions, others suggest the opposite for other reactions.8 Caution is therefore necessary in making such comparisons. On the other hand, there does seem to be consistency to claims that mixedphase materials such as the commercial TiO2 marketed as P25 by Degussa have higher activity than either of the pure phase polymorphs.9 This has prompted several researchers to ascribe the enhanced activities to peculiarities of the anatase-rutile interfaces in this material.10,11 A finite crystal must necessarily have at its surface a certain number of defect sites and the numbers of such sites must scale with the surface to volume ratio of the crystal. However, it is less obvious whether the type of surface structure and degree of order remains invariant for all particle diameters. As early as 1987, Zhu et al. considered nanoparticles of R-Fe consisting of a crystalline core and a disordered shell or grain boundary12 and were able to empirically test the model based on the modeling of the pair distribution function (PDF) extracted from X-ray data. In bulk single crystals the barrage of surface science

10.1021/jp808358v CCC: $40.75 Published 2009 by the American Chemical Society Published on Web 03/26/2009

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techniques can be readily applied. Techniques such as LEED, HREED, and XPS requiring high and ultrahigh vacuum and clean surfaces have provided detailed structural information, while AFM and STM that require flat surfaces with low interface roughness have afforded exquisite images and insights into the various crystallographic faces and the myriad of defects. Early theoretical investigations indeed evinced the existence of oxygen vacancy defects on TiO2 surfaces and assigned these to gap states below the conduction band edge.13 The surface science of titanium dioxide bulk crystals has since been exhaustively studied both experimentally and theoretically and has recently been comprehensively reviewed.14 In contrast to single crystals, nanoparticles can be far more useful in practice but at the same time complex and elusive in yielding to detailed surface characterization. Nanocrystalline semiconductors can have properties that are quite different to their bulk crystalline counterparts, and it is in fact the scaling of physical properties that is at the heart of nanotechnology. Melting points, structural phase transitions, semiconductor properties, and the like are all affected when the size of a crystal drops below a certain limit.15-18 Among these properties is the band gap of the semiconductor which can be shifted toward higher energy as the crystallite size of the semiconductor is reduced. This is the so-called quantum confinement effect and is well established for many semiconductor nanoparticles. However, anatase has remained contentious19 and our own previous measurements have shown only a relatively modest blue shift of the optical absorption edge from 3.33 to 3.42 eV for a decrease in mean particle diameter from 11 to 3.8 nm as measured by XRD.1 The blue shift of an optical absorption edge may be described by the so-called Effective Mass Model (EMM) first formulated by Brus20,21 and Sandroff22,23 and used by others.24 The model relates the band gap difference for a particular nanoparticle with respect to the bulk value to the particle radius and the effective masses of electrons and holes as follows:

∆Eg )

[

]

h2 1 1 1.8e2 + m*h εR 8R2 m*e

(1)

Here ∆Eg is the band gap difference relative to the bulk value, R is the particle radius, h is Planck’s constant, e is the electronic charge,  is the dielectric constant, and m*e and m*h are the effective masses of electrons and holes.20 By definition nanoparticles have a high surface to volume ratio and high surface energy and hence reactivity. They are often prepared by wet chemical techniques as colloids with varying particle size distributions and therefore are almost always highly hydrated and do not always show well defined crystal facets. These materials can not be experimentally studied easily in their native state and therefore little is known about the surfaces of nanoparticles or their interactions with adsorbed species. Such considerations motivated ourselves and others to apply X-ray absorption spectroscopy to shed light on the surface structure of bulk nanocrystalline anatase powders1,25-28 and reach similar conclusions regarding the interpretation of the pre-edge XANES dependence on particle size. For sol-gel anatase materials, in which size was controlled through thermal treatment, an increase in the concentration of more distorted or coordinatively unsaturated Ti sites as a function of shrinking particle dimensions could be observed. Others have more recently subjected the X-ray powder patterns of anatase nanoparticles to Rietveld analysis and observed changes in unit cell

parameters with increasing size.29-34 All of these studies were by no means consistent. The early studies of Bokhimi et al.29 and the most recent studies by Grey et al.35 proposed through Rietveld analysis of X-ray powder patterns a decrease in Ti site occupancy with decreasing TiO2 nanoparticle diameter in the range 4-1.2 nm. A prerequisite to study the surface of nanoparticles is the ability to prepare them in a reproducible manner and study them under controlled conditions with appropriate techniques. The first report of the use of Ti(IV) alkoxides for the production of titania nanoparticles probably dates back to 1982.36 This sol-gel method has subsequently also been utilized for the production of anatase photo electrodes by a plethora of researchers since the first disclosure of dye-sensitized solar cells utilizing TiO2 nanoparticle electrodes.2 The method has therefore become something of a standard. In the preparation of such electrodes nanocrystalline anatase sols are produced from Ti alkoxides through peptization of a hydrolyzate resulting from the addition of the alkoxide to water.37 The mean particle size, and to a lesser extent, the size distribution of the produced particles can then be regulated through either heating of the air-dried sols in air or through hydrothermal ripening.38 When investigating size dependent properties a reliable measure of mean particle dimension or particle diameter distribution is a very basic requirement. There has been significant effort expended in the materials chemistry community in measuring particle size by different methods and understanding the properties of nano crystalline materials and we and others have contributed to this effort.1,25,39,40 In our aforementioned recent article,1 we described the surface properties of titania nanoparticles of various sizes prepared through drying and subsequent calcination of a TiO2 sol prepared through the classical peptization of a titania hydrolyzate using HNO3. We were able to directly monitor through the use of X-ray absorption spectroscopy (XAS) the surface bonding environment in sol-gel anatase nanoparticles as a function of particle diameter. XAS, especially at the Ti K-edge (4.966 keV), has the advantage that it can provide structural information on materials in their native state without the need for high vacuum or extremely energetic beams. It was possible therefore to directly observe through XAS a systematic increase in the degree of distortion or coordinative unsaturation of surface Ti sites as a function of increasing surface-to-volume ratio of the nanoparticles thus implicating directly the nanoparticle surfaces. The present study addresses the question of how the surface structures of anatase nanoparticles grown entirely through wet chemical processes would compare with those grown through calcination such as in our previous work and then how would these in turn compare with rutile nanoparticles? It is also then natural to enquire into how the deduced surface structures of these materials relate to their electronic and other properties. This study aims to furnish answers to such questions and in so doing attention is focused on materials prepared by sol-gel techniques because these are the types of materials in most common use in real applications (viz. solar cells). 2. Experimental Section 2.1. Sample Preparation. The nanocrystalline rutile used in this work was prepared by the wet chemical method of Anpo et al.41 Rutile of increasing particle size was produced from this powder through calcination in air at increasing temperatures (see Table 1). The anatase sol used was prepared by the method of O’Regan et al.42 In brief, this entailed the hydrolysis of titanium

