Structural, Electronic, and Electrical Properties of Y-Doped Cd2SnO4

Article pubs.acs.org/JPCC

Structural, Electronic, and Electrical Properties of Y-Doped Cd2SnO4 Domenico Andrea Cristaldi,† Giuliana Impellizzeri,‡ Francesco Priolo,‡ Tarkeshwar Gupta,§ and Antonino Gulino*,† †

Department of Chemistry, University of Catania and INSTM UdR of Catania, Viale Andrea Doria 6, 95125 Catania, Italy MATIS IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, Via S. Sofia 64, 95123 Catania, Italy § Department of Chemistry, University of Delhi, Delhi 110 007, India ‡

ABSTRACT: Cd2SnO4 shows interesting electrical and optical properties that can be useful for many different applications. Apart from our own previous study, there have been no reports on the influence of rare-earth doping on this ternary oxide. Therefore, the synthesis of Y-doped orthorhombic Cd2SnO4 has been optimized, and the resulting solid solutions have been characterized by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, and electrical measurements. XPS results highlight yttrium segregation to the sample surface, and electrical measurements show that the carrier concentration of yttrium-doped samples increases by an order of magnitude with respect to the undoped Cd2SnO4, where the lowest adopted Y-doping level (0.1% with respect to the cadmium) is the most promising in terms of conductivity, carrier concentration, and mobility. Temperature-dependent electrical measurements highlighted that both undoped and Y-doped Cd2SnO4 are degenerate semiconductors.

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INTRODUCTION Both cadmium oxide (CdO) and dicadmium stannate (Cd2SnO4) show interesting electrical and optical properties to make them useful as photosensitive anode materials for photochemical cells and other solar energy applications.1−16 Their thin films show light transparency in the UV and high reflectivity in the IR regions and this behavior renders them useful as heat reflectors.7 CdO adopts the centrosymmetric rocksalt structure (cubic face-centered system). The post transition-metal oxides, CdO, In2O3 and SnO2, show bandgaps of 2.3, 3.7, and 3.6 eV, respectively, between the O 2p based valence band and the 5s metal-based conduction band minimum.17,18 In addition, CdO represents a material with a large linear refractive index (n0 = 2.49).17,18 This fact, associated with a narrow bandgap, in turn, causes a large third-order optical nonlinearity at the nonresonant region.19−22 As the particle size decreases down to the nanometer scale, its nonlinear optical response is further enhanced due to the quantum size effect.19−22 Polycrystalline Cd2SnO4 adopts an orthorhombic structure in which SnO6 octahedra form chains by sharing edges along the [001] direction, and these linear chains are held together by CdO7 polyhedra. The Sn4+ cation maintains the same coordination environment shown in SnO2, whereas the Cd2+ site geometry deviates from the octahedral symmetry of the CdO. Properties of Cd2SnO4 materials have been extensively studied. Intrinsic stoichiometric defects render the Cd2SnO4 solid solution an n-type semiconductor. It combines the properties of SnO2 and CdO because it has high electron mobility, high electrical conductivity, and low visible absorption. Therefore, many of the properties of both CdO and Cd2SnO4 are originated by their nonstoichiometric composition, which, in turn, strongly depends on the synthetic procedure adopted.13,23 In fact, in these compounds different amounts of n-type defects as cadmium interstitials or intrinsic oxygen vacancies produce donor © 2012 American Chemical Society

states in the bulk bandgap, whose carrier concentration ranges from semiconductors to degenerate metallic conductors (CdO).13,16,23−25 In Cd2SnO4 films obtained by R.F. sputtering, electrical conductivities up to 6500 Ω −1 cm−1 have been achieved.26,27 Instead, the conductivities observed in powder,16 spray-deposited,28 and “electroless” deposited29 Cd2SnO4 films do not show these high values. The observed differences in conductivity are essentially due to carrier mobility values that are approximately one order of magnitude lower than those measured in films obtained by R.F. sputtering.26 Presumably, the reduced mobility is a consequence of the spray and “electroless” techniques that create grain boundary imperfections. Other ways to increase the carrier density are represented by the chemical doping or by syntheses under reducing condition. In these cases, the Cd2SnO4 yellow-green materials resulted in a higher carrier concentration13,23,30 that, in turn, should increase the conductivity and significantly change both electronic and spectroscopic properties.13,23 Already reported experimental results are indicative of the presence in the green phase of a concentration of conduction electrons higher than that found in the yellow phase.5,18−20 Nevertheless, a lower electrical conductance, due to a lower electron mobility, has been observed in the green phase.12,23−25 Concerning the optical properties, an increase in the carrier density will also increase the absorption by free carriers. Therefore, to get both high transparency and conductivity, the mobility must be increased. Moreover, it has been reported that the photoelectrochemical properties of polycrystalline Cd2SnO4 are better than those of single-crystal samples, possibly because of a lower concentration of oxygen vacancies in the latter.15 In addition, Received: October 28, 2011 Revised: December 15, 2011 Published: January 13, 2012 3363

