Article pubs.acs.org/crystal
Structural and Spectroscopic Characterization of Nd3+-Doped YVO4 Yttrium Orthovanadate Nanocrystallites Rafal J. Wiglusz,* Lukasz Marciniak, Robert Pazik, and Wieslaw Strek Institute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw 2, Poland ABSTRACT: Yttrium orthovanadate (YVO4) nanoparticles doped with Nd3+ ions (0.1−10 mol %), thermally treated at 700 to 1000 °C for 3 h were prepared using modified Pechini’s technique. The structure of products were studied using standard X-ray diffraction (XRD) technique and Rietveld method. The average particle size was found to be about 20 nm (both XRD and transmission electron microscopy) for samples annealed at 700 °C, and further rapid growth was noticed above 900 °C. The spectroscopic behavior of the Nd3+:YVO4 was studied in detailed based on the absorption, excitation, emission, and decay times measurements. The effect of the dopant concentration on progressive increase of reabsorption process and emission concentration quenching was observed. The size effect was manifested in the change of the relative intensities ratio for the resonant and laser transitions. In order to explain reverse behavior of the resonant line comparative studies with the Nd3+:YVO4 single crystal were conducted. An increase of synthetic parameters, concentration of Nd3+ cations and sintering temperature, leads to emission spectra comparable with the grinded single crystal. emission properties of the Nd3+ were fully characterized. The behavior of characteristic splitting of the absorption (4I9/2 → 2 P1/2; 4G5/2,7/2; 4F7/2; 4S3/2) and emission (4F5/2; 2H9/2; 4F3/2 → 4 I9/2; 4I11/2) for the Nd3+ f−f transitions was used to support the discussion of structural features of the YVO4 matrix.
1. INTRODUCTION Yttrium orthovanadate (YVO4) is a promising host material for various optical applications, especially doped with trivalent rareearth cations (RE3+−Eu3+, Tb3+, Ho3+, Er3+, Tm3+, and Yb3+). The YVO4 is broadly used as a heart of the solid state lasers or phosphor material due to high thermal, mechanical, and chemical stability and outstanding optical performance.1−8 It crystallizes in a tetragonal zircon structure depicted by the I41/ amd space group (no. 141).9 The RE3+ cations easily substitute the 8-fold coordinated Y3+ site with D2d point symmetry, allowing to obtain highly doped products due to the ionic radii and valence compatibility (i.e., Y3+ 1.019 Å CN 8 and Nd3+ 1.109 Å CN 8).10 Near-infrared (NIR) light-emitting optical materials have a number of potential applications, including friend/foe identification (IFF) in military action,11 night-vision illumination devices,12 chemical and biological sensing,13 consumer electronics,14 spectroscopic analysis,15 etc. The NIR light at 0.8−1.0 μm converges well with the wavelengths of one of the most common types of lasers, like for instance solid state YAG:Nd3+ laser emitting main line at 1064 nm.16 In the case of rare earth cations, the electrons in partially filled 4f inner shell are well shielded by outer 5s2 and 5p6 shells. Therefore, the positions of emission and absorption bands associated with 4f− 4f transitions are not strongly influenced by an external crystal field.17 The main goal of the presented paper is focused on evaluation of physicochemical properties of the YVO4 nanoparticles activated with the Nd3+ ions with special emphasis on luminescence properties of the obtained system. The role of the reaction conditions and influence of dopant concentration on the final form of the YVO4 particles was underlined. The © 2014 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Instruments. The X-ray diffraction (XRD) patterns were measured in a 2θ range of 5−100o with an X’Pert Pro PANalytical Xray diffractometer (Cu Kα 1:1.54060 Å). The mean size of crystallites was calculated using Rietveld refinement as well as using the Scherrer equation for comparison:
D=
kλ cos Θ β 2 − β02
(1)
where the symbols represent D, the grain size; β0, apparatus broadening; β, full width at half-maximum; Θ, angle; k, constant (usually equal to 0.9); and λ, X-ray wavelength.18 The morphology and microstructure of Nd3+:YVO4 nanoparticles were investigated by highresolution transmission electron microscopy (HRTEM) using a Philips CM-20 Super Twin microscope, operated at 200 kV. Samples for HRTEM measurements were prepared by dispersing powders in methanol and further deposition of suspension droplet on a copper microscope grid covered with perforated carbon. The size of particles was estimated using a volume-weighted formula: dav =
Σnidi4 Σnidi3
(2)
Received: May 9, 2014 Revised: September 20, 2014 Published: October 3, 2014 5512
dx.doi.org/10.1021/cg500686h | Cryst. Growth Des. 2014, 14, 5512−5520
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Figure 1. Projection of the YVO4 unit cell with indication of the Y3+ and V5+ coordination polyhedra.
