Preparation, characterization and surface structure of coprecipitated high-area Sr,Ti02 +J 0 <x <1) powders Josh Manuel Gallardo Amores," Vicente Sanchez Escribano," Marco Daturib and Guido Busca*b "Departamento de Quimica Inorganica, Faculdad de Quimicas, Universidad, Plaza de la Merced E-37008 Salamanca, Spain bIstituto di Chimica, Facolta di Ingegneria, Universita, P.le Kennedy, I-1 6129 Genova, Italy Powders with composition Sr,TiO, +, (x=0,0.02,0.11,0.42 and 1)have been prepared by coprecipitation from strontium hydroxide and titanium isopropoxide and have been characterized after drying at 393 K, and after calcination at 773 and 973 K, using XRD, TG-DTA, BET surface area and porosity measurements, FTIR-FTFIR and FT-Raman skeletal vibrational characterization, diffuse reflectance UV-VIS spectroscopy and FTIR spectra of the hydroxy groups and of adsorbed pyridine and CO,.Strontium was found to inhibit anatase crystallization, sintering and transformation to rutile. Strontium tended to be deposited at the surface of anatase and gave rise to a decrease of the surface acidity and an increase of surface basicity. Moreover, it shifted the absorption edge of anatase to higher energies. For x = 1, the truly cubic SrTiO, perovskite phase was produced at room temperature with high surface area. The surface of SrTiO, was definitely basic, with only strontium and oxygen ions exposed at the surface, and hydrogen impurities in the bulk. Perovskite-type metal titanates, e.g. SrTiO,, BaTiO, and PbTiO,, are widely applied in the ceramic and electronics industries, owing to their dielectric, ferroelectric, incipient- ferroelectric and piezoelectric properties.' These materials are also of interest for a number of other properties. SrTiO, is a semiconducting material, acting as a powerful resistivity sensor for ~xygen,~ is active in the photoelectrolysis of water4 and in other photocatalytic proce~ses.~ SrTiO, was also the first ceramic material found to become superconducting, at a very low temperature, below 0.3 K.6 Perovskite-like materials can also be applied in the field of heterogeneous catalysis as active phases7 or as potential catalyst supports.* Some of these properties appear to be peculiar to perovskite-type materials while others (like those related to photoconductivity and catalytic behaviour) appear to concern the nature of the Ti4+ -O2-bond and closely relate perovskite-like metal tita- nates to pure titanias and other titanates characterized by different crystal structures (e.g.ilmenites). Ti0,-anatase and rutile are mainly used in industry as white pigments' while anatase constitutes the active support of the industrial catalysts for o-xylene selective oxidation to phthalic anhydride' and for the selective catalytic reduction of nitrogen oxides by ammonia,'' both based on vanadium oxide. Anatase is also of interest because of the so-called strong metal support interaction (SMSI) phenomenon occurring when some metals are supported on it." Additives to Ti0,-anatase strongly influence several proper- ties, including its stability to sintering and to phase transform- ation to rutile.As a further development of our research on the surface behavior of titanias12?13 and perovskite-like fine powder^'^,'^ we investigated the preparation and the bulk and surface properties of high-area powders in the Sr,TiO,+, (0d xd 1) composition range. The aim of this study is twofold: (i) to test the surface properties and the thermal stability of high-area, nearly stoichiometric, SrTiO, powders; and (ii) to determine the properties of Sr,TiO,+, mixed oxides and the effect of Sr addition to high-area Ti0,-anatase powders. Experimental Preparation procedures Samples with composition Sr,TiO, +,(0dx d 1) were pre-pared via a conventional coprecipitation method, using Sr(OH), 8H,O (Strem Chemicals) and Ti[OCH(CH,),], (Aldrich) as precursors.Stoichiometric amounts of the stron- tium precursor dissolved in water, were added to the titanium precursor under continuous stirring. After water evaporation, the cake was dried at 