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expected because in all synthetic steps there is no possibility to lose matter. X-ray diffraction (XRD) data were recorded on a Bruker D5005 diffractometer operating in a θ-2θ geometry (Cu Kα radiation, 30 mA and 40 kV) over an angular range 20 < 2θ < 80° (0.02° stepsize) with a 20s/channel dwell time.32 XRD crystal parameters have been obtained using the UNIT CELL programm.33 Angle-resolved X-ray photoelectron spectra (AR-XPS) were measured on pellets at 30 and 80° takeoff angles relative to the surface plane with a PHI 5600 Multi Technique System, which offers a good control of the photoelectron takeoff angle (base pressure of the main chamber 2 × 10−10 Torr).34 Samples were excited with a monochromatized Al Kα X-ray radiation using a pass energy of 5.85 eV. No relevant charging effect was observed. Structures due to satellite radiation have been subtracted from the spectra. The XPS peak intensities were obtained after Shirley background removal.34 The atomic concentration analyses were performed by taking into account the relevant atomic sensitivity factors. The instrumental energy resolution was ≤0.3 eV. The remaining adventitious carbon contamination of the surfaces (C 1s at 285.0 eV binding energy) gave a measured XPS concentration of 3%. The surface morphology studies were carried out by atomic force microscopy (AFM), and the images were obtained by an instrument manufactured by the NT-MTD. The noise level before and after each measurement was 0.01 nm. AFM characterizations were performed in a high-amplitude mode (tapping mode). The electrical characterization was carried out by means of fourpoint probe and Hall effect measurements, in the 70−420 K temperature range, by using the BioRad HL5560 equipment. The applied magnetic field was 0.322 T. The samples, patterned according to Van der Pauw geometry, were circular-shaped (1 cm in diameter) and simply contacted by four Au tips. This technique allows the determination of the resistivity, the carrier concentration, and the mobility (the latter determined by the first two quantities). The four-point probe and Hall effect measurements resulted repeatable within 5% and independent of samples handling.

the spinel Cd2SnO4 films have shown conductivity values above 8 × 103 Ω−1cm−1.9 Besides, interesting results on nanoimprinting patterning of thin cadmium stannate films have recently been reported.1 Obviously both CdO and Cd2SnO4 have been doped with different cations.12,23−25 For example, thin CdO films with maximum conductivities of 8540 and 17800 Ω−1cm−1 on glass and MgO (100), respectively, were obtained at 1.2 to 1.3% Y doping.3 Moreover, both In- and Sb-doped Cd2SnO4 have revealed that dopants enhance the number of metal-based (Cd 5s) conduction electrons, even though the conduction band structure seems to be less affected.23−25 Nevertheless, the cadmium indate−cadmium stannate system has shown conductivity values of 3.5 × 103 Ω−1 cm−1.7 We have previously prepared and studied yttrium-doped Cd2SnO4 polycrystalline materials.31 To our knowledge, there has not been any other report on yttrium-doped Cd2SnO4 apart from our above investigation. In the present study, we optimized the synthesis and now report on the electrical and spectroscopic characterization of orthorhombic yttrium-doped Cd2SnO4 to provide further insight into the influence of yttrium (rare earth) doping.