Properties of Anatase and Rutile Nanoparticles

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TABLE 1: Particle Dimension Data from the Scherrer Method for the Various TiO2 Materials

sample

temp. (°C)a

XRD (nm) anatase

AH1 AH2c AH3d AH4 AH5 AH6 AH7 AH8e AH9e RC1 RC2 RC3 RC4 RC5 RC6 RC7 RC8

FDb 25 25 90 155 170 200 Aldrich Fluka 70 200 300 400 500 600 700 900

2 2 3 5 7.6 10.4 12.9 105.9 123 3.63 6.93 10 13 20.95 41.37 57.4 134.8

TEM (nm) anatase 2.2 2.1 3.3 4.9 6.5 8.3 9.8

XRD (nm) brookite

7 3.21 3.99 5.78

120

a

This was the treatment temperature which was the hydrothermal temperature for the AHx samples and the calcination temperature for RCx materials. b Freeze-dried hydrolyzate. c Air-dried hydrolyzate. d Air-dried anatase colloid (hydrolyzate after peptization at 80 °C). e Commercial anatase samples from nominated suppliers.

isopropoxide Ti(OPri)4 (TIP) by adding 83.3 mL of TIP dissolved in 13.3 mL of propan-2-ol to 500 mL of water. The white precipitate was peptized with 3.53 mL of 70% HNO3 by stirring at 80 °C overnight to yield a translucent sol. It is wellknown that this ‘basic’ type of sol-gel method produces a small amount of brookite impurity. Others have devised modifications to the basic method in order to avoid the formation of the brookite impurity.32 However, we have opted to use the original method in order to relate the results to those of relevant studies. Sols with increasing particle size were obtained by hydrothermal ripening portions of the original peptized sol by autoclaving at 90, 155, 170, and 200 °C in Teflon-lined autoclaves according to the method of Shyklover et al..38 The samples are designated AHx for the anatase nanoparticles that had been grown hydrothermally, ACx for the anatase nanoparticles grown by calcination in our previous study, and RCx for the rutile nanoparticles prepared by calcination in this study. The x designation refers to the series number and the conditions in Table 1. 2.2. Spectroscopy and Diffraction. FT-Raman spectra in the range 3500-100 cm-1 were obtained using a Digilab FTRaman II spectrometer, with a liquid-N2-cooled-germanium detector and holographic notch filter. A reproducible sample alignment was used for all samples, which were sealed in 20 mL glass vials. The 1064 nm exciting line of a Spectra-Physics Nd:YAG diode laser was used, with 500 mW power incident at the sample; 1024 scans were added for each unpolarized Raman spectrum (resolution 4 cm-1), while parallel and perpendicularly polarized spectra were measured by coaddition of 2048 scans. X-ray absorption near edge (XANES) spectra were recorded at the Ti K-edge in 0.10 eV steps in transmission mode on beamline 20B at the Photon Factory, Tsukuba, Japan, using a Si(111) double monochromator. An estimate of resolution (∆E/ E) was calculated from the Darwin width of the Si(111) monochromator and the slit opening of 0.4 mm and a value of 1.36 eV was obtained. This value was very similar to the peakto-peak first derivative line width obtained for a Ti foil reference

Figure 1. X-ray powder diffraction patterns of hydrothermally ripened nanocrystalline anatase AHx samples prepared at increasing temperatures (a) 90 (AH4), (b) 155 (AH5), (c) 170 (AH6), and (d) 200 °C (AH7). Particle dimensions above the anatase (101) reflection near 25o 2θ were calculated using the Scherrer equation.

spectrum. Samples were diluted by mixing with boron nitride and then loaded into sample holders with Kapton windows. X-ray powder diffraction patterns were recorded on a Scintag X1 diffractometer using Cu KR radiation, a Peltier detector and θ-θ geometry. The diffraction data were refined using the Rietveld program Rietica.43,44 A fifth order polynomial was generally used for the background. Usually, good fits were achieved using isotropic thermal parameters for all atoms. It was sometimes necessary to restrain these thermal parameters in order to prevent excursions to negative values. The most important structural information required from these refinements were the unit cell parameters that can usually be derived precisely and accurately using powder diffraction data and the Rietveld method. 3. Results 3.1. X-ray Diffraction. To be able to effectively relate electronic and other properties of titania nanoparticles to their size, it is first paramount to have a reliable measure of the mean dimension of those nanoparticles. Numerous methods are available for the measurement of mean particle dimension and in general each method has its biases and can give slightly different results. In the present work therefore, many methods were used in an attempt to obtain consistent values for, or at least to independently corroborate, the dimension of the titania nanoparticles being investigated. A popular and simple method for particle dimension measurement from X-ray diffraction data is through the application of the Scherrer equation to an X-ray reflection. This simple method gives the volume-weighted column length, or in other words, the mean crystallite size in the direction perpendicular to the (hkl) plane and has been found recently to be as reliable as more complex X-ray peak broadening methods such as the Warren-Averbach method.45 Other methods used in the present study included TEM particle counting, laser diffraction, photon correlation spectroscopy (PCS), and small-angle X-ray scattering (SAXS) many of which have been used to corroborate the dimensions provided by the Scherrer method. Figure 1 shows XRD patterns of the powders generated by air-drying nanocrystalline anatase sols ripened hydrothermally

6370 J. Phys. Chem. C, Vol. 113, No. 16, 2009 at increasing temperatures (Table 1 samples AH4-AH7). The XRD patterns of all but the largest AH7 nanoparticles were characterized by a single low-angle reflection around 1o 2θ and high angle Bragg reflections starting from about 25o 2θ due to the anatase crystal structure. The dimension of the AH7 nanoparticles were too large to give a reflection within the range accessible with the diffractometer. The low-angle reflection derives from the correlation between particles of similar size and effectively provides the interparticle correlation. There is broad agreement between the low-angle d-spacing and the mean particle diameters obtained by applying the Scherrer equation to the anatase 101 reflection observed at about 25o 2θ. The fact that this low-angle diffraction or scattering could be observed at all indicates that the nanoparticles prepared by this method had a relatively high degree of monodispersity. As expected, the mean crystallite diameter of anatase was found to vary smoothly with hydrothermal treatment temperature. Wide-angle X-ray diffraction patterns were also measured for the AHx sample series and these were then subjected to Rietveld analysis (see the Supporting Information section Figure A1) where appropriate. The cell volumes, as well as the a- and cdimensions, were determined in the standard manner using the Rietveld technique. All XRD patterns showed evidence of a small amount of brookite and from the Rietveld analysis this was estimated at 10 nm) was observed from anatase nanoparticles prepared by various sol-gel methods followed by calcination (Figure 2c). These investigators also noted that their samples had high concentrations of Ti vacancies which decreased as the particle size increased. A similar trend of increasing cell volume with particle size was later observed in a more recent study of anatase nanoparticles >10 nm prepared using a nonhydrolytic sol-gel approach.32 These researchers used the Williamson and Hall method for determination of lattice strain which they found

Luca

Figure 2. (a) a- and c-dimensions and (b) unit cell volume and as a function of mean particle diameter for hydrothermally prepared anatase (AHx). Panel (c) represents a compilation of all available data. ([) This work, (9) Grey et al.,35 (2) Li et al.,32 (b) Djerdj et al.,34 (×) Swamy et al.,33 (0) Li et al.,51 and (∆) Bokhimi et al.29 Vertical dashed line highlights the 10 nm cutoff.