393 K for 3 h in air. Finally, all samples were subjected to thermal treatments in air at different tempera- tures (773 and 973 K). Characterization techniques Curves were recorded by means of a Setaram TGA 92 12 apparatus, with a heating and cooling rate of 10 K min-'. XRD spectra were recorded on a Philips PW 2256/20 diffractometer (Co-Ka radiation, Ni filter; 40 kV, 20 mA). Cell parameters were calculated using dedicated least-squares software.The crystal size was evaluated using the Scherrer forrnula.l6 Microstructures were measured using N, adsorption at 77 K, with a conventional volumetric BET apparatus. TEM images were obtained using a high-resolution Zeiss instrument with a maximum magnification of 250000 x . The UV-VIS diffuse reflectance spectra were obtained using a Shimadzu spectrophotometer (model UV-240), previously calibrated with two analytical MgO samples. IR spectra were recorded with a Nicolet Magna 750 Fourier transform instrument. The skeletal spectra in the region above 400cm-' were recorded with KBr pressed disks and with a KBr beam splitter, while those in the far IR (FIR) region (400-50 cm -') were recorded using the powder deposited on polyethylene disks, and with a 'solid substrate' beam splitter.FT-Raman spectra were obtained through a Bruker RFSlOO instrument, with an Nd-YAG laser (1064 nm), using 30 mW laser power, 50 scans and 4cm-' resolution. The adsorption experiments were performed using pressed disks of the pure powders, activated by outgassing at 300-1070K into the IR cell. Results DTA and XRD characterization The DTA curves recorded in air for the Sr,TiO, +,(0d x 6 1) samples previously dried at 393 K are reported in Fig. 1. The pure titania sample shows a strong exothermic peak centred J. Muter. Chem., 1996, 6(5), 879-886 879 970 I R+P I 673 773 873 973 1073 1173 1273 TIK Fig. 1 DTA curves for Ti0, (a), Sr, ozTi02oz (b), Sr, llTi02 11 (c), Sr, 42T10242 (d), and SrTiO, (e) (Am =amorphous, A =anatase, R = rutile, P =perovskite at 970 K, with a tail at lower temperatures The XRD spectra of this matenal after drying already show the diffraction peaks of anatase (JCPDS file no 21-1272) with traces of those of brookite (JCPDS file no 29-1360), as reported in Table 1 The XRD patterns of this sample after calcination at 773 and at 973 K are shown in Fig 2(a) and 3(a) respectively At 773 K the material is still essentially composed of anatase with traces of brookite while at 973 K the material is almost totally converted to rutile According to these data and to our previous data12 13 17 the exothermic peak at 973 K relates to the anatase- rutile phase transition, while the previous signal is associated with anatase sintering The DTA curve for Sr, 02T102 02 is shown in Fig 1(b),while the XRD patterns of this sample are reported in Fig 2(b) and 3(b) The sample is still stable as anatase after calcination at 973 K and, correspondingly, the anatase-rutile phase transition is shifted to 1215 K in the DTA curve These data show that the addition of small amounts of Sr stabilizes the metastable form, anatase, towards its transformation to the thermo-dynamically stable form, rutile The data relative to the sample Sr,,,T10211 show a quite different picture In fact, the sample after drying appears amorphous to XRD analysis, while after calcination at both 773 and 973 K, virtually brookite-free and rutile-free anatase is observed [Fig 2(c) and 3(c)] The DTA trace shows a broad exothermic peak near 870 K, that is associated (according to XRD analyses) with the amorphous-anatase transition, occur- ring in the non-isothermal conditions of DTA experiments at higher temperatures with respect to isothermal calcination in the oven A weak peak observed near 1220K in the DTA curve is associated with a more complex phase transition from anatase to rutile +perovskite mixed phases, as deduced from the XRD analyses performed after the DTA run 880 J Mater Chem, 1996, 6(5),879-886 -I I11 73 65 55 45 35 25 2Bldegrees Fig.2 XRD of Ti0, (a), Sr, 02Ti02o2 (b), Sr, 11Ti0211 (c), Sr, 42Ti02 42 (d) and