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EXPERIMENTAL DETAILS Cadmium oxides are remarkably toxic, therefore, all sample manipulations were performed under the hood, using gloves and glasses. The 0.1, 1, 1.5, and 2% Y-doped cadmium oxide (Cd1−xYxO) sample powders were synthesized using the following procedure. Appropriate quantities of CdO and Y(NO3)3·6H2O were dissolved in a 0.05 N HCl solution and water, respectively. For each doping level, their solutions were mixed together and, under continuous stirring, a quantity of citric acid (complexant) corresponding to the double of the total metal amount in mole was added. Then, the resulting mixture was slowly neutralized with conc. NH3, poured in a Teflon-lined stainless-steel sealed autoclave, and maintained at 180 °C for 24 h. Subsequently, the autoclave was naturally cooled to room temperature, and the whole material was dried at 130 °C. The resulting powder was introduced in an alumina crucible, placed in an oven, and heated under a constant oxygen flow (50 sccm) at 850 °C for 120 h. An identical procedure was used to prepare some undoped CdO. After calcination, brown powders were obtained and characterized. Polycrystalline undoped and Y-doped Cd2SnO4 powders were synthesized by thermal reaction, in air at 1050 °C for 10 h, of 2:1 stoichiometric mixtures of the above Cd1−xYxO and SnO2. All thermal treatments were performed in recrystallized alumina crucibles with closely fitting lids, leaving the products to naturally cool to room temperature.30 Both undoped and Ydoped Cd2SnO4 powders were pressed into pellets (49 MPa = 500 kg/cm2), heated in air to 1000 °C for 10 h, and then quenched to room temperature.23 Energy-dispersive X-ray (EDX) measurements were performed to check the stoichiometry of the final pellets. Results indicated compositions identical to the nominal concentrations (Y, Cd, Sn). Moreover, the absolute Y-doping level was also established by inductively coupled plasma mass spectrometry (ICP-MS). The analysis was performed after fusing doped Cd2SnO4 powder in Na2CO3 in a Pt crucible at 1000 °C, followed by dissolution in dilute HCl. The measured Y content was always identical to the value expected from the nominal doping level. This result was highly

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RESULTS AND DISCUSSION XRD Data. XRD results confirm that CdO and all synthesized Y-doped CdO powder materials are in a single cubic phase. The Y3+ ionic radius (1.06 Å) is bigger than the Cd2+ analogue (0.97 Å); therefore, a CdO cell expansion is expected upon yttrium doping and, indeed, observed. In fact, Figure 1a shows the XRD pattern obtained for a representative 1.5% Y-doped CdO material, and Figure 1b shows the related increasing of the lattice parameter a upon increasing the doping level. The a value is 0.47013 nm for the as-synthesized pure CdO and 0.47182 nm for the 2% Y-doped CdO. These results are in good agreement with the ASTM = 5-0640 data, and the observed linear behavior versus the doping level is an indication of the yttrium incorporation within the CdO cubic cell.17,35 Concerning the Cd2SnO4, undoped, 0.1 and 1% Y-doped Cd2SnO4 powders show a yellow color, whereas the 1.5 and 2% Y-doped Cd2SnO4 evidence a yellow color with some palegreen propensity.13 Both undoped and Y-doped Cd2SnO4 show a single phase with an orthorhombic unit cell (ASTM = 20-0188).35,36 Figure 2a shows the XRD pattern obtained for a representative 1.5% Y-doped Cd2SnO4 pellet. 3364

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Figure 2. (a) XRD pattern for a representative 1.5% Y-doped Cd2SnO4 pellet. (b) Orthorhombic cell volume of the Y-doped Cd2SnO4 materials versus the Y content. The R2 of the fit is 0.988. Figure 1. (a) XRD pattern for a representative 1.5% Y-doped CdO. (b) Correlation between the cubic cell lattice parameter a and the Ydoping level for CdO. The R2 of the fit is 0.990.

In addition, as shown in Figure 2b, the orthorhombic cell volume of the Y-doped Cd2SnO4 shows a monotone increase upon the increasing of the Y content, thus confirming that yttrium was incorporated within the Cd2SnO4 unit cell. This trend is expected and agrees well with previously reported data on Sbdoped Cd2SnO4.12 In addition, a comparison between the undoped and 1.5% Y-doped Cd2SnO4 XRD patterns does not reveal any significant preferential orientation upon Y doping. Crystal sizes of both Y-doped CdO and Cd2SnO4 were calculated from XRD data using the Debye−Scherrer equation: S = 0.9 λ/(B cos θB), where λ = 0.154056 nm, B is the full width at half-maximum of the given reflection, and θ is Bragg angle.37 The resulting values confirm that Y doping causes a decrease in the grain dimension being 84 ± 6 and 61 ± 5 nm, those for pure and doped CdO systems, respectively. The same behavior was not observed for Y-doped Cd2SnO4, where values of 82 ± 5 nm were observed not depending on the yttrium content. These relatively large grain dimensions are due to the high sintering temperatures. AFM Measurements. AFM micrographs performed on both undoped and Y-doped Cd2SnO4 pellets show uniform surfaces with grains sometime larger (∼200 nm) than those obtained by XRD, thus suggesting that an agglomeration mechanism is operative (Figure 3). XPS Results. Figure 4 shows the monochromatized Al Kα XPS spectrum for a representative 1.5%Y-doped Cd2SnO4 pellet in the Cd binding energy (BE) region.

