Inorganica Chimica Acta
A convenient preparation of nano-powders of Y2O3, Y3Al5O12 and Nd:Y3Al5O12 and study
of the photoluminescent emission properties of the neodymium doped oxide
Daniela Belli Dell’Amico,a Paolo Biagini,b Giovanni Bongiovanni,c Stefano Chiaberge,b Alessio Di Giacomo,a Luca Labella,a* Fabio Marchetti,a Gianluigi Marra,b Andrea Mura,c Francesco
Quochi,c Simona Samaritania and Valerio Sarritzuc
aDipartimento di Chimica e Chimica Industriale, Università di Pisa, via G. Moruzzi 13, I-56124
Pisa, Italy
bCentro Ricerche per le Energie Rinnovabili e l’Ambiente. Istituto Donegani, ENI S.p.A.,via G.
Fauser 4, I-28100 Novara, Italy.
cDipartimento di Fisica, Università degli Studi di Cagliari, I-09042 Monserrato, (CA), Italy *Corresponding author. Tel.: 0039 050 2219206; E-mail address: luca.labella@unipi.it
ABSTRACT
[Y(O2CNBu2)3], 1, has been prepared by extraction of the Y3+ ions from aqueous solution into heptane by the NHBu2/CO2 system. Exhaustive hydrolysis of 1 produced the yttrium carbonate [Y2(CO3)3·n H2O] (n = 2-3), Ycarb, that was converted to Y2O3 by thermal treatment at 550 °C for 12 h (Yox). The exhaustive hydrolysis of 1 and [Al(OBu)3] (Y/Al molar ratio = 3/5) carried out at room temperature yielded an intermediate mixed carbonate, YAlcarb that, upon heating at 950 °C for 12 h, was converted to Y3Al5O12 (YAlox). The exhaustive hydrolysis of 1 and [Al(OBu)3] was repeated in the presence of [Nd(O2CNBu2)3] (Nd/Y molar ratio = 0.07 ;(Nd +Y)/Al molar ratio = 3:5). The neodymium doped garnet Nd:Y3Al5O12 was obtained,
Nd:YAlox1, through the intermediate formation of a mixed hydroxo-carbonate, Nd:YAlcarb1.
For comparison, the neodymium doped garnet was prepared also starting from N,N-dialkylcarbamato complexes of all three metals, 1, [Nd(O2CNBu2)3] and [Al2(O2CNiPr2)6] (Nd/Y molar ratio = 0.07 ;(Nd +Y)/Al molar ratio = 3:5). Also in this case the intermediate mixed hydroxo-carbonate, Nd:YAlcarb2, after heating at 950 °C for 12 h, evolved to Nd:Y3Al5O12,
Nd:YAlox2.
FTIR, XRD, SEM, TGA measurements were used for the characterization of the obtained materials. Preliminary studies of the photoluminescent emission properties were carried out on
Photoluminescence (PL) dynamics in Nd:Alox1 powder samples have been investigated by means of femtosecond laser pulses and nanosecond temporal resolution up to the millisecond range after excitation. All the luminescence traces have shown a decay time of the order 200 microseconds indicating its potential as a laser medium for infrared emission at 1064 nm.
Keywords: metal carbamato complexes; lanthanides; yttria; YAG; Nd:YAG; luminescence
1. Introduction
The preparation of finely divided oxides via hydrolysis of molecular precursors is currently widely exploited [1]. This method of synthesis allows the use of relatively low temperatures and the possibility to carry out the process in solvents of low polarity which are little involved in the particle aggregation process. Moreover, when multi-metal oxides are required, this route guarantees a homogeneous distribution of the different components of the system. Among the metal complexes chosen as precursors, metal alkoxides or metal carboxylates are often encountered [2].
Our experience in the field of the metal carbamato complexes, [M(O2CNR2)n], drove us to the study of their partial and exhaustive hydrolysis [3]. For instance, silicon or aluminum derivatives undergo complete hydrolysis with formation of dialkylammonium silicates [NH2R2]2SinO2n+1 or aluminates [NH2R2]AlnO(3n+1)/2 [4].