SrT103 (e), all calcined at 773 K Indexing in (c) is for anatase and in (e) for cubic perovskite 101 I I I211 'I0 r 1 7s 65 55 45 35 25 2Bldegrees Fig.3 XRD of Ti0, (a),Sr, ,,Ti02 02 (b),Sr, ,lTiOz 11 (c), Sr, 42T102 42 (d) and SrTiO, (e), all calcined at 973 K Indexing in (a) is for rutile, in (b) for anatase and in (e) for cubic perovskite Table 1 XRD and morphological data for Sr,TiO,+, samples cell parameters/,& composition crystal partic!e BET surface sample T/K phases SrTiO, 393 cubic perovskite +anatase 773 cubic perovskite +anatase 973 cubic perovskite +anatase 1243 cubic perovskite SrO 42Ti02 42 393 773 973 amorphous amorphous amorphous +cubic 1243 perovskite +anatase +rutile cubic perovskite Sr, ll-r-102 11 393 773 amorphous anatase 973 anatase 1243 cubic perovskite +rutile Sro 02T102 02 393 773 anatasef anatase +brookite 973 anatase 1273 cubic perovskite +rutile Ti0, 393 773 anatasef anatase +brookite 973 rutile +anatase (Yo)" a c sizeb/,& size'/A area/m2 g-' DMd/nm VTPe/ml g-' 97 3.9347 - 276 3 20 87 5.5 0.12 trace 98 3.9103 - 322 380 50 10.1 0.12 trace 98 3.9296 - 344 420 20 7.1 0.04 trace 100 3.9067 - 100 - - - - 339 5.1 0.40 100 - - - 60 118 7.4 0.22 41 7.0 0.07 3.9250 - 193 260 70 3.9079 380 100 - - - - 41 1 3.1 0.32 100 3.8012 9.4758 135 145 125 2.4 0.08 100 3.8036 9.47616 118 175 39 8.6 0.08 21 3.9109 380 79 4.5922 2.959 1 158 100 3.81222 9.36248 70 85 236 5.0 0.29 92 3.7889 9.43837 79 100 129 7.2 0.23 100 3.8051 9.5596 135 155 49 8.2 0.10 3.9212 365 92 4.5892 2.9591 241 - 3.8224 9.36817 49 120 240 5.6 0.33 92 3.7852 9.4905 147 160 75 10.9 0.20 94 4.5821 2.9531 538 590 5 9.9 0.01 "Evaluation based on the relative intensities of the XRD diffraction peaks.bFrom XRD data. 'From TEM data. dDM=average pore diameter. eVTp=total pore volume. 'Brookite is also present, according to Raman spectra. The DTA curve of the Sr0.42Ti02.42 sample shows a sharp split peak at 1025-1042 K [Fig. l(d)]. XRD analysis shows that this material is still completely amorphous after calci- nation at 773 K [Fig. 2(d)], while it consists of a mixture of amorphous material and of the perovskite SrTiO, (JCPDS file no.35-734) after calcination at 973 K. After the DTA run a poorly crystallized mixture of anatase, rutile (JCPDS file no. 21-1276) and perovskite is observed [Fig. 3(d)]. The DTA curve relative to the stoichiometric material SrTiO, does not show any definite peak. The XRD patterns show that the well crystallized cubic perovskite phase of SrTiO, (which is thermodynamically stable in air) is already present after drying [Fig. 4(u)], so that only a partial increase of crystal size by further calcination is observed [Fig. 4(b) and (c)]. These data show that the addition of Sr to TiO, at low Sr loadings inhibits the anatase-rutile phase transition while, at higher loadings, it also inhibits anatase crystallization from the amorphous state.In contrast, when Sr :Ti =1 : 1 the copre- cipitation produced the cubic perovskite structure, already well crystallized. This behaviour differs with respect to that of the Sr-Zr system which, at Sr :Zr z 1:1, gives rise to amorphous materials that crystallize to form the orthorhombic perovskite SrZrO, only after calcination at 973 K." This may be related to the larger size ofJhe Zr4+ cation with respect to the Ti4+ cation (0.80 US. 0.68 A, respectively, after Pauling18). Zr4+ does not enter readily into the BO, octahedra of the ABOJ perov- skites, thus inducing an orthorhombic distortion in the SrZrO, perovskite and inhibiting its crystallization. In contrast, owing to its smaller size, Ti4+ enters the perovskite structure more readily, and does not cause distortion when A =Sr2+.No phases other than 'pure' Ti02 and SrTiO, were found in our samples, in agreement with the lack of detection of any phase between TiO, (rutile) and SrTiO, (cubic perovskite) in the phase diagrams of the SrO-TiO, system." Analysis of the unit-cell parameters of rutile, anatase and perovskite phases in our powders (Table 1) does not provide any evidence of reciprocal solubility of TiO, and