Figure 2. XRD patterns of the YVO4 powders as a function of sintering temperature (left) and Nd3+ content (right). where dav is the average particle size, n is the number of particles, and d represents particle diameter. The optical absorption spectra were recorded using a Cary-Varian 5EUV−vis/near-IR spectrophotometer in the 180−2400 nm spectral region. Emission spectra were measured with a Jobin-Yvon THR1000 spectrophotometer equipped with a Hamamatsu R5108 photomultiplier as a detector and 1200 L/mm grating blazed at 500 nm, as the excitation source of the 514 nm line of Argon laser was used. Excitation spectra were measured using a 450 W xenon arc lamp coupled with a 275 mm excitation Spectra Pro 750 monochromator, which used a 1800 L/mm grating blazed at 250 nm. All presented spectra were corrected according to the detector characteristics. For better clarity of presentation, most of the results were normalized where necessary. Luminescence kinetics was the recorder for the 4F3/2 → 4I9/2 transition at 880 nm, using as an excitation source a 532 nm line (second harmonic) of the Nd3+:YAG laser LOTIS TII, operating in the pulse mode and a Jobin-Yvon THR 1000 spectrophotometer equipped with Hamamatsu R5108 photomultiplier as a detector and digital oscilloscope LeCroy WaveSurfer 400 MHz. 2.2. Synthesis of YVO4:Nd3+ Nanoparticles. A general strategy for synthesis of the YVO4 nanopowders has been adopted by us from previous work.19 The 1 mol % Nd3+-doped YVO4 powders were prepared by using 0.5588 g (2.47 mmol) of Y2O3 (99.99% Alfa Aesar), 0.0084 g (0.025 mmol) of Nd2O3 (99.99% Alfa Aesar), and 0.4547 g (2.5 mmol) of V2O5 (99.99% Sigma-Aldrich). Stoichiometric amounts of lanthanide oxides were first digested in excess of HNO3 (ultrapure Avantor Poland) and further on obtained RE3+ nitrates, which were recrystallized three times using MQ-water. Afterward, the V2O5 was
processed in excess of diluted NH4OH (pure Avantor Poland) in order to transform it into soluble ammonium salt and mixed with RE3+ nitrates. Furthermore, 24 g (0.125 mol) of anhydrous citric acid (99.5% Sigma-Aldrich) and 2.5 mL (0.045 mol) of ethylene glycol (extra pure Avantor Poland) were added to the reaction mixture and heated up to 80 °C in order to create a viscous liquid. The viscous solution was left in the laboratory drier at 90 °C for several days until a dark brown resin was formed. The last step of synthesis involved post heat treatment at the temperature range of 700−1000 °C for 3 h. The rest of the YVO4 powders containing different concentration of the Nd3+ ions (0.1−10 mol %) were prepared in exactly the same way.
3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The crystalline phase of the YVO4 has tetragonal zircon structure with lattice parameters a = 7.1183 Å and c = 6.2893 Å,9 whereas substitution of the Y3+ with Nd3+ occurs at site with D2d point symmetry (see Figure 1). The formation of crystalline YVO4 phase doped with Nd3+ was studied either as a function of annealing temperature and dopant concentration by means of XRD technique (Figure 2). Comparison of the resulting diffraction patterns with the reference standard of the YVO4 (ICSD-78074) confirmed the presence of the pure orthovanadate structure without any structural impurities. As can be clearly seen, the crystalline product is already detected at 700 °C and further thermal 5513
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Table 1. Cell Parameters (a, c, V), Grain Size, and Rw of the Nd3+:YVO4 Powder Series
treatment leads only to improvement of crystallinity accompanied by rather rapid grain growth (line sharpening and narrowing). No influence of Nd3+ concentration increase was found for the chosen range of 0.1−10 mol %, confirming good solubility and accommodation of Nd3+ in the vanadate matrix. The Rietveld analysis20 was employed in order to deliver more detailed information regarding structural features using an anisotropic approach.21 Maud 2.3322 was employed to extract cell parameters. For calculation, the reference standard of tetragonal YVO4 ICSD-78074 was taken into account. Results of the fitting (Figure 3) procedure were gathered in Tables 1
Figure 3. Representative XRD pattern (black line) and results of the Rietveld analysis (red, fitted diffraction; blue, differential pattern; column, reference phase peak position) of 0.1 mol % Nd3+:YVO4 treated at 700 °C.