Recently, the development of a facile synthesis of lanthanide carbamato species [5] offered us the availability of a series of attractive precursors for the preparation of lanthanide oxides or mixed oxides containing lanthanide centers. Starting from cerium(III) N,N-dibutylcarbamate, the hydrated metal carbonates obtained by hydrolysis at room temperature [6] were readily converted to CeO2 by treatment at 200 °C in air. The method was extended to the preparation of nanostructured doped ceria affording cerium/lanthanum and cerium/terbium mixed oxides containing a single crystalline phase with the two metals in the preset molar ratio [7]. The method was adopted with success also for the synthesis of the stoichiometric lanthanum cuprate La2CuO4 [8]. The similar lability [9] of the metal centers involved in the processes [Ce(III), La(III), Tb(III), Cu(II)] justify the achievement of these goals. In fact the hydrolysis of the precursors is presumed to proceed at a similar rate, allowing the simultaneous precipitation of the intimately blended early products, probably crucial for the prompt obtainment of the desired oxide by the final thermal treatment.
Yttrium aluminum garnet, Y3Al5O12 (YAG) is an interesting mixed oxide in view of its features like high hardness, strong mechanical resistance, really good chemical stability at elevated temperature, excellent optical properties [10]. In the garnet yttrium ions can be partially substituted by luminescent lanthanide ions without segregation of other phases up to certain lanthanide concentration [11], because no considerable geometrical deformation is involved in view of the similarity of the ionic radii of lanthanide(III) and yttrium ions. These peculiarities make YAG a good material to host lanthanide, suitable also for laser preparation, being able to resist to high intensity radiation. For instance, neodymium doped YAG is commonly used as laser in the IR region. Although single crystals have been widely used in this field, currently a growing interest is addressed to ceramic materials, prepared by powder sintering [12].
The synthesis of pure YAG [Error: Reference source not found] requires to overcome some problems, in part related to other stable phases in the Y2O3-Al2O3 system. This system includes, besides the binary oxides Y2O3 and Al2O3, monoclinic Y4Al2O9 (YAM), hexagonal YAlO3 (YAH), orthorhombic YAlO3 (perovskite, YAP). As a consequence, control over phase purity is not always achieved [13]. Moreover, when the preparation of a ceramic material is the goal, easy to sinter powders are desirable; that means that finely divided solid with a homogeneous microstructure are required and high temperatures processes resulting in inconvenient growth of the crystallite sizes must be avoided.
Among the YAG preparation methods the wet ones are usually preferred and co-precipitation procedures are often used to obtain finely divided powder. When metal salts are chosen as precursors, water is usually the solvent and ammonia or ammonium hydrogen-carbonate (directly introduced or produced by urea hydrolysis) are the agents inducing precipitation of an early solid essentially composed by metal hydroxides [14] or metal carbonates [15, 16, 17], respectively. Indeed metal carbonates turned out to be excellent precursors for the preparation of ceramics based on metal oxide. For instance, yttrium carbonate has been successfully chosen for the production of highly sinterable yttria powder [18].
As precursors of YAG also alkoxides of both metals [Error: Reference source not found, 19] or a mixture of aluminum alkoxide and yttrium carboxylate have been employed [20] by operating in hydro-alcoholic solution.
In this paper we describe the preparation of the yttrium N,N-dibutylcarbamato complex, its exhaustive hydrolysis and conversion to nano-structured yttria, Y2O3, and the preparations of nano-crystalline YAG, and neodymium doped YAG starting from solutions containing N,N-dibutylcarbamato complexes of yttrium and a hydrolysable precursor of aluminum in the Y:Al = 3:5 molar ratio. Although aluminum is a relatively-inert centre with respect to the yttrium and
lanthanide ones [Error: Reference source not found], the germinal precipitates leading to YAG or Nd:YAG by this route show the desired composition and evolve by thermal treatment to pure products (powder-XRD) at a temperature lower than those usually necessary.
2. Experimental 2.1. General methods
Commercial yttrium oxide (Y2O3, Strem Chemicals, 99.9%) and NHBu2 (Sigma Aldrich, 99+%) were
used without further purification. Aqueous solutions of yttrium chloride (0.30 M circa) were
prepared by dissolving the appropriate amount of the metal oxide in diluted hydrochloric acid. The solution was then evaporated to dryness and the solid residue was dissolved in water. Al(OBu)3 (Sigma Aldrich, 95 %) was used directly. In view of the margin of error about composition declared by the supplier, its aluminum content was determined by gravimetric methods after hydrolysis and calcination to Al2O3. [Al2(O2CNiPr2)6] [21] was prepared according to literature.