SrTiO,, according to phase diagrams." Morphology characterization The morphological properties of the materials after drying and after calcination at 773 and at 973 K are summarized in Table 1. The surface area measurements show that the addition of small amounts of Sr to anatase tends to increase its surface area, in agreement with the stabilization of the amorphous phase with respect to the crystallization of anatase and of the anatase phase with respect to its conversion to rutile. This agrees with the previous results which show that the dopants that inhibit the anatase-rutile phase transition also inhibit the loss of surface area of anatase, and with the mechanism we previously proposed for this.', The surface areas are smaller for SrTiO, in the crystalline perovskite form. However, the surface areas obtained for SrTiO, are definitely higher than those obtained with similar preparation procedures for other perovskites, including SrZrO,." This is certainly associated with the low temperature of crystallization of such a cubic phase, in contrast with SrZrO,, which cannot be produced in a crystalline form at low temperature.The parent compound BaTiO,, which can also be obtained in a crystalline, apparently cubic, form at very low temperat~res,'~ is also obtained with rather high surface areas. J. Muter. Chem., 1996, 6(5), 879-886 881 Comparison of the theoretical surface areas, measured from the XRD crystal size assuming cubic particles, with the exper- imental surface areas and those forecast on the basis of TEM images gives satisfactory agreement for SrTiO, According to TEM images, the SrTi0, powder consists, immediately after drying, of well defined globular particles This agreement indicates that the SrTiO, particles are composed essentially of single crystals In contrast, in Ti0,-anatase and in strontium- deficient materials, the 'theoretical' surface areas measured from the XRD crystal size are higher than those observed experimentally On the basis of TEM images, in these cases the particles are more irregular and are probably polycrystal- line, with possibly some amorphous material present also, even after calcination The porosity data for SrXTiO2+, (0<x< 1) samples are summarized in Table 1 The shapes of the experimental Nz adsorption isotherms of the dried samples are intermediate between type I and type IV in the IUPAC classification," corresponding to micro-mesoporous solids They covert to type IV isotherms for samples calcined at 773 K The increasing calcination temperature promotes isotherm transformation from type IV to type I1 and the disappearance of the hysteresis loop The variation of average pore diameter reaches a mini- mum value for the x=O 11 sample, which also shows the highest surface area after drying Bulk vibrational characterization In Fig 5 the FTIR skeletal spectra of the samples Ti02, Sr, 02T102 o2 and Sr, 42Ti02 42, all calcined at 773 K, are reported The spectrum of T102 corresponds with those reported in the literature and with those we discussed pre- viously12 The main absorptions near 700 (very broad shoulder), 425, 330 (shoulder), 260 and 175 cm-' (weak) are typically found in anatase samples with small particle sizes Weak additional components in the 550-650 cm-' region are probably associated with brookite impurities l2 The addition 100 75 65 55 45 35 2s 2Bldegrees Fig.4 XRD of SrTIO, dned at 393 K (a) and calcined at 773 K (b) and 973 K (c) Indexing is for cubic perovskite structure 882 J Muter Chem , 1996,6(5), 879-886 1200 1000 800 600 400 200 wavenumber/cm-l Fig. 5 Skeletal FTIR-FTFIR spectra of T102 (a), Sr, 02T102o2 (b), and Sr, 42T10242 (c),all calcined at 773 K of small amounts of Sr seems to cause mainly the decrease of the bands associated with brookite and of the lowest frequency band However, the general spectrum of anatase remains almost intact In contrast, the spectrum of Sr, ,,Ti02 42, which is X-ray amorphous, is definitely different, being dominated by a very broad band centred near 570 cm-' and a sharper band at 255 cm -The FTIR spectra of the