to be minimal and therefore justified particle size determination using peak X-ray peak broadening which they confirmed by TEM (Figure 2c). In another relatively recent study of lattice parameter variations in nanocrystalline anatase prepared by an aqueous sol-gel route samples with particle diameters in the range 5 to 14 nm an initial slight increase in cell volume with increasing particle size was observed (b symbols in Figure 2c) followed by a power law decrease resulting an increase in cdimension and a linear decrease in a- dimension.34 It was concluded that the most applicable model was one in which defects were uniformly distributed throughout individual grains but whose concentration was proportional to the inverse grain size. The defects present in the investigated materials were predominantly dislocations, the existence of which was proved by HRTEM analysis. Swamy et al.33 have also recently made measurements of the particle size dependence of lattice parameters for sol-gel derived anatase nanoparticles. In comparing their results with those of the aforementioned previous studies, they concluded

Properties of Anatase and Rutile Nanoparticles that subtle variations in unit cell dimensions of nanoparticles were dependent on preparative methods. The nonhydrolytic method of Li et al.32 gave rise to lattice expansion with increasing particle size beyond 9 nm whereas most other studies observed the opposite trend. They singled out the possible variations in Ti vacancy values resulting from the different preparative procedure. Most recently however Grey et al.35 observed an initial decrease in cell volume of about 0.3% as the particle size increased from 3.5 to about 6 nm, and a subsequent increase in cell volume as the dimensions of their sol-gel nanoparticles increased from 6 to 25 nm when using conventional Rietveld refinement. When a Debye function approach was used, a similar trend of decreasing cell volume with increasing particle dimension to that obtained here was observed over a comparable particle size range (i.e., ∼2-100 nm). While these observations align with those of Li et al.32 over a comparable particle size range, their materials were prepared using a hydrolytic sol-gel process. This tends to suggest that conditions of preparation may not be as important as at first thought. The data presented here and the literature cited have the unifying feature that below a particle diameter of about 10 nm, unit cell volume expansion is consistently observed as a function of decreasing particle dimensions. Above this 10 nm cut off, either the cell volume remains about constant as observed here or an increase is observed in accordance with the data of some other researchers.29,32,35 Unit cell contraction as a function of increasing nanoparticle dimension has been observed in many other nanoparticulate systems for which data are available including Fe2O3,47 CeO2, and BaTiO3.49 For the rutile samples prepared by calcination, a much more consistent picture has emerged. A smooth exponential increase in crystallite dimension with calcination temperature was observed (see Figure A2 of the Supporting Information for the XRD patterns of RCx samples). The variations in both unit cell volume and a and c dimension displayed by the RCx series of samples (Figure 3) resembled those of the AHx samples investigated in this study and were in line with the recent observations made by Bokhimi et al.50 and Li et al.51 for rutile materials. Bokhimi et al.50 noted the same decrease in Ti vacancy concentration with increasing rutile particle size as they observed in the case of anatase. In summary, although our results were obtained on materials prepared by different methods to all previous studies, similar variations in cell parameters were observed for both small anatase and rutile nanoparticles (less than about 10 nm). This indicates that nanoparticle cell parameters below about the 10 nm cut off are not on average affected by small differences in preparation conditions and this further confirms that the small variations observed are in fact genuine properties of the nanocrystalline titania particles as their size is reduced. 3.2. Electron Microscopy. TEM images of both the AHx and RCx nanoparticle samples are provided in Figure 4. The AHx nanoparticles prepared at all reaction temperatures were roughly spherical. In comparison, the RCx nanoparticles retained an overall spherical shape but showed a small degree of faceting even for the smallest sizes examined. For the AHx nanoparticles, it was possible to generate particle diameter distributions by TEM analysis and extract mean particle diameters (Table 1). Good agreement was obtained between the XRD determined particle diameters (Scherrer) and those obtained by TEM. This agreement was particularly good for particles below about 10 nm which fit very well on a straight line (see Figure A3 of the Supporting Information). However, when the commercial ana-

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Figure 3. (a) a- ([) and c-dimension (9) and (b) unit cell volumes for the rutile (RCx) series of samples ([) this work (0) from Bokhimi et al.50

tase material was included, a different line of best fit was obtained. Such an effect could be due to the well-known limitations of the Scherrer approach in determining dimensions of particles exceeding about 100 nm, where the line-broadening is approaching instrumental broadening as is the case for our commercial material. 3.3. Raman Spectroscopy. Another useful technique for determining nanoparticle identity and indirectly estimating size is Raman spectroscopy. The Raman spectra of various titania polymorphs are very distinctive (see Figure A4 of the Supporting Information) making it a very simple matter to identify each of the polymorphs. The Raman investigation of TiO2 polymorphs has been ongoing for some time,52 but Kelly et al.53 and Bersani et al. 54 appear to have been the first to observe the dependence of the shift of the anatase Raman band at 150 cm-1 on crystallite size suggesting the band position could be used to determine particle size. Others have since followed this methodology55 and the use of Raman for this purpose has become well established. The variation in frequency shift of the intense Eg Raman band of anatase (∼142 cm-1) as a function of particle size for the present anatase samples is shown in Figure 5a and these are plotted as a function of the particle diameter determined from XRD measurements using the Scherrer method in Figure 6a. A good correlation is observed between the Raman shift of this band and crystallite diameter determined by XRD. Weak bands from the brookite impurity that was identified by XRD can be discerned in the Raman spectra of the anatase nanoparticles. Therefore in terms of their Raman signature, the present suite of AH samples conform to previous observations. For the rutile nanoparticles, we observe both frequency shifts and dramatic changes in relative intensity of the various bands

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Figure 4. Bright field TEM images of anatase nanoparticles of different size prepared by hydrothermal ripening at (a) 90 (AH4), (b) 170 (AH6) and (c) 200 °C (AH7) and rutile nanoparticles of different size prepared through (d) air-drying the hydrolysate and sintering at (e) 300 (RC3) and (f) 600 °C (RC6).

Figure 6. Dependence of Raman shift of selected bands on nanoparticle dimensions for (a) anatase AHx and (b) rutile RCx nanoparticles. ([) band near 444 cm-1 and (9) band near 608 cm-1.

Figure 5. Raman spectra of (a) anatase AHx and (b) rutile RCx nanoparticles as a function of particle dimension. Nanoparticle dimensions are given to the right of each spectrum.