All manipulations except extraction were performed under dinitrogen atmosphere unless otherwise noted. FTIR spectra in the solid state were recorded with a Perkin–Elmer ‘‘Spectrum One” spectrometer, with ATR technique. The metal content of yttrium in [Y(O2CNBu2)3] was determined according to this procedure: a sample of the product was treated in a platinum crucible with diluted HNO3 and the mixture gently warmed; the resulting solution was then evaporated to dryness. After calcination, the weight of the solid residue [corresponding to the metal oxide Y2O3] was determined. Volumetric analyses via EDTA titration were also carried out on the aqueous layer discarded after extraction.
Thermogravimetric analyses were performed on a NETZSCH STA Jupiter F1 instrument. A weighted amount (about 30 mg) of solid sample was placed in an alumina crucible and analyzed using a temperature program from 30°C to 950°C with a heating rate of 10°C/min in dry air atmosphere. The sample weight was recorded and plotted vs Time /Temperature. According to the weight variation, the amount of water can be roughly determined and the final weight of the sample should be related to the residual metal oxide. The morphology of the samples was characterized using scanning electron microscopy by a Field Emission Jeol F7600f by using an 5 keV energy electron beam.
Samples were characterized by X-Ray diffraction by means of a Powder Diffractometer Panalytical X’Pert Pro in Bragg-Brentano geometry using Cu K radiation (=1.5416 Å) with the X-ray tube set to 40 V and 40 mA. The spectra were collected in the range 5-90°(2) with
step size 0.02° and time acquisition set to 15 sec/step. Qualitative phase analysis of powder samples has been carried out by means of standard method [22]. Full profiles fitting procedures implemented in TOPAS program suite allowed structural refinements of any recognized phases. Accurate values of the coherent scattering domains lengths have been obtained by means of FPA (Fundamental Parameter Approach) implemented in Topas Suite [23].
The PL emission from Nd:Alox1 powders was investigated by both integrated and time-resolved photoluminescence (TPRL) spectroscopy. In the former case, PL emission was investigated by exciting the samples with a CW Ti: Sapphire laser operating at 800 nm (Spectra Physics Tsunami). The signals, dispersed by a spectrometer, were detected with a cooled InGaAs diode array camera (Andor iDUS 491A). TRPL measurements were obtained by optically pumping with 790 nm laser pulses of about 150 fs width (FWHM) and 1 kHz repetition rate from a Ti: Sapphire amplified laser system (Quantronix Integra-I). PL emission was dispersed in a spectrometer equipped with a cooled fast phototube with a 1 ns rising time (Hamamatsu NIR-PMT H10330B-75). The signal was then analyzed with a 1 GHz oscilloscope (Textronix TDS 5104).
2.2. Syntheses
2.2.1. Extraction of yttrium(III) from aqueous solution into heptane by the NHBu2/CO2 system.
Synthesis of Y(O2CNBu2)3, 1.
A solution of dibutylamine (10 mL, 59.3 mmol) was saturated with carbon dioxide and then 20 mL (6.00 mmol) of an aqueous solution of YCl3(aq), at 0 °C, were added to the mixture. Upon shaking for a few seconds at 0 °C the organic layer became pale yellow, while the aqueous phase became colorless. The organic layer was separated and quickly evaporated at reduced pressure (1,0 x 10-3 Torr) at 40 °C. By long drying (5h) a colorless solid (2.47 g, 71.6% yield) was obtained. Anal. Calc. for C27H54N3YO6: Y, 14.7%; Found Y, 14.1%. FTIR, ATR: (the most significant bands in the range 1700-1250 cm-1): 1528s, 1488s, 1423s, 1376m, 1313s.
2.2.2. Exhaustive hydrolysis of 1.
To a solution of [Y(O2CNBu2)3] (6.42 g, 10.6 mmol) in 50 mL of anhydrous toluene a solution of water (9.60 mL, 0.53 mol) in THF (25 mL) was slowly added dropwise. The mixture was vigorously stirred for 3 h at room temperature and the formation of a colorless suspension was observed. The liquid phase was decanted and the solid was washed with a THF/toluene solution (25 mL/50 mL) for two times. The mixture was stirred for 5 h for each washing procedure. The
solvent was every time removed after decantation and added to the mother liquor and the solution, once evaporated to dryness, left a negligible amount of residue that was discarded. The solid was dried in vacuo at room temperature for 1 h and at 40 °C for 5 h (Ycarb, 2.09 g; 95.2% yield). Anal. Calc. for [Y2(CO3)3 · 3 H2O] , Y2C3H6O12: Y, 43.2%; Found Y, 42.9%. FTIR, ATR: see Results and discussion. X-Ray powder diffraction pattern showed the presence of a single crystalline phase: orthorhombic tengerite(-Y) [Y2(CO3)3 · 2-3 H2O, JCPDS card no. 16-0698] [24]. By heating Ycarb at 550 °C for 8h, Yox was obtained. X-ray powder diffraction measurements showed the complete conversion to the cubic crystalline phase of Y2O3 (space group Ia3, JCPDS card no. 41-1005). Further heating did not cause significant mass reduction or appreciable changes in the XRD pattern of the product.
