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J. Phys. Chem. C 2008, 112, 14508–14512
Lanthanum Oxyfluoride Sol-gel Thin Films by a Simple Single-Source Precursor Route Lidia Armelao,*,† Gregorio Bottaro,*,‡ Giovanni Bruno,‡ Maria Losurdo,‡ Michele Pascolini,† Evelyn Soini,§ and Eugenio Tondello§ ISTM-CNR and INSTM, Department of Chemistry, PadoVa UniVersity, Via Marzolo, 1-35131 PadoVa, Italy, IMIP-CNR and INSTM, Department of Chemistry, Bari UniVersity, Via Orabona, 4-70126 Bari, Italy, and Department of Chemistry, PadoVa UniVersity and INSTM, Via Marzolo, 1-35131 PadoVa, Italy ReceiVed: May 16, 2008; ReVised Manuscript ReceiVed: June 27, 2008
Highly transparent and homogeneous nanostructured LaO1-xF1+2x (0e x e 1) thin films have been synthesized by a simple sol-gel procedure using La(hfa)3 · diglyme (Hhfa ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme ) bis(2-metoxyethyl) ether) as source compound for both lanthanum and fluorine. While thermal treatments up to 500 °C resulted in highly fluorinated nanosystems, i.e., LaF3 at 300 °C and LaF3 + LaOF at 500 °C, pure and single-phase nanostructured lanthanum oxyfluoride thin films with a mean crystallite size of ∼15 nm have been obtained at 700 °C. The microstructure and composition of the samples and their interplay with the synthesis procedure were investigated by glancing incidence X-ray diffraction, X-ray photoelectron, and X-ray excited Auger electron spectroscopy. Finally, the optical properties of the films were studied by UV-vis-NIR spectroscopy and spectroscopic ellipsometry. Introduction Rare earth (RE) fluorides and oxyfluorides are being increasingly investigated due to their unique electrical, optical, and electrochemical characteristics, which make them suitable candidates for applications in catalysis, sensing, optoelectronics, and photonics.1-4 Moreover, such materials, when opportunely doped with other RE cations, have drawn particular attention owing to their potential uses in lighting and displays, optical amplifiers, lasers, bioprobes, and NMR and MRI relaxation agents.5-7 Besides the above-mentioned applications, rare earth oxyfluorides display promising properties as oxide ion conducting solid electrolytes, with appreciable higher conductivity than that of stabilized zirconia.8-10 Furthermore, it has been reported that LaOF presents catalytic activity in the oxidative coupling of methane, oxidative dehydrogenation of ethane, or both.8,11-13 In order to improve the functional performances of these systems, the possibility of obtaining high-quality nanosystems with tunable phase and chemical composition has become one of the most challenging issues in materials chemistry. Several methods have been employed for the synthesis of LaF3 or LaOF, both in the form of powder and thin films.8,14-16 Usually, the liquid-phase synthetic routes followed a multisource approach in which F- ions derived from fluorides such as NaF, NH4F, and KBF46,17-20 or from CF3COOH.1,2,21-23 Only recently, few works employing rare earth trifluoroacetates as single-source precursors for the synthesis of REF3 or REOF have appeared in the literature.5,24 However, none of them was devoted to the preparation of thin films. In this fashion, the present work focuses on the original sol-gel synthesis of lanthanum fluoride and oxyfluoride thin films, employing the fluorinated β-dichetonate compound La(hfa)3 · diglyme (Hhfa ) 1,1,1,5,5,5-hexafluoro-2,4-pentanedione; diglyme ) bis(2-metoxy* To whom correspondence should be addressed. E-mail: lidia.armelao@ unipd.it;
[email protected]. † Padova University, ISTM-CNR, and INSTM. ‡ Bari University, IMIP-CNR, and INSTM. § Padova University and INSTM.