(Figure 5b). In addition, as the calcination temperature and consequently the particle size of the rutile nanoparticles

increased, a weak sharp feature at 150 cm-1 due to a small anatase impurity was observed. The anatase impurity must have very low concentration as it could not be observed in the XRD patterns. The rutile B1g putative surface mode identified by Swamy et al.56 at about 105 cm-1 does not seem to be observed in the spectra of the present RC nanoparticles. The analysis of Swamy et al. was based on a complete analysis of the Raman spectrum of rutile SnO2 by Dieguez et al.57 and their Raman shifts as a

Properties of Anatase and Rutile Nanoparticles

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Figure 7. Thermo gravimetric (left) and differential thermal analysis traces (right) for the various particle sizes of sol-gel anatase ripened hydrothermally (a) freeze-dried hydrolysate ∼2 nm (AH1), (b) airdried hydrolysate ∼2 nm (AH2), (c) 5 nm (AH4), (d) 7.6 nm (AH5), and (e) 10.4 nm (AH6).

function of particle size were in qualitative agreement with those of the present study. However, additional bands in the Raman spectrum of SnO2 nanoparticles labeled S1-S3 and appearing at 568.1, 485.9, and 705.8 cm-1 for particles heated to 400 °C were assigned to a disordered shell of surface atoms up to two or three unit cells thick in accordance with model derived from PDF analysis of X-ray diffraction data by Zhu et al.12 Most recently Mazza et al.58 in investigating the Raman spectrum of anatase and rutile nanoparticles observed a blue shift of the anatase Eg mode of some 3 cm-1 and a red shift of the bulk rutile Eg and Ag modes of some 10.7 and 4.2 cm-1, respectively in agreement with earlier observations and suggested the phonon confinement model not to be appropriate to describe the particle size dependence.52 These observations are in reasonable agreement with those of the present study (Figure 6b). A band at about 120 cm-1 assigned as a B1u mode is observed here and its relative intensity is strongly size dependent. 3.4. Thermal Analysis. Thermal analyses of the hydrothermally ripened anatase nanoparticles are shown in Figure 7. The hydrolyzate powder prepared by freeze-drying (no hydrothermal ripening) containing 2 nm anatase nanoparticles (AH1) gave a single steep weight loss of about 30% starting from about 50 °C which corresponded to the loss of adsorbed water (Figure 7a). The DTA of this sample showed an intense endotherm at about 100 °C corresponding to the loss of this adsorbed water and an exothermic peak at around 400 °C without any corresponding weight loss. This exotherm could be due to the crystallization of poorly crystalline oxo-hydroxy clusters. At about 650 °C an additional weak exotherm was observed and this is most likely due to the anatase-to-rutile transformation which is known to occur around this temperature. The hydrous titanium oxide hydrolyzate dried in air (AH2) resulted in a TGA trace (Figure 7b) that was qualitatively similar to the freeze-dried material (AH1) but there were considerable differences in the DTA. For the freeze-dried AH2 hydrolyzate material, exotherms at both 400 and around 650 °C were much more pronounced. This could be due to a higher proportion of amorphous or very small particles (proto-particles) in this sample. For all of the hydrothermally ripened materials (Figure 7c-e), the anatase-to-rutile transformation underwent a progressive shift to higher temperatures with increasing particle size as gauged by the shift in the endotherm. The dependence of anatase-to-rutile transformation temperature on particle size has been addressed by Ding et al.,59 who studied sol-gel anatase materials and similarly reported a decrease in the rate of anatase-

Figure 8. (a) Diffuse reflectance spectra and (b) Tauc plots of nanocrystalline AHx samples prepared by hydrothermally ripening the anatase sol.

to-rutile transformation as the particle size increased by monitoring the fraction of anatase converted to rutile at various preset temperatures above 650 °C. Unlike the case of the unripened hydrolyzates (AH1 and AH2), the nanoparticles (AH4-6) showed substantial weight losses between 200 and 400 °C. This is likely to be due to condensation reactions involving surface OH groups and the elimination of more strongly bound water. For similarly prepared hydrolyzate materials others60 have observed an endotherm at 70 °C and exotherms at 325 and 540 °C which was suggested could be attributed to evaporation of adsorbed water, the transition from hydrous titanium oxide to TiO(OH)2 and the transition from TiO(OH)2 to TiO2. These results are similar to our observations with variations in intensity of these thermal transitions. Thermal analyses of the rutile nanoparticles (see Figure A5 of the Supporting Information) were found to be similar to those of the anatase nanoparticles. 3.5. Optical Absorption Edges. The spectral dependence of the band edge is characterized using the absorption coefficient, R, where ν is the frequency in cm-1 and k is the extinction coefficient R ) 4πνk. At photon energies above the band gap Eg ∝ (hν - Eg)1/2 for a direct gap semiconductor while Eg ∝ (hν - Eg)2 for an indirect band gap semiconductor. Bandstructure calculations predict a direct gap at energies just below the onset of indirect transitions. In a detailed study of the absorption edge of single crystal anatase TiO2, Tang et al.61 reported an absorption edge corresponding to Eg ) 3.420 eV and tentatively assigned it to a direct gap. In conventional fashion, and under the assumption of a direct transition, in Figures 8 and 9 are plotted for both AHx and RCx samples respectively, (F(R) hν)2 (F(R) is the Kebulka-Munk function) as a function of photon energy, hν. Again in conventional fashion a straight line was fitted to the linear part of the edge and extrapolated to zero ordinate to obtain the direct band gap Eg. Refer to the paper by Murphy for a more detailed account of these procedures.62 For the highly crystalline anatase compound with particle diameter of about 130 nm, Eg was about 3.42 eV which is consistent with the aforementioned values for pure anatase. However, a wide range of Eg values are actually reported in the literature from the absorption spectra. For instance, Simpson et al.63 reported a direct gap of 3.6 eV

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Figure 9. (a) Diffuse reflectance spectra and (b) Tauc plot for rutile RCx nanoparticles prepared by sintering.

determined from the optical spectrum for the direct band gap of pure anatase TiO2 film grown on SrLaGaO4 using pulsed laser deposition. It is to be noted that the absorption onset of the smallest anatase and rutile nanoparticles is not abrupt. This characteristic Urbach tail is often taken as an indicator of surface defect states. The Eg values for the entire range of materials are plotted as a function of nanoparticle size in Figure 10a. In the case of the AHx nanoparticles (O) the expected general trend of increasing Eg with decreasing particle diameter held roughly. However, there was considerable scatter in the data. Especially for the smallest hydrothermally ripened nanoparticles Eg was significantly blue-shifted from the generally accepted value of around 3.2 eV for bulk anatase. In contrast, for the ACx nanoparticles of our earlier study (0) there appeared to be a better trend if the datum for the large diameter commercial particles was excluded. For the rutile nanoparticles prepared by calcination (4) the trend in Eg with increasing particle diameter was smoother than that of either AHx or ACx nanoparticles and appeared to show either a logarithmic or power law dependence. The dependence of Eg on the particle dimensions of the ACx nanoparticles did show a somewhat similar behavior to that of RCx nanoparticles although the scatter in the data was greater. For the RCx samples the direct band gap varied from 3.38 to 3.20 eV (∆Eg ) 0.18 eV) as the particle size increased from 3.6 to 41.4 nm. Anpo et al.41 did not report direct gap values determined using a Tauc plot although they did provide the absorption onset wavelength with particle diameter and these varied from 398.0 to 410 nm as the particle size varied from 5.5 to 200 nm. These absorption onset values translate to a band gap range from about 3.12 to 3.02 eV which represents a much smaller variation than observed here (∆Eg ) 0.0934 eV). Fine structure on the absorption edge has been previously assigned.64 For bulk anatase and rutile, the band gaps are usually accepted as being 3.2 and 3.0 eV, respectively.65 These values derive from older work on anatase66 and rutile single crystals.67-69 As already mentioned however, Tang et al.61 reported a band edge with Eg ) 3.42 eV for anatase single crystals which they assigned to a direct transition. Most recently, based on optical measurements, Simpson et al.63 reported a direct band gap value

Figure 10. (a) Relationship between the particle diameters measured by XRD and the optical gaps (Eg) determined from the Tauc plots for the various materials with lines being only a guide to the eye and (b) difference between Eg of nanoparticle and the bulk values (∆Eg) and particle diameters. Solid lines are the fits using the effective mass model. The determined values of me and mh are provided near the best fits. SF is a scaling factor.