2.2.3. Synthesis of Y3Al5O12, (YAlox) with intermediate formation of YAlcarb.
A solution of [Al(OBu)3] (4.0 g; 15.42 mmol) in DME (15 mL) and BuOH (5 mL) was added to a solution of [Y(O2CNBu2)3] (5.47 g; 9.03 mmol) in toluene (50 mL) with formation of an opaque mixture that was gently warmed up to 40 °C with obtainment of a clear solution. A solution of H2O (1.53 mol) in THF (25 mL) was then added dropwise at room temperature. Gas development was observed. The mixture was vigorously stirred for 3 h with formation of a colorless suspension. The liquid phase was decanted and the solid, gelatinous in aspect, was washed with a THF/toluene solution (25 mL/50 mL) for two times. The mixture was stirred for 5 h for each washing procedure. The solvent was every time removed after decantation and added to the mother liquor and evaporated to dryness leaving a negligible amount of residue that was discarded. After the washing procedures the colorless solid, that had acquired a treatable form, was dried in vacuo at room temperature for 1 h and at 40 °C for 8 h. A portion of the product, YAlcarb, was heated at 950 °C with a mass loss of 49.0 %. FTIR-ATR: (most significant bands, cm–1): 3300m (broad), 1506m, 1404m, 1328w (sh). The product appeared amorphous after X-Ray diffraction measurement. TGA (in air, 30-900 °C, TG = 10 °C/min) showed Δm = 48.1%. X-Ray diffraction measurements carried out on the powder obtained after calcination at 950 °C (YAlox) revealed the presence of a single crystalline phase corresponding to the cubic phase of Y3Al5O12.
2.2.4. Synthesis of Nd:YAOx1 and Nd:YAOx2
a) With Al(OBu)3 as precursor, Nd:YAOx1. To a solution of [Y(O2CNBu2)3] (5.58 g, 9.22 mmol) and [Nd(O2CNBu2)3] (44 mg, 0.067 mmol) in 50 mL of toluene a solution of Al(OBu)3 (4.00 g, 15.42 mmol) in 15 mL of DME and 5 mL of nBuOH was added. A solution of water
(28.1 mL, 1.56 mol) in 25 mL of THF was added dropwise to the mixture warmed at 40 °C. A pale grey suspension was formed. The mixture was refluxed for 4 h, then the suspension was decanted, the mother liquor was removed and the sticky solid residue was washed with a THF/toluene solution (25 mL/50mL) for two times. The suspension was stirred for 5 h in the course of each washing procedure. The solvent was every time removed after decantation and added to the mother liquor: this solution, once evaporated to dryness, left a negligible amount of residue that was discarded. The solid was finally dried in vacuo at room temperature for 1 h and at 40 °C for 6 h, Nd:YAlcarb1, (2.77 g). FTIR, ATR (most significant bands, cm–1): 3300m (broad), 1506s, 1404s. By heating the product at 950 °C Nd:YAlox1 was obtained. X-Ray powder diffraction measurements carried out on a sample of Nd:YAlox1 showed the presence of a single crystalline phase corresponding to the cubic phase of Y3Al5O12.
b) with [Al2(O2CNiPr2)6] as precursor, Nd:YAOx2. A solution of [Y(O2CNBu2)3] (13.1 g, 21.6 mmol), [Nd(O2CNBu2)3] (0.086 g, 0.13 mmol) and [Al2(O2CNiPr2)6] (16.6 g, 18.0 mmol) in 350 mL of anhydrous toluene was treated under vigorous stirring with a solution of H2O (52.9 mL, 2.94 mol) in THF (50 mL), added dropwise at room temperature and then the suspension was refluxed for 6 h. The colorless suspension was decanted, the mother liquor was removed and the solid was washed with a THF/toluene solution (25 mL/50mL) for two times. The suspension was stirred for 5 h in the course of each washing procedure. The solvent was every time removed after decantation, added to the mother liquor and evaporated to dryness. The negligible amount of residue was discarded. The colorless solid was dried under vacuum (Nd:YAlcarb2). FTIR, ATR (most significant bands): 3300w, broad, 1500s, 1404s. This product was treated at 950 °C with conversion to Nd:YAlox2. X-Ray powder diffraction pattern of Nd:YAlox2 showed the presence of a single crystalline phase corresponding to the cubic phase of Y3Al5O12.