ethyl) ether)14 as single-source precursor. Such complex was recently employed in the preparation of LaO1-xF1+2x (0 e x e 1) films by chemical vapor deposition,8,15 whereas, to the best of our knowledge, it has never been used before in the synthesis of such systems by soft solution phase routes. Lanthanum-based thin films were prepared by sol-gel spincoating and annealed in air between 300 and 900 °C. The effects of different thermal treatments on composition, phase structure, and optical properties of the obtained layers have been investigated by glancing incidence X-ray diffraction (GIXRD), X-ray photoelectron (XPS), and X-ray excited Auger electron (XE-AES) spectroscopy, UV-vis-NIR spectroscopy and spectroscopic ellipsometry. Relevant results are reported and critically discussed. Experimental Section Chemicals. La(hfa)3 · diglyme was synthesized according to the literature8,14 and employed as a lanthanum and fluorine source. Synthesis. The La-based thin films were prepared starting from ethanolic solutions of La(hfa)3 · diglyme. The sol-gel reactions occurred under acid conditions by adding water and trifluoroacetic acid to the precursor solutions with the following molar ratios: 1La:87C2H5OH:3H2O:0.8CF3COOH. The solutions were stirred at 60 °C for 6 h and successively employed for film deposition by spin-coating (SCS P6700 SpinCoater) on Herasil silica slides (Heraeus, Quarzschmelze, Hanau, Germany). Finally, the films were annealed up to 900 °C in air for 1 h. Films analyzed in the present study had a thickness ranging from 50 to 10 nm and decreasing with increasing the annealing temperature. Structural Characterization. Glancing incidence XRD measurements were carried out by means of a Bruker D8 Advance diffractometer equipped with a Go¨bel mirror and a Cu KR source (40 kV, 40 mA), in the 20-55° 2θ range at a fixed incidence angle of 0.5°. The average crystallite dimensions were estimated from line broadening by means of the Scherrer equation.
10.1021/jp804351y CCC: $40.75 2008 American Chemical Society Published on Web 08/23/2008
Lanthanum Oxyfluoride Sol-Gel Thin Films
Figure 1. GIXRD patterns for La-based thin films annealed for 1 h at different temperatures in air. In the inset is reported a comparison between XRD patterns for specimens prepared employing HNO3 (i) and CF3COOH (ii) annealed at 700 °C for 1 h in air.
Chemical Composition. XPS measurements were performed on a Perkin-Elmer Φ 5600ci spectrometer using a nonmonochromatized Al KR radiation (1486.6 eV), at a working pressure lower than 10-9 mbar. The specimens, mounted on steel sample holders, were introduced directly into the XPS analytical chamber by a fast entry lock system. Survey scans were run in the 0-1350 eV range. Detailed spectra were recorded for the following regions: La 3d, F 1s, F KLL, O 1s, C 1s, Si 2s. The reported Binding Energies (BEs; standard deviation, (0.2 eV) were corrected for charging effects assigning to the adventitious C 1s line a BE of 284.8 eV.25 The analysis involved Shirleytype background subtraction and, whenever necessary, spectral deconvolution, which was carried out by nonlinear least-squares curve fitting, adopting a Gaussian-Lorentzian sum function. The atomic composition of the samples was calculated by peak integration, using sensitivity factors provided by the spectrometer manufacturer (Φ V5.4A software) and taking into account the geometric configuration of the apparatus. Depth profiles were carried out by Ar+ sputtering at 2.5 kV and 0.5 mA cm-2 beam current density, with an argon partial pressure of 5 × 10-8 mbar. Optical Spectroscopy. Transmittance spectra were recorded in the UV-vis-NIR range by a Cary 5000 spectrophotometer. Ellipsometric measurements were performed using a phasemodulated spectroscopic ellipsometer (UVISEL, Jobin-Yvon) in the 0.75-6.5 eV spectral range.26 Spectroscopic ellipsometry is also sensitive to the presence of an interface layer as well as of microscropic surface roughness and bulk inhomogeneity. All films were found to be rather homogeneous, and the optical functions were described by a Lorentzian oscillator; a model considering an interface layer with the substrate and the presence of a surface overlayer representing the surface microroughness was used. Results and Discussion The effects of the thermal treatments on the evolution of LaO1-xF1+2x (0 e x e 1) thin films were initially studied by GIXRD. The formation of crystalline phases at temperatures as low as 300 °C (Figure 1) is clearly indicated by the presence
J. Phys. Chem. C, Vol. 112, No. 37, 2008 14509 of three weak and broad peaks centered at 2θ ) 24.4°, 27.8°, and 44.6° indexed as the [(002), (110)], (111), and [(300), (113)] reflections of the hexagonal structure of LaF3.27 The observed change in the intensity ratios with respect to those reported in ref 27 is likely to be addressed to the presence of some degree of preferential orientation15,28 as suggested from the unusual high intensity of the (002) peak with respect to the 100% intensity expected for the (111) reflection. Peak attribution was not straightforward in this case due to the large full width at half-maximum (fwhm) likely due to the presence of nanocrystalline structures and structural disorder. Annealing at 500 °C promoted the system crystallization as evidenced from the intensity