Eg ) 3.6 eV for pure anatase thin films made by pulsed laser deposition. These rather higher values for the direct gap of anatase are consistent with the values observed here for both the hydrothermally ripened and sintered anatase materials. Most recently Li et al.70 reported the preparation and optical properties. Kuznetsov and Serpone71 recently attributed absorptions at 2.90 eV (427 nm), 2.55 eV (486 nm), and 2.05 eV (604 nm) on the band edge of P25 TiO2 to color centers. As these absorptions are all lower in energy than the first observed peak in the spectra of both the anatase and rutile specimens examined here we can discount the generation of such defects due to residual organic. Attempts to fit the difference between Eg for the nanoparticles of different size and the bulk values (∆Eg) for these nanoparticles using the EMM (eq 1 in the introduction) gave the results of Figure 10b. Clearly for both AHx and ACx poor fits were obtained across the particle size range investigated. In obtaining these fits, expression 1 was used in conjunction with an offset which displaced the curves vertically to obtain the best fit. The electronic masses me ranged between about 0.2 and 2m0, whereas the hole masses mh ranged between about 0.5 and 3m0 where m0 represents the electron rest mass. These values are in the range of those reported for anatase and rutile materials. For instance, Enright et al.72 report 10 and 0.8me respectively for holes and electrons for anatase. Toyoda et al.73 estimate mh at 0.01me. 3.6. X-ray Absorption Spectroscopy. In the author’s previous work,1 a smooth variation was observed in the intensity

Properties of Anatase and Rutile Nanoparticles

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6375

Figure 11. Fitted pre-edge XANES of (a-c) calcined sol-gel anatase (from Luca et al.1), (d-f) hydrothermally ripened sol-gel anatase (AH2, AH5, and AH9), and (g-i) calcined sol-gel rutile samples (RC1, RC4, and RC6) of selected particle diameter.

ratio of the A2 and A3 pre-edge XANES features as a function of crystallite size for ACx nanoparticles. Indeed the IA2/IA3 ratio was found to be directly proportional to the surface area-tovolume ratio of the particles.1 Thus, it was of interest to see if such variations would also be apparent in the hydrothermally ripened materials. The XANES data of selected ACx, AHx, and RCx nanoparticles are collected in Figure 11. Even casual inspection of these data reveals the close similarity between the variations of the AHx nanoparticles with those of the previously reported ACx sample series. That is, as the particle size increased the ratio of the intensity of the A2 to A3 preedge features progressively decreased. This shows that the surface structure of the hydrothermally ripened anatase nanoparticles was the same as that of the corresponding calcined materials. An attempt was made to reproduce the pre-edge XANES of the anatase nanoparticles by summing the pre-edge of crystalline anatase with that of brookite since this phase was present as an impurity during preparation. Using this combination, it proved impossible to achieve an even qualitative reproduction of the XANES of the AHx nanoparticles (see Figure A6 in the Supporting Information). It also proved impossible to simulate the pre-edge of the AHx nanoparticles by summing the preedge of amorphous TiO2 and crystalline anatase. These results support the hypothesis that the AHx nanoparticles are not simply an admixture of two phases but that the increase in the relative intensity of the A2 feature as particle size decreases is indeed a function of the greater contribution of the particle surfaces as the surface/volume ratio increases. Figure 11 shows that the pre-edge XANES of the rutile nanoparticles was invariant with crystallite size and that only three pre-edge features labeled A1, A3, and B in order of increasing energy with a complete absence of the A2 transition previously assigned to distorted 5-fold coordinated Ti in anatase. The A1, A3, and B features are in excellent agreement with what has been previously reported for bulk highly crystalline rutile samples.74,75 Wu et al.76 noted some minor differences in the pre-edge of nanoparticulate rutile samples and highly crystalline rutile and observed a very slight shift to lower energy of the A3 peak and a slight reduction in the intensities of the B

Figure 12. XANES of rutile samples with increasing particle size in the range 3.6-14.6 nm (a) RC1, (b) RC2, (c) RC3, and (d) RC4. The dashed line superimposed on the spectrum of the RC4 sample is the spectrum of the RC1 sample to emphasized that the pre-edge region from 4.97 to 4.98 keV remains unchanged as the particle size increases.

peak. In their samples they also noted some variations in the intensity of the edge peaks labeled B1-B3. These subtle differences can not be observed in the present series of samples. To make this even more apparent the rutile pre-edge XANES data are stack plotted in Figure 12. Yeung et al.77 have also reported XANES of anatase nanoparticles and essentially are in agreement as to the assignment. In Figure 13 are shown the fitted R-space EXAFS of RC1 (3.6 nm) and RC6 (41.4 nm) nanoparticles. These spectra are very similar except for the absolute intensity of the spectra which is lower for the smaller nanoparticles, reflecting the larger vibrational contribution to the mean square relative displacement. Notably there is no significant difference in the relative intensity of the Fourier transform peaks as has been observed previously in the case of anatase nanoparticles.1 Fitting of the spectra yielded values of coordination number, Ti-O and Ti-Ti distances in accordance with the crystal structure of rutile.

6376 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Figure 13. Fitted magnitude FT spectra of samples (a) RC1 (3.6 nm) and (b) RC6 (41.4 nm).