Figure 3. AFM image of a representative 1.5% Y-doped Cd2SnO4 pellet.

The two sharp and well-resolved Cd 3d spin−orbit components lie at 404.5 (3d5/2) and 411.3 (3d3/2) eV. These values are typical of Cd2SnO4.23 Figure 5 shows XPS of the representative 1.5% Ydoped Cd2SnO4 pellet in the Sn 3d B.E. region. Both spin−orbit components 3d5/2 = 485.9 and 3d3/2 = 494.3 eV are in tune with values previously observed for pure SnO2 and Cd2SnO4.17,23−25,31 No XPS evidence of relevant species consistent with any Sn4+ → Sn2+ reduction is present in these sharp spectra. The Y 3d spectrum (Figure 6) shows the two spin−orbit peaks at 157.8 (3d5/2) and 159.1 (3d3/2) eV. Similar B.E. values have been already observed for other parent oxide systems.17,38,39 3365

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Y-doped Cd2SnO4. This observation indicates a pronounced segregation of yttrium to the surface.17,41−43 Nevertheless, no XRD evidence of any Y2O3 surface phase have been detected, thus inferring that the segregation involves a very thin surface layer. Within a simple model based on n layers uniformly enriched in Y, the I(Y 3d)/I(Cd 3d) XPS intensity ratio, corrected for the relevant atomic sensitivity factors, depends on the Y occupancy in the n layers through the following equation41,44

I(Y3d) (θ + xH1) = I(Cd3d) [(1 − θ) + (1 − x)H2]

Figure 4. Monochromatized Al Kα excited XPS at 80° photoelectron takeoff angle of the 1.5% Y-doped Cd2SnO4 pellet in the Cd 3d binding energy region.

H1 =

K1 ; K1 = e−nd / λ1 sin α (1 − K1)

H2 =

K2 ; K2 = e−nd / λ 2 sin α (1 − K2)

(1)

H1 and H2 parameters depend on the Y 3d and Cd 3d photoelectron inelastic mean free paths (λ1 and λ2, respectively) and the photoelectron takeoff angle relative to the surface plane (α); x represents the nominal doping level, θ is the Y occupancy of the Y-enriched layers, and d is the separation between adjacent Cd-containing planes. The 2.834 Å value related to the most intense XRD reflection, corresponding to the separation between adjacent atomic planes in the (130), direction has been taken for d. The λ values have been estimated using the Gries algorithm, and results indicate 2.03 nm for Cd 3d, 1.92 nm for Sn 3d, and 2.38 nm for Y 3d photoelectrons.45 The intensity ratio I(Y 3d)/I(Cd 3d) (0.16 at 80° photoelectron takeoff angle), observed for the 1.5% Y-doped Cd2SnO4 pellets fired at 1000 °C, is consistent with θ = 0.92 and n = 1. This result indicates that a Gibbs segregation phenomenon is operative for yttrium that occupies the 92% of the Cd surface sites and is also consistent with XRD results that do not evidence any Y2O3 surface phase because only the topmost ionic layer is involved. This conclusion is confirmed by XPS measurements performed at 30° photoelectron takeoff angle. In fact, the experimental Y/Cd ratio 0.32 is in agreement with θ = 0.94 and n = 1. Both θ values are largely comparable once the limitation due to the polycrystalline ceramic nature of the Y-doped Cd2SnO4 analyzed samples, instead of a smooth (130) single crystal surface, has been taken into account.41 The effective heat of Y segregation in Cd2SnO4 has been estimated using the following equation46,47

Figure 5. Monochromatized Al Kα excited XPS at 80° photoelectron takeoff angle of the 1.5% Y-doped Cd2SnO4 pellet in the Sn 3d binding energy region.

ΔHsegr = − RT ln

(Y/Cd)surf (Y/Cd)bulk

(2)

The present samples have been annealed at 1000 °C and quenched to room temperature after taking them outside the furnace. Therefore, XPS probed the equilibrium surfaces at 1000 °C. Therefore, the obtained ΔHsegr value for the 1.5% Ydoped Cd2SnO4 is −33 kJ mol−1. This negative value is also consistent with the strong tendency of yttrium to segregate on the surface of Cd2SnO4. At variance to the above observation, the Y/Cd intensity ratio = 0.009 obtained at 80° photoelectron takeoff angle for the 0.1% Y-doped Cd2SnO4 pellets, fired at 1000 °C, indicates a θ = 0.06 and n = 1. This suggests only a moderate surface

Figure 6. Monochromatized Al Kα excited XPS at 80° photoelectron takeoff angle of the 1.5% Y-doped Cd2SnO4 pellet in the Y 3d binding energy region.