increase and sharpening of the reflections belonging to crystalline LaF3 (φ ) 11 nm). However, the diffraction pattern of this sample is dominated (Figure 1) by the presence of a peak centered at 2θ ) 26.7° and indexed as the (110) reflex of the rhombohedral LaOF (φ ) 14 nm) crystal phase.29 Besides the intensity increase of the diffraction peaks, the major effect of the annealing at this temperature was the formation of oxygen-containing species probably originated by a thermally induced reaction between lanthanum fluoride and atmospheric oxygen. It is worth highlighting that longer thermal treatments, up to 5 h at T e 500 °C, did not induce remarkable variations on the sample microstructure. Conversely, LaF3 to LaOF conversion was almost complete after annealing at 700 °C, as clearly evidenced in the corresponding diffraction pattern (Figure 1) showing that all the reflections belonging to crystal phases different from lanthanum oxyfluoride had disappeared. Finally, the GIXRD pattern of the samples annealed at 900 °C is characterized by the presence of several reflections in the 20-55° 2θ range, which cannot be indexed according to any of the previously detected crystal phases. It is likely that chemical reactions between the lanthanum-based layers and the glassy substrate took place at high temperature, thus leading to the formation of lanthanum silicates with not well-defined composition. In order to check whether the acid catalyst CF3COOH used in the precursor solution was responsible for the formation of F-containing crystalline phases, one sample was prepared by employing HNO3 while keeping constant all the other synthesis parameters. The diffraction pattern of this specimen, annealed at 700 °C for 1 h in air, is reported in the inset of Figure 1 and compared to that of a LaOF thin film annealed in the same conditions but deposited from solutions containing CF3COOH. Since no difference between the two diffraction patterns is observed, it is likely that the employment of trifluoroacetic acid has no remarkable effect on the microstructural evolution of the systems. On this basis, the choice for CF3COOH was made taking into account that CF3COOH and CF3COO- species already easily decompose into volatile compounds under annealing at low temperature (T ) 280 °C for La(OOCCF3)3).2,5 On the other hand, the use of acetic acid, which is commonly adopted in sol-gel synthesis, was disregarded in this specific case, due to the low quality of the deposited layers from the corresponding solutions. The surface and in-depth chemical composition was studied by X-ray photoelectron spectroscopy. In agreement with GIXRD results, pointing out the formation of oxygen-rich crystal phases on increasing the temperature of the thermal treatments, XPS analyses evidenced a progressive fluorine decrease from 47 atom % at 300 °C to disappearance at 900 °C, whereas the O content increased up to 60 atom % at the highest temperature (Figure 2a), thus clearly indicating the formation of oxygen-rich compounds.
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Figure 3. La 3d XPS spectra of La-based thin films annealed in air at different temperatures for 1 h. The peak highlighted in the figure is the F KLL Auger signal.
Figure 2. (a) F and O surface atomic percentage as a function of the annealing temperature. (b) F 1s XPS peak for the film annealed at 300 °C for 1 h in air. In the inset is reported the C 1s peak for the same sample.
Interestingly, in the film annealed at 300 °C the F 1s XPS peak is sharp (fwhm ) 1.8 eV) and symmetric (Figure 2b) pointing out an homogeneous chemical surrounding for fluorine atoms. The peak position at BE ) 684.9 eV is typical for inorganic fluorine.8 It is worth highlighting that no signal was detected in the BE region typical for CFn groups (BE g 687 eV),8 thus pointing out the formation of La-F bonds at low temperatures at the expense of the C-F bonds present in the lanthanum precursor. The presence of F- anions in an inorganic environment was further confirmed by the evaluation of the fluorine Auger R parameter calculated as the sum of the F 1s peak BE and the F KLL peak kinetic energy yielding R ) 1342.5 eV, in full agreement with the literature value for LaF3.30 Moreover, the analysis of the C 1s XPS peak (BE ) 284.8 eV, fwhm ) 1.5 eV), reported in the inset of Figure 2b, confirmed the absence of CFn (n e 3) residuals at the specimens surface (BE g 286 eV),8 originally present in the lanthanum precursor. The La 3d photoelectron peak (Figure 3) shows the typical complex structure of core-level photoemission spectra of the light rare earth compounds; i.e., in addition to the well-known spin-orbit multiplet splitting, a characteristic satellite structure is present, which has been mainly attributed to final-state effects or to charge-transfer coexcitations.31-34 However, on raising the annealing temperature to that corresponding to a progressive decrease of the fluorine content, severe modifications were observed in the band shape. The 3d satellite structure of the samples annealed at 300 °C closely resembles that of the LaF3 one; i.e., each of the 3d5/2 (BE ) 837.1 eV) and 3d3/2 (BE ) 854.1 eV) signals8 shows a relatively weak satellite on the high
Figure 4. XPS depth profiles of La-based layers annealed for 1 h at 300 (a) and 700 °C (b).