4. Discussion The answers to two fundamental questions were sought as part of the present study. How do the surface structures of nanocrystalline anatase and rutile differ and why do the band gaps of rutile nanoparticles follow reasonably well the effectiVe mass model while those of anatase do not? Quantum confinement is a property of many nanocrystalline materials that has attracted considerable interest. Yet despite this interest there has been relatively little agreement as to whether or not nanocrystalline TiO2, and in particular the anatase polymorph, displays such confinement. While some studies report small blue shifts in the optical absorption onset for anatase24 others have claimed the shift to be negligible.19,78 From the EMM described in equation 1 of the introduction, and using appropriate values of m*e and mh* in TiO2, it can be predicted that to observe a significant blue shift would require particles below about 10 nm in diameter. Monticone et al.19 observed no blue shift in the band gap of sol-gel anatase studied anatase nanoparticles which ranged in diameter from 1.5 to 3 nm. For investigations of size-dependent properties to be reliable it is important that measurements of average particle dimensions, or better still, particle size distribution measurements, be reliable. In the present study numerous techniques were therefore used to corroborate the particle dimensions measured by XRD including TEM, SAXS and N2 adsorption-desorption measurements (see the Supporting Information). As previously mentioned, the sol-gel anatase nanoparticles of variable dimensions that were prepared here both through hydrothermal treatment (AHx series) and calcination at increasing temperature (ACx series) showed definite changes in the direct gap, Eg, with particle diameter. However, the variation in Eg with mean particle diameter did not follow the EMM very well in either of the anatase nanoparticle preparations, although there was better agreement for the ACx nanoparticles. Also, the AHx and ACx materials did not give the same values of Eg for the same particle diameter. In contrast, the RCx rutile nanoparticles prepared through calcination were observed to show a steep decrease in Eg with increasing particle diameter that was in reasonable accord with the EMM and returned values of 0.196m0 and 1.30m0 respectively for the electron and hole effective masses. To explain these differences between anatase and rutile, it is necessary to have a good understanding of the surface and bulk properties of the nanoparticles.

Luca In the present study the unit cell volumes of the anatase and rutile nanoparticles were measured to provide a first insight into their structures and facilitate comparison with crystalline materials. The results for anatase were compared with those reported by other investigators in Figure 2c. At first glance the results are apparently contradictory in that some investigators report decreasing cell volume with decreasing particle dimension while others report the opposite. Closer inspection of the compilation in Figure 2c shows that for particle dimensions above a 10 nm cutoff the trend is either constant or slightly increasing cell volume with increasing particle dimension while for particles below 10 nm the trend is clearly an increase in cell volume with decreasing particle dimension. To understand the volume expansion with decreasing particle dimension, it is necessary to recall that the free energy of small particles is a product of the free energy of the bulk, surface, edges and corners of the crystallites and is therefore dependent on particle size and shape. Barnard et al. have calculated ab initio the surface free energy, ∆G, for anatase and rutile nanoparticles using a model that takes as input the possible geometries of the nanocrystal morphology, the surface free energy γ and the surface tension σ. Both anatase and rutile show decreasing free energy with increasing particle size as expected.79 This trend is mirrored by the variation in unit cell volume with particle diameter observed here and in other studies cited already. For small particles, the volume dilation, e, is determined by the effective pressure, Peff, and the material compressibility, β.

∆V ) e ) Peffβ V The surface pressure is in turn determined by the surface tension σ and the material compressibility.

Peff )

2σx R

This increase in surface free energy with decreasing particle diameter is therefore at least in part due to an increase in surface tension which in turn results in an increase in surface pressure and volume dilation. It is this volume dilation that appears to be directly observable in the present data as an increase in unit cell volume with decreasing particle size. Such a volume expansion for very small nanoparticles with respect to bulk materials has also very recently been modeled by Iacomino et al.80 for nanocrystals with hydrogenated and hydroxylated surfaces. It is hypothesized here that the enhanced surface energy of anatase compared with rutile is the result of a greater surface tension. This surface tension is relieved in order to reduce the free energy to a minimum value through the breaking of bonds and the creation of a greater proportion of distorted fivecoordinate sites present mainly within the interphase or shell region of the core-shell nanoparticles (Figure 14a). Although the DFT calculations of Barnard et al.79 did not include any hydrated surface layer they showed that all the common crystal faces of both crystalline anatase and rutile nanoparticles are expected to contain some 5-fold coordinated Ti and for some of the faces 6- and 4-fold Ti coordination is also present. Hydrogenation of these surface oxygen atoms was not observed to change this coordination although it did appear to result in displacements. Hydrogenation of the less frequent

Properties of Anatase and Rutile Nanoparticles

Figure 14. Model for (a) core-shell sol-gel anatase nanoparticles, (b) alternative with discrete phases, (c) core-shell sol-gel rutile displaying thin shell and minor faceting, and (d) the anatase sol showing accretion of solution species.

(110) surfaces of anatase nanoparticles were found to deteriorate on relaxation becoming disordered. More recent studies by Koprade81 have attempted to model water adsorption on 2.5 nm rutile nanoparticles and have found that about 60% of the Ti in these nanoparticles are in 6-fold coordination, 30% in 5-fold coordination, and 12% in 4-fold coordination. Relatively little influence was observed on these coordination numbers between the nanoparticles in vacuum or under ambient conditions suggesting water was not able to coordinate to these sites to any significant extent. In fact only about 5% of the initial 5-coordinate Ti could be converted to 6-coordinate Ti even when the nanoparticles were immersed in water. To the author’s knowledge, no comparable theoretical studies have been reported for anatase nanoparticles as yet. In the present study X-ray absorption spectroscopy was the principal technique used to provide insight into the surface structure of TiO2 nanoparticles. The pre-edge XANES has been investigated in detail theoretically in the past and numerous studies have attempted to reproduce the pre-edge using a variety of theoretical approaches. As a consequence it appears that the pre-edge structure of both anatase and rutile are reasonably well understood. For highly crystalline specimens of anatase, four features predominate in the pre-edge. We label these as A1-A3 and B in accordance with previous studies. There appears to be general agreement that A3 and B features in the anatase preedge are due to formally dipole forbidden transitions to core electron hybridized states.82,83 The A1 transition has been shown to have predominantly quadrupole character on the basis of orientation dependence and together with multiple scattering theory, has been attributed to transitions to 3d-4p hybridized states. There less certainty regarding the assignment of the A2 feature. Whereas Wu et al.82 give A2 a similar origin to the other A series transitions, Parlebas et al.83 do not distinguish the A2 and A3 peaks in their theoretical spectra, although they make specific mention of the fact that the experimental preedge of anatase needs four Lorentzian lines for satisfactory decomposition. However, the spectra of Parlebas et al. do reproduce the experimental spectra rather well in terms of both energy and intensity. The fact that the anatase pre-edge very much depends on medium range structure between 50 and 90 Å from the embedded Ti atom is emphasized through both of these diverse theoretical approaches. In contrast to anatase, rutile appears to show only three preedge transitions A1, A3 and B and Aifa et al.84 attributed the A1 and A3 transitions to dipolar transitions to t2g and eg, respectively, of the absorbing octahedra. These transitions are normally forbidden but are made allowed by thermal vibrations.

