Preparation and characterization of luminescent films of Pr3+-doped CaTiO3 processed by sol–gel technique

Journal of Alloys and Compounds 374 (2004) 202–206

Preparation and characterization of luminescent films of Pr3+-doped CaTiO3 processed by sol–gel technique Eric Pinel, Philippe Boutinaud∗ , Geneviève Bertrand, Christophe Caperaa, Joël Cellier, Rachid Mahiou Laboratoire des Matériaux Inorganiques, UMR CNRS 6002, Université Blaise-Pascal et ENSCCF, Ensemble Scientifique des Cézeaux, 63174 Aubière Cedex, France

Abstract Luminescent films of Pr3+ -doped CaTiO3 have been prepared by dip-coating (DC-films) and spray techniques (S-films) from stabilized sols and diluted gels, respectively. The structure and microstructure of these films have been characterized by X-ray diffraction and by electron microscopy and their luminescence properties have been evaluated at room temperature, under ultraviolet pumping. Both kinds of films exhibit the typical single red photoluminescence of powder CaTiO3 :Pr3+ after a thermal treatment at 500 ◦ C for 1 h. For all films, the photoluminescence intensity increases along with the increase of the sintering temperature to 800 ◦ C, but, while the brightness of DC-films is still increased by raising the temperature of preparation up to 1000 ◦ C, the brightness of S-films is drastically reduced. These behaviors are interpreted by considering mainly structural and microstructural arguments. © 2003 Elsevier B.V. All rights reserved. Keywords: Thin films; X-ray diffraction; Scanning electron microscopy; Luminescence

1. Introduction Orthorhombic Pr3+ -doped calcium titanate is known for several years as a red emitting phosphor characterized by a single emission peak with chromaticity coordinates positioned at x = 0.68, y = 0.311, i.e. very close to those of the NTSC “ideal red” [1]. In recent years, several works have been carried out to determine some critical parameters for the improvement of the brightness of these phosphors. Especially, it has been observed that the preparation of the powders by sol–gel procedures leads to significant reinforcement of the red photoluminescence intensity [2] and that the stoichiometry, the degree of crystallization, the sintering conditions and the microstructural characteristics of the powders affect also strongly the performances [3]. In powder materials, it is now clear that the brightest Pr3+ -doped calcium titanates are obtained for a doping level of 0.1 mol% and for Al3+ charge-compensated materials processed by sol–gel procedure and sintered at 1000 ◦ C. In this paper, we report on the preparation

∗ Corresponding author. Tel.: +33-4-7340-7100; fax: +33-4-7340-7108. E-mail address: [email protected] (P. Boutinaud).

0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2003.11.093

and characterization of Pr3+ -doped CaTiO3 films processed by sol–gel technique. Two different kinds of films were prepared either by dip-coating technique from stabilized sols (DC-films) or by spray technique from gels (S-films). The structural, microstructural and optical properties of these films are studied and compared to the characteristics of CaTiO3 :Pr3+ powders processed by sol–gel route. 2. Experimental DC- and S-films of CaTiO3 :Pr3+ were prepared by the sol–gel process using alkoxides as precursors. As alkoxides require water-free atmosphere, the first step of the synthesis was carried out under anhydrous conditions in a dry argon atmosphere, and then for preparation of S-films the hydrolysis reaction was performed by adding a certain amount of distilled water. Experimentally, two solutions were separately prepared: a solution I made up of anhydrous calcium chloride (0.7520 g, 1 eq.), aluminum chloride (0.0009 g, 1 eq.) and praseodymium chloride (0.0017 g, 1 eq.) dissolved in isopropanol (anhydrous solvent, 60 ml), and a solution II of potassium isopropoxide (0.6377 g of metallic potassium, 1.2 eq., 20 ml of i PrOH). Chunks of potassium needed to be

