ARTICLE IN PRESS Journal of Luminescence 128 (2008) 1787– 1790
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Cr3þ and Cr4þ luminescence in glass ceramic silica Marco A.U. Martines a,,1, Marian R. Davolos a, Miguel Jafelicci Ju´nior a, Dione F. de Souza b, Luiz A.O. Nunes b a b
Instituto de Quı´mica, Universidade Estadual Paulista-UNESP, P.O. Box 355, 14801-970 Araraquara, SP, Brazil ˜o Paulo-USP, P.O. Box 369, 13560-970 Sa ˜o Carlos, SP, Brazil Instituto de Fı´sica, Universidade de Sa
a r t i c l e in fo
abstract
Article history: Received 3 February 2006 Received in revised form 9 April 2008 Accepted 11 April 2008 Available online 26 April 2008
This paper reports on the effect of glass ceramic silica matrix on ½CrO4 4 and Cr2 O3 NIR and visible luminescence. Chromium-containing silica was obtained by precipitation from water-glass and chromium nitrate acid solution with thermal treatment at 1000 1C. From XRD results silica and silica–chromium samples are crystalline. The chromium emission spectrum presents two main broad bands: one in the NIR region ð1:121:7 mmÞ and other in the visible region ð0:620:7 mmÞ assigned to Cr4þ and to Cr3þ , respectively. This thermal treated glass ceramic silica–chromium sample stabilizes the ½CrO4 4 where Cr4þ substitutes for Si4þ and also hexacoordinated Cr3þ group probably as segregated phase in the system. It can be pointed out that luminescence spectroscopy is a powerful tool for detecting the two chromium optical centers in the glass ceramic silica. & 2008 Elsevier B.V. All rights reserved.
Keywords: Silica Chromium(III) and (IV) VIS and NIR luminescence
1. Introduction The study of materials based on inorganic crystals doped with 2 d transitions metal ions has recently received a great deal of interest due to near infrared (NIR) laser action around 1:2 mm in chromium-doped crystals [1–5]. The characteristic infrared emissions of Cr-doped inorganic crystals are presently associated with tetrahedrally coordinated Cr4þ that has the electron structure 2 Arð3d Þ. Cr4þ -doped materials present wide bands and the emission can be assign to spin allowed transition ð3 T2 ! 3 A2 Þ [3,6]. The Cr4þ -doped compounds correspond to weak-field situations for which Dq=B ¼ 1:221:6, whereas the Mn5þ and Fe6þ ones correspond to situations for which Dq=B ¼ 1:922:2. On the Tanabe–Sugano diagram (TSD) this situation corresponds to positions below and above the crossing point of 1E and 3 T2 states [3,6–9], respectively. Chromium-doped forsterite samples show fluorescence emission from both Cr4þ and Cr3þ ions. Cr4þ replaces Si4þ at tetrahedral sites, whereas Cr3þ ions are found in two types of octahedral sites of Mg: site one with inversion of symmetry and site two with mirror symmetry, resulting in high and low field sites, respectively [10]. 2 The energy-level scheme of Cr4þ ion with 3d configuration can be described with the TSD [7]. Moreover, the optically active
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[email protected] (M.A.U. Martines). Actual address: Departamento de Fı´sica e Quı´mica, FEIS, UNESP, P.O. Box 31, 15385-000 Ilha Solteira, SP, Brazil. 1
0022-2313/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.04.011
Cr4þ center occupies the tetrahedrally coordinated lattice [3–7,9]. However, from the TSD three spin-allowed and broad band transitions are expected to be observed in absorption spectrum of Cr4þ ion. Generally, a crystal-field analysis for tetrahedrally coordinated Cr4þ ion is difficult, the absorption spectra are dominated by triplet–triplet transitions, because the transition to the crystal-field components of the 3 T2 energy level are weak and overlapping each other. Then, the peak position and bandwidth depend on the crystal-field strength, and on the crystal-field splitting symmetry [10,11]. The two NIR emission bands observed on Cr-doped Zn2SiO4 were attributed to tetrahedrally coordinated Cr4þ [12]. Moreover, three sharp lines from Cr4þ in the NIR were observed in Cr-doped forsterite system and were assigned to the triplet–triplet transition [10]. The luminescence decay of transparent forsterite nanocrystalline glass-ceramic doped with chromium reveal the presence of Cr3þ and Cr4þ centers in both glass and crystal phases [13]. The incorporation of chromium into different sol–gel system results of the oxidation state of the chromium precursor, octahedrally coordinated Cr3þ is the preferred coordination and oxidation state in silicate, germanate and a number of mixed component gel. Although, after calcination, tetrahedrally coordinated Cr6þ is preferred geometry and oxidation state in SiO2 gels, while Cr4þ exits in germanate, aluminate and mixed composition gels. On the other hand, tetrahedrally coordinated Cr4þ is stabilized in the silicate-aluminate and germanate glasses prepared from sol–gel process [14]. The purpose of this work is to obtain chromium broad and/or sharp luminescence bands in glass ceramic silica powder.