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6377 Jeanne-Rose85 has used multiple scattering theory to try to shed light on the pre-edge or rutile. Most recently Chaboy et al.75 have carried out detailed ab initio multiple scattering computations of the Ti K-edge XANES spectrum in the case of rutile focusing on the edge and have shown that with the use of clusters of 105 atoms the main edge features C1, C2, and D could be well reproduced. Therefore apart from the A2 transition in anatase, which does not appear to be present in rutile to any significant extent, the two polymorphs have very similar pre-edge and edge structures. Since it is the unique A2 feature which distinguishes anatase from rutile, and it is this feature which shows size dependent relative intensity variations in anatase nanoparticles, this suggests that the nanoparticle surface are implicated. Nevertheless, it is worth reconsidering whether A2 could be attributed to an ancillary phase such as depicted in Figure 14b. While the previously mentioned correlation in IA2/IA3 as a function of surface area-to-volume ratio seems to argue strongly against this possibility it is also unlikely on the basis that A2 is clearly discernible in even the high temperature commercial sample investigated here and in our previous work as well as in the sample investigated by Parlebas et al.83 Any remnant amorphous material would most certainly be expected to be converted to crystalline anatase on high temperature treatment. Although the exact origin of the sample used in study of Parlebas et al. was not stated one presumes it to be highly crystalline material prepared at high temperature rather than a nanocrystalline specimen. In the very early study of what was presumably also a highly crystalline phase-pure anatase sample, A2 was not clearly identified although its observation depends on the resolution with which the data is recorded.86 In further support of the genuine existence of A2, Choi et al.27 have recently also observed the feature in what was definitely a highly crystalline anatase sample. Further evidence in favor of attribution of A2 to the structure of the anatase nanoparticle surfaces derives from the fact that the pre-edge of the anatase nanoparticles can not be modeled through a combination of the pre-edge of crystalline and amorphous anatase or brookite as reported in section 3. The notion of a substantial contribution of distorted and coordinatively unsaturated Ti sites on anatase has gained further support in very recent molecular dynamics studies of anatase and rutile nanoparticles.87 These studies support surface bond lengths deduced from the fitting of the EXAFS of the nanoparticles although it is also claimed that lower coordination Ti is only found at the nanoparticle surface. Explicit inclusion of a simple adsorbed surface layer appears to have first been taken into account only in very recent computational studies of the surface and electronic properties of TiO2 nanoparticles having 29 Ti atoms.80 The striking, and perhaps surprising, observation that is now being made through the XAS of rutile nanoparticles is the lack of an A2 feature in the pre-edge over the entire size range investigated, and this suggests the virtual absence of coordinatively unsaturated Ti in rutile nanoparticles. Therefore, there is little difference in the rutile nanoparticle surface structures as a function of particle dimension insofar as the degree of Ti undersaturation is concerned as compared to anatase. This perhaps points to the putative overall greater average reactivity of anatase surfaces. Since as previously discussed all the common crystal faces of both anatase and rutile are likely to contain both fivecoordinate Ti the question must be asked as to why the preedge XANES of rutile nanoparticles ranging from 3-130 nm in diameter show no similar A2 feature and no variation with particle dimension as was observed for anatase nanoparticles

6378 J. Phys. Chem. C, Vol. 113, No. 16, 2009 over a similar size range. One possible explanation could be in the accessibility of the under-saturated Ti sites in rutile nanoparticles to atmospheric water. Clearly reactive undersaturated or Lewis acid sites will result in differing reactivity of the surfaces to different molecules which will in turn influence the photocatalytic activity. Interestingly, for bulk samples, Bezrodna et al.88 have recently undertaken a study of the surface acidity of the interactions of pyridine with anatase and rutile nanoparticles surfaces. The samples investigated were anatase and rutile with surface areas of about 100 m2/g which means the crystallite sizes were about 15 nm assuming spherical particles and an average density of 4.1 g/cm3. For anatase they observed two types of Lewis acid centers as well as Bro¨nsted acidity whereas for rutile no Bro¨nsted acidity was observed. This suggests that at least as far as pyridine is concerned coordinative unsaturated Ti sites are accessible on both materials. Therefore sol-gel anatase nanoparticles could have a core-shell type structure with the shell or interphase region having a higher concentration of distorted coordinatively unsaturated sites with the core being more crystalline or bulklike. Indeed, such a core-shell structure has been proposed for sol-gel nanoparticulate materials of various sorts as far back as 198789 and, in more recent times, by Dieguez et al. for nanocrystalline rutile-structured SnO2 based on the appearance of Raman bands due to disordered material that is not usually observed in bulk SnO2.57 The present study clearly indicates that the precise method by which sol-gel anatase nanoparticles of increasing size are synthesized (calcination or hydrothermal ripening) has little or no influence on the surface structure of the nanoparticles as gauged from the similarity of the XANES. Even so, the anatase nanoparticles prepared through calcination show optical band gaps that appear to be better behaved in terms of variation with particle size than those of sols ripened hydrothermally. If the core-shell model represents an appropriate description of sol-gel derived anatase nanoparticles then one must consider how such a structure develops during the synthesis process. Finnegan and Banfield90 have concluded that hydrothermal coarsening of anatase nanoparticles at low pH as is applicable here occurs through a combination of Oswald ripening and oriented attachment pathways. In the former, small particles dissolve and deposit on larger particles whereas the latter requires the aggregation of smaller particles to form larger particles. The pH used for the preparations of the present nanoparticles (pH < 1) is very low and therefore solubility of small particles is likely to be significant. Colloidal anatase sols are therefore likely to consist of an equilibrium distribution of species ranging from large to small nanoparticles and oligomeric and mononuclear hydroxo complexes such as Ti(OH)n(4-n)+.60 If this is the case, the outer surfaces of the particles could be undergoing a continuous dissolution reprecipitation process with limited scope for condensation either in the colloidal solution or after drying of the colloid. Indeed the thermal analysis results presented in the results section did indicate the possibility of dehydroxylation in the 200-400 °C temperature range that may at least in part be attributable to this poorly condensed surface structure consisting of the Ti(OH)n(4-n)+ species accreted from the solution phase and which are still present on the air-dried hydrothermally ripened particles as depicted in Figure 14d. If the core-shell model of anatase is correct (Figure 14a), and the sol-gel anatase nanoparticles can indeed be considered as a mixture of a crystalline core and a disordered shell with different electronic properties, then the observed variation in Eg with nanoparticle core dimension, which is what is measured