The XPS atomic concentration analysis was used for quantitative surface studies.40 The surface Cd/Sn ratio is 1.9 ± 0.1 for the undoped Cd2SnO4 pellets, and that is close to the theoretical value. The intensities of the Y 3d peaks, at all present doping levels, are much greater than expected on the basis of the nominal doping levels being, for example, the Y/Cd ratio = 0.16 (at 80° photoelectron takeoff angle) for 1.5% 3366

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ion will occupy a 5s-6p metal based conduction band. This is indeed observed because in all of the present doped samples the carrier concentration is an order of magnitude higher than that observed for the undoped material. In contrast, if Y3+ substitutes for Sn4+, then the overall effect will be a p-type doping because Y3+ behaves as an acceptor. Therefore, the increased carrier concentration upon yttrium doping confirms that Y3+ substitutes for Cd2+, thus acting as an n-type dopant, and the observed suppressing of it above 1.5% of Y is an indication that the Y-doping level has exceeded that useful for n-type doping. A similar conclusion has been proposed for some In-doped Cd2SnO4 thin films obtained by spray pyrolysis and Y-doped CdO (at 1.2 to 1.3% Y) films obtained by MOCVD.2,3 The observed resistivity values upon doping is linked to the Hall mobility trend. In fact, the increasing carrier scattering with donors can explain the increased resistivity (or decreased mobility) above the 0.1 Y % doping level, where the highest is that for the 1.5% Y-doped sample. In addition, results of Figure 7a underline that the better properties of our Y-doped Cd2SnO4 solid solutions are shown at the lowest doping level. In fact, the 0.1% Y-doped Cd2SnO4 shows a conductivity of ∼160 Ω−1cm−1, a ∼1.0 × 1019 cm−3 carrier concentration, and a ∼100 cm2/(V s) Hall mobility. This latter effect is of relevance. In fact, the observed Hall mobility (∼100 cm2/(V s)) for the 0.1% Y-doped Cd2SnO4 is an excellent value compared with those recently observed for some In-doped thin films obtained by spray pyrolysis (i.e., 45 cm2/(V s)).2 Electrical measurements from 70 to 420 K (Figure 7b,c) point out that the resistivity, the carrier concentration, and the mobility are practically constant over the entire investigated temperature range, thus indicating that both undoped and Y-doped Cd2SnO4 are degenerate semiconductors. We estimated the free carrier mean free path l using the following equation, valid for a degenerate electron gas2

yttrium segregation because yttrium substitutes for the 6% of the surface cadmium ions. It is important to point out that our present study does not really concern with the investigation of possible structure defects as cation vacancies, interstitial oxygens or more complex defect clusters.41 In fact, this study is not devoted to the evaluation of possible mode of charge compensation upon yttrium doping.41 Nevertheless, even though we do not really have the evidence to distinguish between the above cases, the possibility of a Cd2+ vacancy upon two yttrium ions would be hard to reconcile with the observed increase in lattice parameters. Therefore, the presence of interstitial oxygen or more complex defects upon yttrium doping could be more appropriate.41 In the case of interstitial oxygen, their energy levels generally lie just above the oxide valence band and could influence the conductivity. Electrical Measurements. Four-point probe and Hall measurements at room temperature and versus T have been performed for both pure and Y-doped Cd2SnO4 pellets, soon after the 1000 °C sintering. The sign of the charge carriers was n-type for all samples, in good agreement with the literature results.16,26,29 The undoped material shows a ∼5 Ω−1 cm−1 conductivity, ∼1.0 × 1018 cm−3 carrier concentration, and ∼30 cm2/ (V s) Hall mobility. These values are consistent with those already reported in the literature for powder, spray-deposited, and “electroless” deposited Cd2SnO4.1,2,16,28,29 In all instances, the carrier concentration of yttrium-doped samples increases by an order of magnitude with respect the undoped analogues, where that of the 2% Y-doped Cd2SnO4 is somewhat lower (∼6.6 × 1018 cm−3) than that the others (Figure 7a).

l = 2.02 × 10−15n1/3μ (cm)

(3)

where n is the carrier concentration and μ is the mobility. The best l value of 4.5 nm was observed for the 0.1% Y-doped Cd2SnO4 pellets, and this is better than that recently reported for In-doped Cd2SnO4 films (2.3 to 3.6 nm).2