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3. Results and discussion The XRD results of SiO2 and SiO2:Cr are shown in Fig. 1. Tridymite phase (Joint Committee on Powder Diffraction Standard—JCPDS 18-1170) is present in the samples prepared at 1000 1C, in this way, the heating promote the formation of glass ceramic in both systems, but in chromium–silica system the crystallization is less intense. The DTA curves of SiO2 and SiO2:Cr, Fig. 2, are divided into four regions: the first one, under 300 1C, is attributed to desorption of physically adsorbed water; the second region can be associated to silica polymerization and structural relaxation; the third one, upper 650 1C, the change in the base line can be associated to glass transition of silica; and finely, the fourth region can be attributed to initial of crystallization of silica or new phase formation [18]. The high resolution 29Si-NMR spectra of silica and chromium-containing silica heated at 1000 1C are present in Fig. 3. The NMR spectrum of silica sample shows three lines with chemical shifts at 91:3, 104:2 and 112:7 ppm. This lines can be, respectively, assigned of silicon atoms of silanediol
∗ b
∗ ∗ ∗
∗ ∗ ∗
a
10
20
∗ ∗∗ ∗
∗
30 40 2 Theta (degree)
50
60
∗∗
70
Fig. 1. X-ray diffraction of samples, undoped (a) and chromium-doped (b) silica.
b
a
EXO
ΔT (arbt. unit.)
The silica and chromium-containing silica powders were obtained by precipitation from soluble sodium silicate (waterglass) previously diluted in deionized water in the mass proportion of 1:5 and 3 M aqueous solution of nitric acid to have initially 3% (w/w) chromium(III) oxide in relation to silicon dioxide. Chromium acid solution at room temperature was slowly added drop to drop into diluted silicate solution contained in a Kettle vessel under mechanical stirring with controlled temperature at 50 1C [15]. The final pH value was reached to 1 and was allowed for 1 h. Products were treated by different methods, static dialysis and solid liquid extraction using Sohxlet system equipped with a sintering plate cylindrical vessel which provides the removal of silica sol and gel formed [16]. Solvents used for extraction system were a nitric acid solution and deionized water. Powders were dried in a microwave oven which was adapted in our laboratory [17] to have a controlled time period of microwave generation. The obtained powders were thermally treated at 1000 1C under static air atmosphere for 4 h in order to obtain glass ceramic water-absent silica. The powder was characterized by XRD using a diffractometer SIEMENS D5000 by using Ni-filtered CuKa radiation generated at 30 mA and 40 kV, integration time at 5 s and 0.0201 step. Differential thermal analyses (DTA) was performed under nitrogen atmosphere at 10 K min1 heating rate from 100 to 1200 1C, in TA Thermal Analyst 3100. The solid-state NMR measurements were performed on a Varian Unity Inova spectrometer operating at a field of 7.05 T. The 29Si MAS NMR spectra were recorded at a frequency 121.43 MHz, using radio frequency pulses of 5:4 m, recycle delays of 30 s and 4000 transients. Samples were packed in zirconium rotors and spun at 5000–5500 Hz. Absorption spectrum at room temperature was achieved by using the diffuse reflectance spectra—DRS—with Guide Wave model 260 spectrophotometer. NIR luminescence spectra, at room and liquid nitrogen temperatures, were excited by a 488 nm Coherent Innova 400 Ar laser. The emission was dispersed by a Jarrell-Ash 34 monochromator and the signal detected by a N2(l) cooled germanium detector of Judson Infrared. Visible luminescence spectrum at liquid helium temperature was excited by a 488 nm Coherent Innova 400 Ar laser. The emission was dispersed by a Spex 1403 monochromater and the signal was detected by a GaAs photomultiplier RCA 31034 connected to a EGG PAR 124A Lock-in amplifier. The pure silica, used as standard in luminescence spectroscopy, confirms the absence of matrix emission.