3. Results and discussion
The preparation of the N,N-dibutylcarbamato complex of yttrium was carried out by extraction of the yttrium ion from the aqueous solution of its chloride into heptane by NHBu2 saturated with CO2, according to a method previously used with success with lanthanides [Error: Reference source not found]. By this approach, the homoleptic derivatives [Ln(O2CNBu2)3] or the carbonato-carbamato species [NH2Bu2]2[Ln4(CO3)(O2CNBu2)12] or [Ln4(CO3)(O2CNBu2)10] are obtained, in dependence of the reaction conditions and/or the lanthanide identity. With yttrium, according to the Y content, a product with composition [Y(O2CNBu2)3], 1, has been obtained with good reproducibility. The IR spectrum of the compound is similar to the ones of
other lanthanide derivatives with the same formula, [Ln(O2CNBu2)3] (see Table 1) and shows strong bands, due to the O2CN stretching vibrations, in the 1600-1300 cm–1 region.
Table 1. IR spectra of [Y(O2CNBu2)3], 1 and [Ln(O2CNBu2)3] complexes in the region 1700-1300 cm–1: the most significant bands (cm–1)
Y 1528 1488 1423 1376 1313 This work Ce 1520 1482 1423 1375 1314 [Error: Reference source not found] Nd 1528 1490 1430 1378 1325 [Error: Reference source not found]
The exhaustive hydrolysis of [Y(O2CNBu2)3], 1, was carried out in toluene/THF at room temperature. The yttrium content of the colorless solid, Ycarb, was in agreement with the formula Y2(CO3)3·3 H2O. Moreover, its XRD pattern (Figure 1) showed the presence of a single crystalline phase, corresponding to tengerite(-Y), with composition Y2(CO3)3 · 2-3 H2O, the variable water content in the mineral being due to its zeolite-type structure [Error: Reference source not found]. The powder spectrum shows an evident anisotropic broadening of the peaks width that cannot be described as quadratic function of tan(2). This aspect is more evident along the (0l0) peaks and probably is due to the elongated fibrous morphology of spherulities of the sample as well shown by SEM micrograph (Figure 2).
Figure 2. SEM image of Ycarb
Also the IR spectrum of our compound (ATR mode) corresponded to the one reported for
tengerite(-Y) (KBr disk) [Error: Reference source not found]. The most significant bands were
observed at 3300 (m, broad) and 1650 (sh) cm–1 attributable to the H2O stretching and bending vibrations, respectively, and at 1507 (s), 1466 (m), 1417 (s) cm–1 attributable to the CO3 group stretching ones [Table 2].
Figure 3. TGA of Ycarb (in air, 30–900°C, Heating rate = 10 °C/min)
Products with composition Y2(CO3)3·2-3 H2O have been reported as obtained by a hydrothermal procedure exploiting the urea promoted hydrolysis of yttrium nitrate [25] or by treatment of yttrium nitrate with ammonium hydrogencarbonate in water [26].
Ycarb was completely converted to Y2O3 by heating at 550 °C for 8 h with a mass loss corresponding to 45.0 %, in agreement with the theoretical value (45.2 %) expected for the complete dehydration and decarboxylation of Y2(CO3)3·3 H2O. In view to the zeolite nature of the product, the variable water content in the samples is reasonable due to their preliminary treatments before analysis. The TGA study on Ycarb carried out in air in the temperature range
30-900 °C (Figure 3) showed a slow and diffuse weight loss (43.6 %), completed just above 800°C, in agreement with the formation of Y2O3 starting from Y2(CO3)3·2.5 H2O, while the transition discerned at around 220°C (weight loss around 20 %) might be ascribed to the complete loss of water and one molecule of CO2. Similar behavior in TGA analysis of Y2(CO3)3·2-3 H2O were already observed by Fedorov and coworkers [24a].