SrTiO, samples (Fig 6) present a typical spectrum with the main bands at 550-555, 450 (shoulder), 405,250 and 150 cm-' The spectrum is similar but with significant band shifts with respect to those discussed previously for a commercial low-area SrTi0, sample,I2 and is also in approximate agreement with the spectra of different SrTi03 samples reported by Diaz-Guemes et aZZ1In our case, the calcination treatment does not change the spectrum of SrTi0, very much, thus showing that the crystal shape is not modified significantly The sharp band at 880-860cm-' for samples with x30 42 is due to traces of SrCO, (out-of-plane deformation of the carbonate ion) The FT-Raman spectra of Ti02 and of Sr, llTi02 11 calcined at 773 K are reported in Fig 7 The FT-Raman spectrum of the strontium-free sample (and of Sr, ,,T102 02, which is very similar) show the very intense peaks of Ti0,-anatase 639, 517, 398, 196 and 144cm-', which correspond to the six funda- mental modes of this structure, because of the superimposition of two of them However, weaker features already assigned to brookite can also be found at 453, 366, 323 and 246 cm-' l2 The FT-Raman spectrum of Sr, ,,TiO2 11 presents the anatase peaks, with nearly the same intensity as pure TiO,, but the brookite peaks decreased Moreover, the scattering baseline increases progressively from ca 1000 cm-' towards lower frequencies, and a broad component can be found in the region 950-700cm-' These features may be associated with the increased disorder corresponding to the decrease of crystal- linity Further increase of the Sr content causes the almost 1200 1000 800 600 400 200 wavenumberfcm-l Fig. 6 Skeletal FTIR-FTFIR spectra of SrTiO, calcined at 393 (a), 773 (6) and 973 K (c) y 0.25.1200 1000 800 600 400 200 wavenumberfcm-1 Fig. 7 Skeletal FT-Raman spectra of TiO, (a)and of Sr,,,,Ti02.1, (b), calcined at 773 K (two different scale expansions) complete disappearance of the anatase peaks (for samples calcined at 773 K). Very weak and broad Raman peaks are observed for Sro.42Ti02.42 calcined at 773 K, near 870 and 240 cm-I [Fig. 8(a)], which could be associated to scattering from amorphous materials. The broad peaks are also found in -~ k , , a 1 , , , , , , 1 1200 lo00 800 600 400 200 wavenumberkm-1 Fig.8 Skeletal FT-Raman spectra of Sro,42Ti02,42[(a),(c)] and SrTiO, [(b),(d)]calcined at 773 K [(a),(b)] and at 973 K [(c),(d)] the case of SrTiO, calcined at 773 K [Fig. 8(b)]with additional peaks at 549cm-' (rather broad) and at 180 and 149cm-1 (both sharp). Further heating at 973 K causes the almost complete disappearance of the broad peaks near 870 and 240 cm-' [Fig. 8(c) and (41,so that the Raman patterns are even weaker. However, in the case of SrTiO,, weak scattering peaks are still apparent at 550, 470, 178 and 148crn-'. The sharp peak near 1070 cm-' (out-of-plane deformation of the carbonate ion) confirms the presence of traces of SrCO, in the samples with x20.42.Note that the 'cubic perovskite structure' of SrTiO,, space group Pm3m =Ot,2 = 1, is first-order Raman-silent. In fact, the irreducible representation for the optical modes is: To,,=3 Flu(IR)+F,, (La.) This means that only three triply degenerate IR-active modes are expected, with no Raman-active fundamentals. However, Raman-active combinations and overtones can be found using low-temperature monocrystal mesurements.22 The peaks we observe, however, do not correspond to such a second-order spectrum. Nevertheless, the peaks we found correspond nicely to those assigned by Nilsen and Skinner22 at 551,450, 176 and 146cm-', for an 'impure' SrTiO, monocrystal, to the three IR-active translational fundamental optical modes (vq 55 1, v2 J.Muter. Chem., 1996, 6(5),879-886 883 176 and v1 146cm-l) and to the longitudinal component of v4 (450 cm-l) These authors attnbute the appearance of these peaks in the Raman spectra to a distorsion of the impure crystal at low temperature In our case, these modes may appear due to imperfect crystallinity and to the presence of defects The extremely weak Raman spectra show that