Intensity (arbt. unit.)
2. Experimental procedures
200
400
600 Temperature ( C)
800
1000
Fig. 2. Differential thermal analysis of samples, undoped (a) and chromium-doped (b) silica.
groups ½ðOHÞ2 SiðOSiÞ2 ; silanois groups ½ðOHÞ SiðOSiÞ3 ; and silicon–oxygen tetrahedral of the silica framework ½SiðOSiÞ4 or Q 2 , Q 3 and Q 4 units, where the superscript indicates the number of siloxane bonds [19]. Although the characteristic peak of tridymite ð111 ppmÞ [19] is not evident in this spectrum, the presence of Q 2 and Q 3 units in the 29Si-NMR spectrum of silica heated at 1000 1C can be an indicative of silica crystallization that it is associated with breaking of cross link of silica framework. The NMR spectrum of chromium-containing silica shows one wide band with chemical shift at 111:6 ppm that is attributed at silicon–oxygen tetrahedral of the silica framework ðQ 4 Þ, it is an indicative of non-crystalline silica or silica with low crystallinity. The absorption spectrum of chromium-doped non-crystalline silica powder, Fig. 4, shows three absorption wide bands peaking at around 26 100, 21 900 and 16 200 cm1 (383, 457 and 617 nm). The last band clearly includes a shoulder in the lower energy side at 13 600 cm1 (735 nm). The wide bands showed in Fig. 4 are an indicative that this spectrum involves the Cr3þ and Cr4þ absorptions. Therefore, the deconvolution of this spectrum was fitted to better assignment. The two wide absorption bands of the sample at 21 900 and 16 200 cm1 are attributed to 3 A2g ! 4 T1g ðFÞ and 3 A2g ! 4 T1g ðFÞ transitions of Cr3þ (Oh notation), respectively, given a 1620 cm1 Dq value. These results are compared with theoretic values of transitions Cr3þ hexacoordinate for ligand field
ARTICLE IN PRESS M.A.U. Martines et al. / Journal of Luminescence 128 (2008) 1787–1790
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Table 1 Theoretic values of transitions bands Cr3þ hexacoordinate for ligand field strength, 10Dq ¼ 16 160 cm1 ; Racah’s interelectronic repulsion parameter, B ¼ 557 cm1
b
Method
4
ELEa TSDb
16 160 15 875
a b
A2g ðFÞ ! 4 T2g ðFÞ
4
A2g ðFÞ ! 4 T1g ðFÞ
19 321 21723
4
A2g ðFÞ ! 4 T1g ðPÞ
37 514 36 650
4
A2g ðFÞ ! 4 T1g ðGÞ
12 072 12 254
ELE, energy level equations. TSD, Tanabe–Sugano diagram.
a Wavelength (nm) 555
0
-50
Fig. 3. The (b) silica.
-100 Chemical Shift (ppm)
-150
-200
29
Si-MAS-NMR spectra of samples, undoped (a) and chromium-doped
Wavelength (nm) 444
500
571
667
800
Absorption (arbt. units)
400
Luminescence Intensity (arbt. units)
500
20000
571
714
833
16000
14000
12000
(b) (a) 18000
Wavenumber (cm-1) Fig. 5. Luminescence spectra in the visible region of SiO2:Cr, lexc ¼ 488 nm, at room (a), and liquid N2 (b) temperatures. The inset is the deconvolution of the N2 (l) spectrum.