The XRD study of the powder obtained after calcination (Yox) showed the formation of a single crystalline product corresponding to the cubic phase of Y2O3 (Figure 4). The average size of the scattering coherent domains were estimated to be 13(2) nm. Heating to higher temperatures (up to 1100 °C) led only to a narrowing of the peaks in the XRD pattern due to the gradual increasing of the mean crystallite size.
Figure 4. XRD pattern of Yox (550) (black) compared with cubic-Y2O3 (black, JCPDS no 41-1105). Body-centred cubic structure (bcc), S.G. Ia3 a = 10.620(2) Å
SEM micrographs of Yox are presented in Figure 5; the sample shows a complicated morphology in which there are broken oblate shaped spherulities extended till to 10 µm, which are constituted by elongated micrometric fibrous aggregates. Each aggregate is composed by a stack of sub-micrometric thinner fibers of the same length of the aggregate.
Figure 5. SEM images of Yox
With the aim to prepare YAG in form of a nano-powder, [Y(O2CNBu2)3] and [Al(OBu)3] in the appropriate molar ratio were dissolved at 40 °C in a mixture of solvents (DME/BuOH/toluene/THF) and hydrolyzed at room temperature. The germinal precipitate, after the usual work-up, was labelled as YAlcarb. The IR spectrum of YAlcarb showed bands at 1506 and 1404 cm–1 attributable to stretching vibrations of the carbonate moiety (Table 2).
Table 2. IR spectra of the early hydrolysis products (most significant bands, cm–1) compared with literature data
Ycarb 3300 (broad) 1650 1507 1466 1417 This work
Tengerite Y2CO3)3·2-3 H2O 3500-3000 1650 1510 1463 1425 [Error: Reference source not found]
YAlcarb 3300 (broad) 1506 1404 This work
Nd:YAlcarb1 3300 (broad) 1650 1505 1393 This work
Aln(OH)3n-2(CO3)·hH2O 3400-3300 (broad) 1500-1300 (broad) [27]
[NH4][Al(OH)2CO3] 3450, 3170,, 3020, 2850 1540 1450 [28]
It is not easy to assign a composition to the product. The formula Y3Al5(CO3)5(OH)14·12 H2O is in good agreement with the mass loss measured after heating the product at 950 °C with conversion to the cubic phase of Y3Al5O12, as discussed below (48.6 % calc. versus 49.0 % exp). Any way, also other formulae of the type Y3Al5(CO3)x(OH)24-2x·n H2O, could fit the measured mass loss [29]. The TGA study on YAlcarb, carried out in air in the temperature range 30-900 °C, is reported in Figure 6. The weight loss (48.1 %) proceeds slowly and without any discrete transition, till to about 500 °C agreeing with the formation of Y3Al5O12 starting from the supposed early formulation.
Figure 6. TGA of YAlcarb (in air, 30–900°C, TG = 10 °C/min)
The XRD pattern of the powder revealed that the material was amorphous and a study on samples progressively treated at higher temperatures showed that in the course of they hydration and decarboxylation they remained amorphous up to 850 °C. After heating at 950 °C for 4 h
YAlcarb transformed into a derivative, YAlox, showing an XRD pattern corresponding to the
one of cubic YAG (Figure 7).
Figure 7. XRD pattern of YAlox (black) compared with YAG peaks (JCPDS no 00-033-0040). Body-centered cubic structure (bcc), S.G. Ia3d , a = 12.0170(4) Å
The average size of the scattering coherent domains were estimated to be 42(2) nm. Heating to higher temperatures (up to 1100 °C) did not lead to appearance of other crystalline phases, but only to a narrowing of the peaks due to crystal growth.
On the base of this favorable outcome, similar procedures were tried for the synthesis of neodymium doped YAG. A former attempt was performed starting from 1, [Al(OBu)3] and [Nd(OCNBu2)3] in the 2.98 : 5 : 0.02 molar ratio. The hydrolysis was carried out in the solvent mixture DME/BuOH/toluene/THF, in this case at the reflux temperature (about 110 °C) to accelerate the process. The solid obtained after washing and drying procedures, Nd:YAlcarb1, transformed to Nd:YAG by heating at 950 °C with a mass loss of 38.8 %. A derivative with the composition Y3Al5(OH)16(CO3)4 · n H2O (n about 4) has been reported in the literature as produced by hydrolysis of aluminum and yttrium salts in the presence of urea carried out in water at 95 °C [Error: Reference source not found]. We suggest the composition Y2.98Nd0,02Al5(OH)16(CO3)4 · n H2O (n about 4) for our compound since it is in good agreement with the measured mass loss due to the thermal dehydration and decarboxylation. It is reasonable to presume that the presence of a small percentage of neodymium does not significantly modify the process of precipitation, therefore Nd:YAlcarb1 should differ from
YAlcarb essentially since it is formed at higher temperature.