E. Pinel et al. / Journal of Alloys and Compounds 374 (2004) 202–206

handled in non-polar and highly anhydrous solvent such as distilled cyclohexane, and cleaned from the oil of storage prior to the addition of i PrOH. The solution II was slowly added to solution I with vigorous stirring, the reaction was highly exothermic and KCl precipitated immediately. After a 2-h reflux of (I + II) at 85 ◦ C, a solution of titanium isopropoxide (2 ml, 1 eq.) was added dropwise with a syringe. A homogeneous solution was obtained after refluxing (85 ◦ C) and stirring vigorously for 20 h. After cooling, chloride potassium was separated by centrifugation and a clear sol was isolated [4]. The sol was deposited by dip-coating technique [5,6] on a substrate to obtain thin DC-films. To avoid a too-fast gelation, the sol was stabilized by adding acetylacetone. Depending on annealing temperature, different kinds of substrates were used, such as ordinary glass slide, Pyrex plate or fused silica plate. After filtering with a 0.2-␮m filter, the sol was kept in a Teflon container. Then, the substrate was slowly dipped and withdrawn into the viscous sol. After coating, each layer was dried at 100 ◦ C for 10 min and then heat-treated at 400 ◦ C for 10 min in a silica tube. Repeating the same procedure, multicoated DC-films of CaTiO3 :Pr3+ were prepared. The films selected for this study were obtained after 19 coatings. To prepare S-films, the sol was hydrolyzed by adding water and the colloidal solution was deposited on a substrate using a home-made spray system. The spray pyrolytic method is one of non-vacuum synthesis methods which can be used to obtain thin or thick films. It has been shown to be a simple method, inexpensive, with high deposition rate and excellent film chemical homogeneity. The process control is very easy and leads to a very good reproducibility [7–9]. The spray solution was transported (rate: 2 ml min−1 ) to the deposition zone with a nitrogen flow of 3 l min−1 . The substrate was placed on a resistance heated stage maintaining the temperature at 280 ◦ C. The distance between nozzle and substrate was kept 15 cm and the nozzle was moved backward and forward over the substrate at a frequency of 0.5 Hz. All the nozzle movements (x, y, z) were controlled by computer. A typical procedure involved five repeated cycles composed including 1 min of spray, 1 min of pyrolytic decomposition at 280 ◦ C and 30 min in an oven at 350 ◦ C. The pyrolytic decomposition of the colloidal solution led to continuous film with good adhesion to substrate. This process was repeated until the desired thickness was reached. The structural characteristics of the S-films were measured by X-ray diffraction (XRD) either at room temperature, versus temperature up to 1100 ◦ C, or as a function of time at a given temperature. The high temperature XRD (HTXRD) patterns have been recorded at several points (each 20 ◦ C between 380 and 500 ◦ C, each 50 ◦ C between 500 and 1100 ◦ C, a step of 10 h is realized at 1100 ◦ C). Measurements of in situ HTXRD were carried out in air in the temperature range mentioned before, using an X-ray diffractometer (Philips X’Pert) equipped with a high-temperature attachment. We used a sequential temperature rising rate of 5 ◦ C/min and a temperature holding time of 1 h before

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each measurement. Temperature was measured by means of Pt/Pt-Rd thermocouple in direct contact with the films deposited on a Pt ribbon. The Cu K␣ line was used as the X-ray source with a monochromator positioned in front of a scintillation detector. In the case of DC-films, room temperature XRD patterns were recorded with an incident θ angle of the X-ray beam fixed at 1.5◦ and by monitoring the detector in a 2␪ range from 20 to 50◦ . The microstructure of the different films was checked by scanning electron microscopy with a Cambridge Scan 360 microscope operated at 10 kV. The luminescence properties of DC- and S-films were measured at room temperature upon continuous or pulsed UV pumping, by using, respectively, a monochromatized 250-W xenon lamp or a nitrogen laser as excitation sources. The incident UV beam was focused at right angle with respect to the films and the fluorescence was analyzed in the same way with a set-up described elsewhere [3]. The temporal decay curves were recorded with the help of a 400-MHz Lecroy digital oscilloscope with the input impedance at 50 . For intensity measurements, special care was taken to control the beam power of the xenon source and to keep constant the illuminated surface of the sample. The photoluminescence intensity was evaluated by integrating the red emission band in a range from 600 to 650 nm.