binding energy side.35 On increasing the annealing temperature and then the oxygen amount, the satellite peaks gain intensity due to a more efficient O2p f RE4f charge transfer with respect to the F2p f RE4f one.34,35 Of particular interest was the study of XPS depth profile for samples annealed at different temperatures in the 300-900 °C range (Figure 4). At low temperature (Figure 4a), a complex in-depth composition was detected, characterized by the presence of La, F, C, and O as minor element. A similar elemental distribution is consistent with the formation of fluorine-based
Lanthanum Oxyfluoride Sol-Gel Thin Films compounds in which the considerable carbon content is justified by the low treatment temperature (300 °C), which is not sufficiently high for a complete precursor decomposition. According to the surface composition (see above), no signals related to CFn residuals were detected in the inner sample layers. Moreover, under these conditions, the La-O coordination is partially retained as pointed out by the presence of oxygen in the whole film thickness. It is worth highlighting that the occurrence of structural disorder revealed by XRD (see above) could be due to the presence of precursor residuals in the samples. Increase of the annealing temperature (700 °C) induced remarkable modifications in the sample in-depth composition. In this case (Figure 4b), the carbon signal fell to noise level after the first sputtering cycle, clearly indicating a complete precursor decomposition and the formation of pure materials. Moreover, La, O, and F were homogeneously distributed along the film thickness as evidenced from the similarity of their indepth profiles, suggesting a common chemical origin of these elements and thus the formation of lanthanum oxyfluoride phases. The deviation from a La:O:F ) 1:1:1 composition can be explained taking into account the formation of nonstoichiometric lanthanum oxyfluoride phases, preferential sputtering effects induced by the Ar+ beam, or both. Finally, in the depth profile of the 900 °C-treated samples (not reported), fluorine is absent whereas the presence of the silicon signals both in-depth and at the specimen surface clearly indicated the formation of lanthanum silicates through thermally induced film/substrate reactions. The observed evolution in the specimen composition and microstructure can be explained taking into account that, in the precursor compound, La3+ ions are oxygen coordinated; therefore, it is rational that the retention of the La-O bonds would occur during film formation. The formation of La-F bonds takes place as a consequence of thermal treatments at 300 °C through subsequent replacement of the metal-oxygen bonds with Fions produced by the cleavage of the C-F linkages present in the CF3 groups of the hexafluoropentanedione. A similar mechanism has been recently reported in the synthesis of rare earth fluorides and oxyfluoride nanoparticles using trifluoroacetate groups as fluorine sources.5 Conversely, the formation of oxyfluoride compounds was progressively favored at higher annealing temperatures (T g 500 °C),2 probably through thermally induced chemical reactions with atmospheric oxygen. A similar trend has been recently evidenced in the CVD deposition of LaF3 or LaOF thin films starting from the La(hfa)3 · diglyme precursor.8,15 Concerning optical properties, Figure 5 shows the refractive index and the extinction coefficient of the films annealed at various temperatures. Figure 5a shows that the refractive index increases with the annealing temperature; however, while the lower refractive index for the film annealed at 300 °C might be due to a less compact and dense microstructure, upon annealing at 500 and 700 °C, the refractive index approaches that reported for La-F-based thin films (e.g., LaF3),36 consistent with the XRD data. Correspondingly, Figure 5b shows that the lowest spectrum for the extinction coefficient is obtained upon annealing at 700 °C, indicating the higher purity of the fluoride phase, as also indicated by XRD. Furthermore, the higher refractive index (1.95-2.30) and extinction coefficient for the film annealed at 900 °C are consistent with the formation of a silicate phase due to intermixing and interdiffusion occurring at such a high annealing temperature. The intermixing increasing with
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Figure 5. Spectra of (a) the refractive index, n, and of (b) the extinction coefficient, k, of films annealed at temperatures of 300 (I), 500 (II), 700 (III), and 900 °C (IV). The inset in (b) shows the thickness of the interface layer for films annealed at the various temperatures.