Nd:YAlcarb1 showed in its IR spectrum (table 2) absorptions attributable to water and
carbonato groups. The XRD study of the powder deriving by its calcination at 950 °C (Nd:YAlox1) (Figure 8) revealed the presence of only one crystalline phase with the peaks essentially congruent with the ones of cubic body centered YAG. The average size of the scattering coherent domains were estimated to be 42(2) nm.
Figure 8. XRD pattern of Nd:YAlox1 (black) compared with YAG peaks (blue, JCPDS no 00-033-0040). Body-centered cubic structure (bcc), S.G. Ia3d a = 12.0416(2) Å
The comparison of the powder diffraction pattern of Nd:YAlox1 with the reference pattern of YAG (JCPDS 33-0040) suggests a substantial equivalence of the structure. A profile fitting on the YAG peaks leads to a lattice parameter of 12.0416 Å, greater than the value estimated for
YAG, as expected for the partial substitution of Y3+ by the larger Nd3+ ions. This interpretation is consistent with the observed progressively greater shift of the peaks at lower values of 2θ with respect to the YAG ones.
The composition of the product, in view of the complete precipitation of the metal ions, can be considered very close to Y2.98Nd0.02Al5O12.
Low magnification SEM micrographs of YAlox and Nd:YAlox1 are reported in Figure 9 and 10, respectively, in both cases compacted powders with dimensions ranging from tens to hundreds µm are observed without any particular internal fine morphology.
Figure 9. SEM images of YAlox
Figure 10. SEM images of Nd:YAlox1
An alternative preparation of a neodymium doped garnet was carried out with a procedure similar to the one described above but with the use of freshly prepared [Al2(O2CNiPr2)6] instead of commercial aluminum tri-butoxide. Also in this case the yttrium, aluminum and neodymium precursors were used in 2.98:5:0.02 Y:Al:Nd molar ratio in order to produce again, as ultimate
product, a species with the composition of Y2.98Nd0.02Al5O12. The hydrolysis, carried out in toluene/THF at the reflux temperature, produced an early product, Nd:YAlcarb2, that underwent a loss mass by heating at 950 °C analogous to the one observed for Nd: YAlcarb1 described above. The IR spectra of the two products resulted very similar, too. Moreover, the XRD study of the powder deriving by its calcination at 950 °C (Nd:YAlox2) revealed the presence of only one crystalline phase with a pattern superimposable to the one of Nd:YAlox1. Although the hydrolysis processes are expected to proceed with a different path and to involve different intermediates, the use of the aluminum carbamato complex in substitution of commercial [Al(OBu)3] did not affect the nature of the products in the described conditions.
Optical measurements on Nd:YALox1
The PL emission properties of the Nd:Alox1 powders were investigated by CW and transient PL spectroscopy. The aim was to gain insight into the energies and lifetimes of the excited states of the doping ion Nd3+ in Nd:Alox1, which is a candidate for applications such as laser media. According to literature, the 4F3/2 → 4I11/2 transition (∆E = 1,165 eV, 1064 μm) is reported to be the most intense among all the other radiative decays. The radiative lifetime associated with this transition is in the range of hundreds of μs, depending on the preparation method of the sample [30]. Furthermore, one of the main features of Nd-doping in the field of solid state lasers (NIR spectral range) is related to the dense packaging of levels placed at higher energies than the emitting level 4F3/2, which can extend from NIR to UV. Excitation from these levels quickly relaxes to the 4F3/2 level and thus the 4I9/2 → 4F5/2 absorption band can be used as efficient optical pumping band in this kind of four-level laser system [30]. In this work excitation of the PL of
Nd:Alox1 samples was achieved using pulsed or CW radiation at about 800 nm as reported in
the experimental section. In order to avoid overheating of the sample, the excitation power density was always kept below 100 mW/cm2.
The PL spectrum of Nd:Alox1 is shown in Figure 9 in the 1000–1500 nm spectral window. A narrow intense peak stands out at 1064 nm, that can be ascribed to the main transition 4F3/2 → 4I11/2. The signal centered at 1124 nm is also related to the same transition, while the ones around 1330 nm are related to the 4F3/2→ 4I13/2 transition. The structured nature due to crystalline field effects of these signals, is also well recognizable [31] and all these results are in good accordance with the literature [32].