3. Results and discussion 3.1. X-ray diffraction (XRD) Both kinds of films (S- and DC-films) were characterized by X-ray diffraction. For the S-films, two different thicknesses were considered: 1 ␮m coating for S1-films and 5 ␮m coating for S5-films. These thicknesses were measured by talystep technique. Fig. 1 shows the room temperature XRD patterns of S-films sintered at different temperatures for 1 h under air, from 500 to 1000 ◦ C and the inset shows the room temperature pattern of a DC-film treated at 1000 ◦ C. This latter pattern shows the characteristic peaks of orthorhombic CaTiO3 host lattice (JCPDS 22-153, +), with some textural effects, as indicated by a strong quenching of (1 2 1) peak. In the case of S1-films, the crystallization of orthorhombic CaTiO3 is detected after thermal treatment at 500 ◦ C and appears more clearly at 600 or 700 ◦ C. The reflexions observed are (1 2 1), (0 4 0) and (2 4 0). The broad band at 20◦ is connected with the silica substrate. After thermal treatment at 1000 ◦ C, the XRD features of orthorhombic CaTiO3 are no longer observable but new peaks are detected. The dominant peak indicated by a black circle corresponds to the re-crystallization of the silica substrate (JCPDS 85-621, 䊉) and the peaks indicated by a star and a white circle are related, respectively, to rutile (JCPDS 21-1276, ∗) and possibly to a tetragonal form of CaTiO3 (JCPDS 39-145, 䊊). The reasons of these structural modifications are not yet clearly understood but will be further studied. For S5-films, the thicker coating makes it possible to observe unambiguously

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+

*

+

a.u .

a.u.

(101)

S5-film

DC-film (040)

a. u.




+

(121)

+




+

20

* +

25

+

*

30

+

35 40 2θ(˚)

45

+

*

50

+

1100 (10h) 1100 1000 950 900 850 800 750 700 650 600 500 450 400

(1000˚ C) (700˚ C) (600˚ C) (500˚ C)

(1000˚ C) (700˚ C) (600˚ C)

S1-film

20

25

30

20

30

40

50

60

70

2θ( ˚) Fig. 1. XRD patterns recorded at 25 ◦ C of S-films of CaTiO3 :Pr3+ heated for 1 h at different temperatures. The inset shows the XRD pattern at 25 ◦ C of a DC-film treated at 1000 ◦ C.

the XRD features of orthorhombic CaTiO3 after thermal treatment from 500 ◦ C and to avoid the observation of the amorphous signal of the silica substrate. The reflexions observed are now (1 0 1), (1 2 1), (0 3 1), (0 4 0) and (2 4 0). The film is single phased only after a treatment at 500 ◦ C. After being treated at 600 ◦ C, peaks related to TiO2 appear (i.e. rutile and anatase, JCPDS 21-1272, ) together with unidentified weak reflexions. In the same time, peaks related to orthorhombic CaTiO3 increase. As the sintering temperature of the film is increased up to 1000 ◦ C, new sharp XRD peaks appear at the expense of the XRD peaks of orthorhombic CaTiO3 . These peaks are in good agreement with the titanite CaTiSiO5 (JCPDS 11-142, 䉱) which could be formed by interaction at high temperature with the silica substrate. In the same time, partial re-crystallization of the SiO2 substrate is also observed. For a better understanding of the phases transformation occurring during the thermal treatments, XRD patterns of S1-films were recorded versus temperature in a range from 400 to 1100 ◦ C, then as a function of time at a temperature fixed at 1100 ◦ C. The results are shown in Fig. 2. From 400 to 600 ◦ C, the major peak of the patterns is in agreement with cubic Ca2 Ti2 O6 (JCPDS 40-103, 䉬), then this peak disappears at the profit of the reflexions of orthorhombic CaTiO3 . It is therefore assumed that the crystallization of the titanate perovskite starts at 650 ◦ C in these films. As the temperature of the film is raised up to 750 ◦ C and above, another structural form of CaTiO3 is observed (JCPDS 39-145, 䊊) together with a small peak of CaO (JCPDS 37-1497, 䊏). Finally, for temperatures above 1000 ◦ C, the characteristic peaks of rutile appear and their intensity increase at the expense of the orthorhombic CaTiO3 peaks by maintaining the films at the same high temperature for 10 h.

35

40

45

50

2θ (˚)

(500˚ C) 10

T /˚ C

Fig. 2. HTXRD patterns of S1-film of CaTiO3 :Pr3+ in the temperature range from 400 to 1100 ◦ C.