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CONCLUSIONS Y-doped Cd2SnO4, has been synthesized and characterized by XRD, AFM, XPS, and electrical measurements. XRD measurements confirm the presence of a single orthorhombic Cd2SnO4 phase at all adopted doping levels. XPS atomic concentration analyses indicate a pronounced surface segregation of yttrium involving only the topmost ionic layer, confirmed by the negative ΔHsegr value. Electrical measurements show that the carrier concentration of yttrium-doped samples increases by an order of magnitude with respect the undoped Cd2SnO4. The lowest adopted Y-doping level (0.1%) generated the better properties in terms of conductivity, carrier concentration, and Hall mobility. Also, the yttrium surface segregation affects the conductivity of Y-doped Cd2SnO4 above the 0.1% Y doping because the topmost ionic layer of each grain could act as an insulating layer. In these solid solutions, Y3+ substitutes for Cd2+ sites with the donation of the extra valence electron provided by Y3+ into the conduction band up to the 1.5% doping level. Above this level, some Y3+ appears to substitute for Sn4+ sites. Temperature-dependent measurements highlighted that both undoped and Y-doped Cd2SnO4 are degenerate semiconductors.

Figure 7. (a) Four-point probe and Hall measurements at room temperature. (b) Measurements versus T for the 0.1 Y-doped Cd2SnO4 pellets. (c) measurements versus T for the 1.5 Y-doped Cd2SnO4 pellets.

These results are also totally in agreement with both our previous EPR and IR specular reflectance investigations that were indicative of increased carrier concentration in Cd2SnO4 upon light Y doping (1 to 1.5%).31 This overall behavior can be accounted for by considering the mode of Y bulk substitution. Stoichiometric Cd2SnO4 should be insulating with a O 2p filled valence band and a 5s-5p Cd/Sn empty conduction band. Nevertheless, the presence of oxygen vacancies confers to this material semiconducting properties. Yttrium shows only its 3+ oxidation state. Therefore, if Y3+ substitutes for Cd2+, then the overall effect will be an n-type doping and the extra electron per 3367

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(34) Egdell, G.; Gulino, A.; Rayden, C.; Peacock, G.; Cox, P. A. J. Mater. Chem. 1995, 5, 499. (35) American Society for Testing and Material, Powder Diffraction Files, Joint Committee on Powder Diffraction Standards, USA, Set 1−5 Revised, Inorganic (1974). (36) Shannon, R. D.; Gillson, J. L.; Bouchard, R. J. J. Phys. Chem. Solids. 1977, 38, 877. (37) Warren, B. E. X-Ray Diffraction; Addison-Wesley: Reading, MA, 1969. (38) Majumdar, D.; Chatterjee, D. J. Appl. Phys. 1991, 70, 988. (39) Hughes, A. E. J. Am. Ceram. Soc. 1995, 78, 369. (40) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; Wiley: Chichester, U.K., 1994. (41) Gulino, A.; Taverner, A. E.; Warren, S.; Harris, P.; Egdell, R. G. Surf. Sci. 1994, 315, 351. (42) Egdell, R. G.; Parker, S. C. In Science of Ceramic Interfaces; Nowotny, J., Ed.; Material Science Monographs 75; Elsevier: Amsterdam, 1991; p 41. (43) Mackrodt, W. C.; Tasker, P. W. J. Am. Ceram. Soc. 1989, 72, 1576. (44) Gulino, A.; Egdell, R. G.; Baratta, G.; Compagnini, G.; Fragalà, I. J. Mater Chem 1997, 7, 1023−1027. (45) Gries, W. H. Surf. Interface Anal. 1996, 24, 38. (46) Gulino, A.; Condorelli, G. G.; Fragalà, I.; Egdell, R. G. Appl. Surf. Sci. 1995, 90, 289. (47) Cao, L.; Egdell, R. G.; Flavell, W. R.; Mok, K. F.; Mackrodt, W. C. J. Mater. Chem. 1991, 1, 785.

AUTHOR INFORMATION

Corresponding Author

*Tel: +39-095-7385067. Fax: +39-095-580138. E-mail:agulino@ unict.it.

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ACKNOWLEDGMENTS A.G. thanks the FIRB project ITALNANONET (RBPR05JH2P) and PRIN (2008KHW8K4).

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dx.doi.org/10.1021/jp2103676 | J. Phys. Chem. C 2012, 116, 3363−3368