Figure 9: Emission spectra (range 1500 – 1000 nm) of Nd:Alox1 powders (λexc = 800 nm).
The PL spectrum is in good agreement with the data reported in literature for the Nd:YAG species [Error: Reference source not found, Error: Reference source not foundd], confirming the presence of Nd3+ excited ions in the solid matrix. PL measurements on different spots of the same sample and in different samples of the same species showed similar results to those reported in Figure 9, ensuring the homogeneity of the Nd:Alox1 product.
Figure 10 shows the time resolved photoluminescence decay for the transition 4F3/2→ 4I13/2 in the Nd:Alox1 powders after pulsed excitation at 800 nm with 1KHz repetition rate. From this figure it can be easily seen that the residual photoluminescence signal after 1 ms from the excitation pulse is negligible. The best fit of the experimental data (the red line in figure 10) is in good agreement with a mono exponential decay with a 2=5·10-10 over the 500 points (gray dots) reported in the plot. The luminescence intensity as a function of the time I(t), can be thus written as
where I(0) is the time intensity immediately after excitation, and τ is the average lifetime of the transition 4F3/2→ 4I13/2. The decay constant τ obtained from the fitting is τ = 217 ± 1 µs.
Figure 10: Photoluminescence decay 1064 nm of Nd:Alox1 powders . The red line represents the mono-exponential best fit.
This result is in good agreement with those (τ ~260 µs) reported in literature for Nd:YAG single crystals[Error: Reference source not founda]. The small difference between the values can be ascribed to the different nature of the samples (powder, as in this work, or single crystals) [Error: Reference source not founda].
The experimental decay constant τ measured for the Nd:Alox1 powders are also in good agreement with the ones reported for Nd:YAG (237.6 μs ,1,0%) ceramics [33], showing that the
Nd:Alox1 powders can be a good candidate for the fabrication of new ceramic media for lasers
operating in the NIR region.
Conclusions
The preparation methods of YAG and neodymium doped YAG here described offers some advantages as reported below.
1) The molecular nature of the complexes where the metal centers are surrounded by ligands with large lipophilic moieties allows the dissolution of the precursors in a scarcely polar medium where the interactions between the germinal hydrolysis products and the solvent are negligible. As a consequence a low solubility of the products is expected that leads to their complete precipitation in form of finely divided powder.
2) Alkyl-ammonium alkylcarbamate formed in the course of the metal carbamato complex hydrolysis (Scheme 1) can act as a surfactant preventing a significant aggregation of the particles. At the same time it is easy to remove it under vacuum.
3) The endogenous production of alkyl-ammonium hydrogen-carbonate (Scheme 1) in the course of hydrolysis favors the formation of metal carbonates, hydroxo- or oxo-carbonates. 4) When two or more metal precursors are used in a certain molar ratio, the early precipitates are probably mixtures of carbonato-, hydroxo- and hydroxocarbonato derivatives, but in the raw mixture an intimate blend of the different fine particles is achieved that favors the evolution towards the desired composition of the mixed oxide upon the appropriate thermal treatment.
[M(O2CNR2)n] + H2O → metal oxide/metal carbonate + CO2 + NHR2
CO2 +2 NHR2 [NH2R2][O2CNR2]
[NH2R2][O2CNR2] + H2O [NH2R2][HCO3] + NHR2
Scheme 1- Reactions and species involved in metal carbamato complex hydrolysis
As the rare earth carbamato complexes here used are easily and promptly obtained by the metal oxides, their choice is particularly convenient. About aluminum precursors the carbamato complexes have to be prepared in strictly anhydrous conditions, therefore the alternative use of a commercial easily hydrolysable product was attractive. Nevertheless, the comparison with the preparation carried out with the use of a carbamato complex also for the aluminum was considered interesting. The preparation of the neodymium doped YAG was then conducted in both ways, either starting with [Al(OBu)3] or with [Al2(O2CNiPr2)6] obtaining analogous results. Photoluminescence spectra and lifetime measurements confirmed the good optical quality of the Nd:YAG powders, hence demonstrating their potential for the development of new NIR ceramic laser materials.
Acknowledgements
This work was supported by the Università di Pisa (Fondi di Ateneo 2015 and PRA_2016_50 Materiali Funzionali, Progetti di Ricerca di Ateneo) and by MIUR PRIN 2015.
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