Luca by XRD, will depend on the relative proportions of the core and shell material. However, for rutile the shell dimension must be relatively small (Figure 14b) since the differences in the XANES with particle dimension are undetectable. In this case Eg depends directly on only the crystalline particle dimension. Because of the negligible influence of reduced particle size on the rutile XANES it is reasonable to hypothesize that for rutile nanoparticles the interphase region has to either be very thin and/or contain few reduced coordination Ti sites compared with anatase nanoparticles (Figure 14c). In support of this notion is the fact that disorder bands are not observed in the Raman of rutile nanoparticles as they have been observed in the rutilestructured form of SnO2 and attributed to an interphase region of constant width. Because of the ostensible absence of an interphase region of significant thickness, rutile nanoparticle surfaces appear to be terminated more cleanly and are smoother than in the anatase polymorph. They are consequently able to be fully coordinated by water molecules in the ambient state. It is hypothesized that this cleaner termination is at least partly responsible for the fact that rutile nanoparticles display quantum confinement characteristics that are more akin to those of more ideal semiconductors. The differences observed in the surface structures between anatase and rutile are made all the more remarkable when one considers also that similarly sized nanoparticles of both polymorphs should intuitively have a similar proportions of their total Ti on the surface as estimated using the relation 12.5/d put forward by Chen et al.91 This relation gives the fractions of exposed TiO2 as 3% for 40 nm, 6% for 20 nm, 25% for 5 nm, and 41% for 3 nm diameter particles. The apparent absence of 5-coordinate Ti sites on nanoparticulate rutile could be the result of the ability of water to complete the Ti coordination on the Ti surfaces whereas access to all 5-coordinate Ti in a thicker interphase region in anatase nanoparticles is simply not possible. The virtual absence of 5-coordinate Ti could also be partly responsible for the claimed lower photoactivity of rutile although this claim should be viewed with some skepticism being reliant on comparison of similar materials. In any case, the persistence of coordinative under-saturation of anatase nanoparticles even in the presence of atmospheric moisture does not necessarily preclude the more effective binding of other molecules such as phosphonate and/or carboxylates to the anatase nanoparticles relative to rutiles. This preservation of coordinative under-saturation of Ti even in ambient air for anatase may be responsible for differences in activity. Of course the exact nature of the interphase region in these two materials may depend on the methods used for preparation although the present results suggest that at least in the case of anatase there is little dependence on synthetic procedure used for nanoparticle preparation. 5. Conclusions Anatase and rutile nanoparticles prepared using sol-gel chemistry and grown both hydrothermally and through calcination have been studied by a range of techniques. Rutile nanoparticles do not show significant concentrations of coordinatively unsaturated surface Ti sites whereas anatase nanoparticles do irrespective of the preparation method. On the other hand rutile nanoparticles show a smooth dependence of Eg on particle diameters whereas anatase nanoparticles do not. Such significant discrepancies between two polymorphs that otherwise show very similar properties is rather surprising, and it is hypothesized that this is due to a difference in the ability of water to access under-saturated Ti sites on the anatase nanoparticle surfaces under ambient conditions.

Properties of Anatase and Rutile Nanoparticles Acknowledgment. The author is grateful to ANSTO colleagues, Darren Attard of ANSTO for the TEM particle size determinations, Dr, Tracey Hanley for the SAXS data, and Dr, Lou Vance for his advice on the preparation of the manuscript. X-ray absorption spectroscopy measurements were undertaken at the Australian National Beamline Facility and were supported by the Australian Synchrotron Research Program, which is funded by the Commonwealth of Australia under the Major National Research Facilities program. The author thanks Dr. James Hester and Dr. Garry Foran for their assistance with these measurements. Supporting Information Available: A considerable body of ancillary data is included. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Luca, V.; Djajanti, S.; Howe, R. F. J. Phys. Chem. B 1998, 102, 10650. (2) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (3) Muscat, J.; Swamy, V.; Harrison, N. M. Phys. ReV. B: Condens. Matter 2002, 65, 224112. (4) Yan, M. C.; Chen, F.; Zhang, J. L.; Anpo, M. J. Phys. Chem. B 2005, 109, 8673. (5) Park, N.-G.; van de Lagemaat, J.; Frank, A. J. J. Phys. Chem. B 2000, 104, 8989. (6) Carp, O.; Huisman, C. L.; Reller, A. Prog. Solid State Chem. 2004, 32, 33. (7) Yanagisawa, K.; Ovenstone, J. J. Phys. Chem. B 1999, 103, 7781. (8) Wang, J.; Guo, B. D.; Zhang, X. D.; Zhang, Z. H.; Han, H. T.; Wu, J. Ultrason. Sonochem. 2005, 12, 331. (9) Van Der Meulen, T.; Mattson, A.; Osterlund, L. J. Catal. 2007, 251, 131. (10) Li, G. H.; Chen, L.; Graham, M. E.; Gray, K. A. J. Mol. Catal. A: Chem. 2007, 275, 30. (11) Hurum, D. C.; Agrios, A. G.; Gray, K. A.; Rajh, T.; Thurnauer, M. C. J. Phys. Chem. B 2003, 107, 4545. (12) Zhu, X.; Birringer, R.; Herr, U.; Gleiter, H. Phys. ReV. B: Condens. Matter 1987, 35, 9085. (13) Munnix, S.; Schmeits, M. Phys. ReV. B: Condens. Matter 1985, 31, 3369. (14) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (15) Henglein, A. Chem. ReV. 1989, 89, 1861. (16) Chen, C. C.; Herhold, A. B.; Johnson, C. S.; Alivisatos, A. P. Science 1997, 276, 398. (17) Hodes, G. AdV. Mater. 2007, 19, 639. (18) Fernandez-Garcia, M.; Martinez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Chem. ReV. 2004, 104, 4063. (19) Monticone, S.; Tufeu, R.; Kanaev, A. V.; Scolan, E.; Sanchez, C. Appl. Surf. Sci. 2000, 162, 565. (20) Brus, L. J. Phys. Chem. 1986, 90, 2555. (21) Brus, L. Curr. Opin. Colloid Interface Sci. 1996, 1, 197. (22) Sandroff, C. J.; Kelty, S. P.; Hwang, D. M. J. Chem. Phys. 1986, 85, 5337. (23) Sandroff, C. J.; Hwang, D. M.; Chung, W. M. Phys. ReV. B 1986, 33, 5953. (24) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196. (25) Chen, L. X.; Rajh, T.; Jager, W.; Nedeljkovic, J.; Thurnauer, M. C. J. Synchrotron Radiat. 1999, 6, 445. (26) Choi, H. C.; Jung, Y. M.; Kim, S. B. Vib. Spectrosc. 2005, 37, 33. (27) Choi, H. C.; Ahn, H.-J.; Jung, Y. M.; Lee, M. K.; Shin, H. J.; Kim, S. B.; Sung, Y.-E. Appl. Spectrosc. 2004, 58, 598. (28) Fernandez-Garcia, M.; Belver, C.; Hanson, J. C.; Wang, X.; Rodriguez, J. A. J. Am. Chem. Soc. 2007, 129, 13604. (29) Bokhimi, X.; Morales, A.; Novaro, O.; Lopez, T.; Sanchez, E.; Gomez, R. J. Mater. Res. 1995, 10, 2788. (30) Bokhimi, X.; Morales, A.; Lucatero, M. A.; Ramirez, R. Nanostruct. Mater. 1997, 9, 315. (31) Li, Y.; White, T. J.; Lim, S. H. J. Solid State Chem. 2004, 177, 1372. (32) Li, G. S.; Li, L. P.; Boerio-Goates, J.; Woodfield, B. F. J. Am. Chem. Soc. 2005, 127, 8659. (33) Swamy, V.; Menzies, D.; Muddle, B. C.; Kuznetsov, A.; Dubrovinsky, L. S.; Dai, Q.; Dmitriev, V. Appl. Phys. Lett. 2006, 88. (34) Djerdj, I.; Tonejc, A. M. J. Alloys Compd. 2006, 413, 159. (35) Grey, I. E.; Wilson, N. C. J. Solid State Chem. 2007, 180, 670. (36) Barringer, E. A.; Bowen, H. K. J. Am. Ceram. Soc. 1982, 65, C199.

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