Figure 6. Transmittance spectra in the 200-2500 nm wavelength range for films annealed for 1 h in air at different temperatures.
annealing temperature can also be seen in the increase of thickness of the interface layer, as shown in the inset of Figure 5b. Thus, the films annealed at temperatures lower than 900 °C were highly transparent in a wide wavelength range (200-2500 nm) with a transmittance higher than 90% above 400 nm (Figure 6). Conversely, films annealed at 900 °C present a slight absorption in the visible range, probably due to the formation of lanthanum silicates. Similar absorption features represent a key point for the employment of lanthanum fluoride and oxyfluoride films as host materials for the realization of rare earth luminescent nanosystems. Conclusions Lanthanum-based thin films characterized by different fluorine and oxygen content have been obtained by sol-gel spin-coating using La(hfa)3 · diglyme as precursor for both La3+ and F- ions. After treatment at low temperature (T ) 300 °C), a highly fluorinated compound (LaF3) has been obtained, although the formation of a well-interconnected -(La-A)n- (A ) F- and/ or O2-) network was not completed as evidenced from the broadening and low intensity of the diffraction peaks and from
14512 J. Phys. Chem. C, Vol. 112, No. 37, 2008 the appreciable amount of carbon residuals in the inner sample layers. The increase of the annealing temperature up to 500 °C improves lanthanum trifluoride crystallization (average crystallite size φ ) 11 nm) and concurrently favors the formation of lanthanum oxyfluoride (φ ) 14 nm). Pure and single-phase nanostructured (φ ) 15 nm) LaOF thin films have been obtained after annealing at 700 °C. Higher treatment temperatures promoted LaOF/substrate reactions thus leading to the formation of lanthanum silicates. Lanthanum fluoride and oxyfluoride thin films were highly transparent in the 200-2500 nm wavelength range with a transmittance higher than 90% for λ > 400 nm. Such features open interesting perspectives for the employment of these nanosystems in the realization of visible and infrared emitting materials and devices by doping with a variety of rare earth cations (e.g., Eu3+, Tb3+, Er3+). Acknowledgment. This work was financially supported by research projects CNR-INSTM PROMO, FIRB-MIUR RBNE033KMA “Molecular compounds and hybrid nanostructured materials with resonant and non resonant optical properties for photonic devices”, FISR-MIUR “Inorganic hybrid nanosystems for the development and the innovation of fuel cells”, CARIPARO 2006 “Multi-layer optical devices based on inorganic and hybrid materials by innovative synthetic strategies”, and NanoCharM (NMP3-CA-2007218570). References and Notes (1) Hosono, E.; Fujihara, S.; Kimura, T. Langmuir 2004, 20, 3769. (2) Tada, M.; Fujihara, S.; Rimura, T. J. Mater. Res. 1999, 14, 1610. (3) Vijayakumar, M.; Selvasekarapandian, S.; Gnanasekaran, T.; Fujihara, S.; Koji, S. Appl. Surf. Sci. 2004, 222, 125. (4) Vijayakumar, M.; Selvasekarapandian, S.; Gnanasekaran, T.; Fujihara, S.; Koji, S. J. Fluorine Chem. 2004, 125, 1119. (5) Sun, X.; Zhang, Y. W.; Du, Y. P.; Yan, Z. G.; Si, R.; You, L. P.; Yan, C. H. Chem. Eur. J. 2007, 13, 2320. (6) Diamente, P. R.; van Veggel, F. C. J. M. J. Fluoresc. 2005, 15, 543. (7) Evanics, F.; Diamente, P. R.; van Veggel, F. C. J. M.; Stanisz, G. J.; Prosser, R. S. Chem. Mater. 2006, 18, 2499. (8) Barreca, D.; Gasparotto, A.; Maragno, C.; Tondello, E.; Bontempi, E.; Depero, L. E.; Sada, C. Chem. Vap. Deposition 2005, 11, 426.
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