25000
20000
15000
Wavenumber (cm-1) Fig. 4. Deconvolution absorption spectrum of SiO2:Cr at room temperature.
strength, Table 1, calculated from energy level equations (ELE), 3þ calculated by using ELE of d ion in octahedral symmetry and from TSD [20]. The TSD was obtained from the same ELE, but the transitions values estimated from TSD are different values directly calculated from ELE. The absorption band at 26 100 cm1 is attributed to 3 A2 ! 3 T2 ðPÞ and also to the ligand–metal charge transfer Cr4þ transitions. The shoulder at 13 600 cm1 is assigned to 3 A2 ! 3 T2 ðFÞ Cr4þ transition. Comparing with chromiumdoped barium orthosilicate materials [9] results it can be observed that in this work the spectrum has components from absorption spectrum of Ba2Cr0.01Si0.99O4 at room temperature. The visible luminescence spectra at room and He(l) temperatures are shown in Fig. 5. A wide band with the maximum intensity at ca. 16 000 cm1 is observed at room temperature and a high intensity band appears in the same region at He(l) temperature. The emission band in the visible region is attributed to 4 T2g ! 4 A2g (Oh notation) of Cr3þ and the sharp peaks superimposed on the luminescence spectra are due to artifacts of equipments. The spin forbidden transitions are not observed indicating that 4 T2g level is lower than 4 T2g =2 Eg crossover point. In 3 the 4 A2 ground state the Cr3þ ð3d Þ ion has a strong preference for
octahedral coordination [21]. However, the SiO2 lattice does not offer a cation site with octahedral coordination. It is unlikely that the Cr3þ replaces Si4þ due to large difference in ionic radii, 75.5 pm for octahedral chromium ion and 40 pm for tetrahedral Si4þ , and the necessity of charge compensation [22]. Then, in this work hexacoordinated Cr3þ is probably present as segregated phase in the system [23]. Cr3þ in LaPO4:Cr and in LaMgB5O10:Cr is not built into the host lattice, but is present as a second phase with a glassy character [22]. Therefore, the Cr3þ ion creates its own octahedral coordination probably in the non-crystalline phase. The NIR luminescence spectra at room and N2(l) temperatures are in Fig. 6. Room temperature luminescence spectrum is very weak. At N2(l) temperature the spectrum shows a wide band with the maximum intensity at about 7400 cm1 with a tail at high energy side and a line at 7064 cm1 followed by lower intensity shoulders at lower energy side. The wide band is formed by three narrow peaks at 7373, 7248 and 7064 cm1 . Low intensity shoulders are observed at lower energy side. This wide emission band in NIR region is attributed comparing with Mg2 SiO4 : CrO4 4 result [1,11], assigned to the emission from 3 T2 state caused by tetrahedral deformation where Cr4þ ion occupies Si4þ on ½SiO4 4 sites of Cs symmetry [4]. However, the attribution of Cr4þ based on the TSD is difficult, because Cr4þ transitions corresponds to positions below and above the crossing point of 1E and 3 T2 states [3,6–9], respectively. Furthermore, the transitions of the 1E and 1 A1 levels, which do not depend on the crystal-field strength, are not easily observed [4]. In addition, the emission lifetime at different temperature can give help in interpreting the attribution of the transition of 1E and 3 T2 states. The emission band of N2(l)
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Acknowledgments
Wavelength (nm) 833 909
1111
1250
1429
1667 We gratefully acknowledge Ana Maria Alexiou for absorption measurements assistance and Henrique Eisi Toma from IQ/USP, Sa˜o Paulo, for equipment availability. M.A.U.M. thanks CAPES for scholarship. Financial supports by FAPESP and CNPq are gratefully acknowledged.
Luminescence Intensity (arbt. unit) 12000
1000
References
(b) (a) x 10
11000
10000
9000
Wavenumber
8000
7000
6000
(cm-1)
Fig. 6. NIR luminescence spectra of SiO2:Cr, lexc ¼ 488 nm, at room (a), and liquid He (b) temperatures.
temperature spectrum indicates that the 3 T2 ðFÞ ! 3 A2 dipole electric allowed transition is present in this region with its maximum intensity at ca. 7000 cm1 . Probably, in this system the 3 T2 state position is near 3 T2 =1 E crossover point. The strong broad band emission is similar to that one found in chromium-doped silicate glasses [4].
4. Conclusion Chromium-doped glass ceramic silica powder samples thermal treated at 1000 1C present the three and four valences chromium. The visible emission band indicates that the Cr3þ Oh group is segregated on non-crystalline silica. The structured NIR emission band is due to tetrahedrally coordinated Cr4þ that replaces Si4þ in ½SiO4 4 isolate groups, or coordinated Cr4þ that replaces Si4þ in crystalline phase of glass ceramic silica.
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