the SrTi0, samples we prepared are 'truly' cubic perovskites, since they are essentially Raman-silent This differs strongly from the high-area sample of BaTiO, we investigated previo~sly,~~ that looks cubic according to XRD but gives rise to a very strong Raman pattern similar to that of the 'normal' tetragonal structure, whose tetragonal domains are probably very small and disordered, thus being averaged upon XRD analysis UV-VIS characterization In Fig 9 the UV-VIS diffuse reflectance spectra of TiO,, Sr, 02T102 02, Sr, llTi02 11, Sr, 42T10242, and SrTiO,, all cal- cined at 773 K, are reported All present an absorption edge in the region 350-450 nm, and possibly two main absorptions at lower wavelengths The electronic structure of T102polymorphs and of perovsk- ite-type titanates has been the object of previous experimen- ta15 23 24 and the~retical~~ investigations The valence band is generated by the 2p oxygen orbitals while the conduction band is essentially due to the 3d orbitals of titanium, thus the absorption edge is due to an 02-+Ti4+ charge-transfer transition Our spectra show a progressive shift to lower wavelengths of the edge by increasing Sr content for the samples TiO,, Sr, 02Ti0202, Sr, ,,T102 11 and Sr, ,,Ti02 42, from ca 390 to ca 320 nm Correspondingly, the onset shifts from ca 430 nm to 380nm (E, shifting from 288 to 3 26eV) Examination of the spectra seems to indicate that an higher wavelength component of the absorption shifts to lower wave-length or progressively disappears, while the lower wavelength component remains unaffected However, in the case of the crystalline SrTiO, samples a component grows again and gives rise to a nearly split edge at 320 and 370 nm The UV-VIS spectra of our SrTiO, samples do not change significantly with increased heating temperature and agree well with the spectra of bulk SrTiO, samples reported '24 According to Kutty and Avudaithai,' bulk SrTiO, has a bandgap of 3 2 eV (387 nm), evident as a pro- nounced shoulder similar to that found (at a slightly higher wavelength) in Fig 9, and also shows three absorption compo- nents whose positions depend on the preparation procedure The position of the higher wavelength component found by 200 400 600 BOO wavelengthfnm Fig.9 UV-VIS diffuse reflectance spectra of Ti0, (a), Sr, 02Ti02o2 (b), Sr, 11T10211 (c), Sr, 42T10242 (d) and SrTi03 (e),all calcined at 773 K 884 J Mater Chew, 1996, 6(5),879-886 these authors (300-350 nm) depends strongly on the crystal size, while the other two (265-270 and 210nm) are less sensitive According to the literat~re,~ 23 25 the position of the edge in titanates is also associated with the more or less pronounced deformation of the octahedron around titanium, and with the arrangement of the octahedra in the structure, which modifies the breadth of the lower energy t,, part of the Ti d band According to these data, the shift to lower wavelength of the edge with Sr addition in the SrxTi02+x samples (x<<l) seems to occur in parallel with the progressive decrease of the crystallinity of anatase to an amorphous material In effect, the dried precipitates, which are amorphous or poorly crystal- line, show their edges at even lower wavelengths The data reported above show that the addition of Sr causes a deep modification of the electronic state of anatase, which may be important in relation to the use of Ti0,-anatase as a support for vanadia catalysts These materials need mor- phology and structure stabilizers, in order to inhibit anatase sintering and transformation to rutile, both of which are favoured by vanadium oxide l7 On the other hand, basic dopants should also be added, to improve selectivity in the oxidation of hydrocarbons In both respects, Sr addition may be beneficial However, according to our interpretation, the role of anatase as an optimal support for these catalysts lies in the ability of this semiconductor phase to interact with V centres and exchange electrons with them The significantly different energy gaps in the two T10, polymorphs anatase and rutile (E, anatase >E, rutile) may be a reason for their different behaviour in this respect The addition of Sr to anatase, resulting in a further enhancement of E,, does not necessarily further improve the behaviour Surface characterization by FTIR of adsorbed probe molecules To gain information on the nature of the cationic centres exposed at the surfaces of these solids, we investigated the adsorption of the basic probe pyridine (Fig 10) As is well known, some bands of pyridine are sensitive to the strength of the coordinative interaction involving its own nitrogen lone pair The most sensitive bands are the so-called 8a, 19b, 12 and 1 modes, which are observed in liquid pyridine at 1580, 1438, 1029 and 991 cm-' 26 all of which tend towards higher wavenumbers the stronger the interaction, 1 e the stronger the Lewis acidity of the site 27 According to previous studies,28 the positions of the 8a and 19b modes for pyridine adsorbed on Ti0,-anatase are ca 1608 and 1445 cm-l, which manifest the medium Lewis acidity of Ti4+ cations The 12 and 1 modes fall close to the cut-off limit of T102 By increasing the Sr content, a new band arising from the 8a mode adsorbed on a different site grows progressively at 1595 cm-' and becomes predominant with respect to the band at 1608cm-I in the case of the sample Sr, ,,T102 11 Correspondingly, the cut-off limit shifts to lower wavenumbers and the 12 and 1 modes can be better observed They are both split and their higher frequency components at 1044 and 1012 cm-' decrease in intensity while their lower frequency components at 1034 and 1001 cm-' increase in intensity, caused by increasing the Sr content Meanwhile, the 19b mode shifts progressively to 1441 cm-' The evident splitting of the 8a, 12 and 1 modes in the Sr,TiO,+, samples with 0<x <1 demonstrates that two defi- nitely different sites are present on such surfaces, whose relative amounts progressively invert The 8a, 19b, 12 and 1 modes at 1608, 1446, 1044 and 1012cm-' are typical of pyridine mol- ecules coordinated on sites with medium Lewis acid strength, 1 e on coordinatively unsaturated Ti4+, very similar to those observed on anatase In contrast, the bands at 1596, 1441, 1034 and 1001 cm-' are associated with molecules coordinated I I 1&0 1400 12bo 1000 wavenumber/cm-1 Fig.10 FTIR spectra of the adsorbed pyridine species on pressed disks of T10, (a), Sr, ,,TiO, 02 (b), Sr, llTi02 l! (c), Sr, ,,Ti02 42 (d) and SrTiO, (e), all calcined at 773 K and activated at 773 K, and outgassed at 373 K after pyridine adsorption to sites with low Lewis acidity, which can be identified as Sr2 cations.+ The bands associated with pyridine interacting with Sr2+ ions are already intense for the sample Sr0.02Ti02.02, and are definitely predominant with respect to those of pyridine inter- acting with Ti cations for the sample Sro.llTi02 ll.This should indicate that Sr2+, although coprecipitated with TiO,, is located preferentially at the surface of anatase. We can calculate that for Sr, ,,T~O, ,,,assuming all Sr2+ cations are located at the surface, 88 A2 per Sr atom is available. The same calculation for the sample Sr, 11Ti02 11 gives rise to the a cation density of 1 Sr cation per 17.2A2. These data can be compared with the cation dEnsity for a perfect {OOl} plane of anatase, 1 Ti ion pFr 14.2 A,, and for the {1001 face of SrTiO,, 1 Sr ion per 15.2 A,. This means that, in the case of the sample Sr0.02Ti02.02, all Sr ions can be easily accommodated at the surface; for x= 0.11, Sr ions (if all are located at the surface) would already saturate the surface.For higher x, Sr is necessarily also in the bulk. Interestingly, on SrTiO, only the bands of pyridine adsorbed on Sr2+ are observed. Thus, it is concluded that at the surface of SrTiO, only Sr2+ cations are exposed while Ti4+ cations remain coordinatively saturated in a lower layer. The positions of the main bands of adsorbed pyridine on SrTiO, are similar here with respect to those observed pre- viously on SrZr03,15 on BaTi0,I4 and on lanthanum metal- 1ates.l' It can be concluded that the