3.2. Scanning electron microscopy (SEM) Representative micrographs of the different films and of a Pr3+ -doped CaTiO3 powder processed by sol–gel method are displayed in Fig. 3 at the same scale. The microstructure of the powder (Fig. 3a) will be taken as a reference in the following. After thermal treatment at 1000 ◦ C, the microstructure of this powder consists typically of an almost equi-sized distribution of grains, all situated in the micrometer range. For the same temperature of treatment, the DC-films show a very dense microstructure composed of submicronic grains (Fig. 3b) and appear for this reason transparent by naked eye. Concerning S-films, the observed microstructures depend significantly on the temperature of treatment. After a treatment at 800 ◦ C, we can observe a fine grained microstructure which consists of aggregates of submicronic grains (Fig. 3c). After sintering at 1000 ◦ C for 1 h, the microstructure changes drastically as it is no longer possible to observe any granular structure, whatever the thickness of the films (Fig. 3d). 3.3. Photoluminescence The emission profiles of the films recorded at 300 K under excitation at 337 nm are presented in Fig. 4a in comparison to the typical emission spectrum of Pr3+ -doped CaTiO3 powder. The luminescence signals observed are all very similar and correspond to red 1 D2 → 3 H4 transitions of Pr3+ ions [10]. The half width at middle height of the emission band is estimated at around 300 cm−1 for all the samples. Fig. 4b shows the room temperature photoluminescence intensity of the DC- and S-films for an excitation at 254 nm and an emission at 612 nm, against the temperature of preparation of the films. The S1- and S5-films exhibit similar behavior which consists in the increase of the photoluminescence intensity as the temperature of preparation is raised up to 800 ◦ C, followed by a strong decrease of this intensity

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205

Fig. 3. SEM images of CaTiO3 :Pr3+ . (a) Powder annealed at 1000 ◦ C for 6 h, (b) DC-film heated at 1000 ◦ C for 1 h, (c) and (d) S-films heated, respectively, at 800 and 1000 ◦ C for 1 h.

Intensity : a. u.

for films sintered at temperatures above 800 ◦ C. The shape of the curves in the range 800–1000 ◦ C can be explained by the partial structural transformation of orthorhombic CaTiO3 host lattice evidenced on the XRD patterns at the profit of non-luminescent TiO2 . Similar CaTiO3 + TiO2 phases mixings have been observed previously either by grind-

4b)

S1-film S5-film DC-film

Intensity : a.u.

500

600

700 800 T(˚C)

900 1000

Powder

ing CaTiO3 :Pr3+ samples prepared by solid state reaction or by preparing the samples by sol–gel procedure under acidic conditions. In both cases, the resulting mixed samples exhibited a weaker luminescence due to the presence of non-luminescent TiO2 [3]. For the DC-films, it is observed a progressive increase of the photoluminescence intensity along with the increase of the temperature of treatment of the films. The intensity gain is estimated at around 300% between a film sintered at 500 ◦ C and the same film sintered at 1000 ◦ C. This result can be explained mainly by considering that the orthorhombic CaTiO3 structure of the DC-films is maintained after a thermal treatment at 1000 ◦ C and by assuming that the degree of crystallization of the DC-film increases progressively as the sintering temperature is raised. This interpretation is consistent with a previous study of sol–gel processed CaTiO3 :Pr3+ powders showing also an increase of the brightness of the samples as the crystallinity of the powders was improved by thermal treatments at high temperature [2].

S-film 4a)

4. Concluding remarks DC-film 550

600

650

700

Wavelength (nm) Fig. 4. (a) Emission profiles of CaTiO3 :Pr3+ powder, S-film and DC-film prepared by sol–gel process. (b) Photoluminescence intensity of DC- and S-films.

Luminescent films of CaTiO3 :Pr3+ have been prepared for the first time by dip-coating and spray techniques from, respectively, stabilized sols and diluted gels. The photoluminescence performances of these films depend severely on the sintering conditions, which affect both the structural and microstructural characteristics. For the films processed by

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the spray method, the temperature and/or duration of the thermal treatment and the thickness must be carefully adjusted (typically 800 ◦ C, 1 h for a thickness of 1 ␮m or even lower) to avoid drastic decrease of the brightness. In contrast, the photoluminescence intensity of the films prepared by the dip-coating method is as strong as the sintering temperature is increased. However, as the nature of the substrate used for the coatings limits severely the value of the temperature, further experiments will be carried out to improve the brightness of these films by increasing the duration of thermal treatments at a temperature fixed below 1000 ◦ C.

Acknowledgements The authors acknowledge financial support from the firm DGTec.

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