surfaces of ABO, perovskite-type structures, which are generated by the presence of very large A cations that cannot enter a close-packed array of oxygen ions where, instead, the B cation can enter, are dominated by the presence of A cations. These sites, due to their large size and relatively small charge, are very weak Lewis acids, even if they are coordinatively unsaturated.Correspondingly, studies of CO, adsorption on Sr,TiO, + show that, by increasing x, carbonate species in increasing amounts and adsorbed with increasing strength are formed. The desorption of such carbonate and bicarbonate species from TiO, can be obtained by outgassing at room or only slightly higher temperature, according to the literat~re.~~?~' In contrast, on SrTiO,, to desorb partially carbonated species outgassing at a high temperature such as 973 K must be accomplished. Similar data have been found over other perovskite-type corn pound^.'^^^^ This demonstrates that the very weak acidity of such surfaces goes together with a very strong basicity and/or nucleophilicity, reasonably due to highly uncoordinated exposed oxide ions.py,,,.,--7 400 i900 5600 3'00 3600 Ed0 3400 I300 3200 3130 wavenumberkm-1 Fig. 11 FTIR spectra of the OH groups of TiO, (a), Sro ,,T102 11(b), Sr, 42Ti0242 (c) and SrTiO, (d), all calcined at 773 K and outgassed at 773 K IR evidence of bulk hydrogen impurities in SrTiO, In the IR spectra of all samples (pure powder pressed disks) bands in the OH stretching region 3800-3000cm-1 are observed (Fig. 11). We can distinguish in all cases a weak complex band in the region 3730-3680cm-1. This is the typical region for the free surface OH groups on metal oxides, and, in particular, on anatase and rutile Ti0228.31 as well as on alkaline-earth-metal oxides.31 However, this absorption seems to be decreased in intensity by increasing the Sr content.This can be interpreted, assuming that Sr2+ exchanges H+ at the OH groups, according to its location on the surface, as deduced by pyridine adsorption experiments, and to its effect on sintering. At lower frequencies a strong sharp band is observed only in the case of SrTiO, at 3403cm-1 which may have a component on its lower frequency side. This band, which is definitely unusual for both binary and ternary metal oxide^,^"^^ corresponds to the band observed at 3477cm-1 in the IR spectrum of high-area BaTiO, ~0wders.l~ This band has also been observed by several authors in SrTiO, monocrystals, and was assigned to bulk hydrogen It is well known that hydrogen impurities can penetrate several perovskite structures36 as Hf bonded to a lattice oxygen in the form of an OH-group.These protons can compensate the cation charge defect due either to reduced centres like Ti3+ (or trivalent dopants) or to cation vacancies in non-stoichio- metric samples. Conclusions The conclusions from the present study are as follows. (i) The addtion of Sr to TiO, hinders both the crystallization of anatase from the amorphous state and the phase transform- ation to rutile. (ii) Sr addition also hinders the sintering of anatase, thus allowing higher surface areas to be obtained. (iii) Sr addition to anatase causes a significant shift of the absorption edge to higher energies, thus evidencing an import- ant electronic perturbation of the solid.(iv) Although coprecipi- tated with titanium, strontium cations tend to cover the TiO, surface, possibly by exchanging with H+ of the surface hydroxy groups, and this leads to a drastic modification of the surface chemistry of anatase. (v) When Sr :Ti =1:1 the sample crys- tallizes at room temperature into the cubic perovskite phase, with surface areas approaching 90 m2 g-' that progressively decrease by calcination. (vi) The Raman spectrum shows that this phase is truly cubic, since it is essentially Raman silent. Only extremely small distortions give rise to very weak Raman peaks. This behaviour differs from that of high-area BaTiO, which appears cubic to XRD but is 'microscopically' tetragonal J.Muter. 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