and 2 and compared with reference data. The quality of structural refinement is generally checked by R-values (Rw, Rwnb, Rall, Rnb, and σ) determining the fit goodness.23 On the basis of the fit results presented in Table 1, general trends can be found. The unit cell parameters increase mutually with increase of the Nd3+ concentration, which is actually expected since smaller Y3+ cation is replaced by larger Nd3+, leading to the cell expansion. Obviously the effect of annealing temperature is reflected directly in an increase of the grain size, since the Ostwald ripening mechanism of particle growth is involved upon increase of sintering temperature. In the case of the powder samples heated at 700 °C, the grain size is around 20 nm and after exceeding 900 °C, particles grow intensively above 1.2 μm. No inhibitive influence of the dopant concentration increase on particle size was found well confirming incorporation of the Nd3+ ions in the YVO4 structure. The primary size and morphology of the Nd3+:YVO4 particles were estimated on dry powders using TEM microscopy (see Figure 4). Hence, in accordance with representative TEM images of 0.1 mol % Nd3+:YVO4 treated at 700 °C, the samples contain fairly agglomerated and rounded particles with average particle size of 20 nm. The agglomeration process is typical for the nonsurface blocked nanoparticles as the response for the high surface energy of the single nanoparticle leading to its minimization. Analysis of the SAED pattern reveals the presence of well-developed spotty rings at positions expected for the tetragonal YVO4 (Figure 4b). 3.2. Spectroscopic Analysis. The absorption reflectance and excitation spectra of the Nd3+:YVO4 were measured at 300
sample
a (Å)
c (Å)
V (Å3)
grain size (nm)
Rw (%)
single crystal
7.118(3)
6.289(3)
318.681(4)
−
−
0.1 mol % Nd3+ 0.5 mol % Nd3+ 1 mol % Nd3+ 5 mol % Nd3+ 10 mol % Nd3+
7.119(5)
700 °C 6.292(8) 318.964(9)
16.49
1.70
7.119(5)
6.291(4)
318.893(9)
24.78
1.85
7.119(3)
6.292(7)
318.941(9)
16.91
1.84
7.133(0)
6.301(2)
320.603(1)
91.85
2.32
7.140(7)
6.309(5)
321.718(9)
29.09
1.60
0.1 mol % Nd3+ 0.5 mol % Nd3+ 1 mol % Nd3+ 5 mol % Nd3+ 10 mol % Nd3+
7.119(7)
800 °C 6.296(7) 318.964(9)
39.27
1.71
7.124(2)
6.294(9)
318.757(7)
62.10
2.44
7.123(0)
6.295(0)
319.390(2)
92.29
2.77
7.133(0)
6.300(7)
320.577(6)
993.13
2.84
7.143(0)
6.307(9)
321.844(5)
395.61
3.10
0.1 mol % Nd3+ 0.5 mol % Nd3+ 1 mol % Nd3+ 5 mol % Nd3+ 10 mol % Nd3+
7.121(7)
900 °C 6.293(5) 319.197(6)
552.30
3.02
7.116(0)
6.295(3)
319.492(8)
172.75
2.22
7.121(8)
6.296(3)
319.348(6)
76.21
1.71
7.132(0)
6.300(2)
320.554(6)
565.87
3.01
7.142(4)
6.306(7)
321.729(2)
759.51
2.47
0.1 mol % Nd3+ 0.5 mol % Nd3+ 1 mol % Nd3+ 5 mol % Nd3+ 10 mol % Nd3+
7.121(4)
1000 °C 6.293(4) 319.145(3)
471.34
3.29
7.123(3)
6.294(9)
319.412(1)
1209.76
3.52
7.123(1)
6.294(2)
319.358(6)
688.36
2.54
7.132(2)
6.300(7)
320.505(7)
357.43
3.10
7.142(4)
6.307(5)
321.770(0)
700.22
3.42
K as a function of dopant content and sintering temperature in order to estimate energy levels of the Nd3+ ions (Figure 5). In fact, the representative absorption and excitation spectra consist of typical absorption bands of the Nd3+ ion with the broad band emission in the UV spectral region ascribed to the charge transfer band (CT) O2− → V5+ (dominant) centered at 30985 cm−1 (322 nm), whereas an abundance of sharp lines were depicted to the intraconfigurational f−f electron transition. Thus, the group of lines with maximum at 23047 cm−1 (433 nm) were attributed to the 4I9/2 → 2P1/2 electron transition (very weak), at 21010 cm−1 (476 nm) to the 4I9/2 → 2K15/2, 4 G9/2,11/2 (very weak), at 18770 cm−1 (532 nm) to the 4I9/2 → 2 K13/2, 4G7/2,9/2 (weak), at 16844 cm−1 (593 nm) to the 4I9/2 → 4 G5/2,7/2 (medium), at 15 862 cm−1 (630 nm) to the 4I9/2 → 2 H11/2 (very weak), at 14542 cm−1 (687 nm) to the 4I9/2 → 4 F9/2 (very weak), at 13459 cm−1 (743 nm) to the 4I9/2 → 4F7/2, 4 S3/2 (medium), at 12371 cm−1 (808 nm) to the 4I9/2 → 5514
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Table 2. Atomic Parameters for Representative 0.1% Nd3+:YVO4 Treated at 700°C 0.1 mol % Nd3+:YVO4; Z = 4 tetragonal I41/amd (141) a = 7.1192(1) Å c = 6.2930(1) Å V = 318.95(4) Å3 1.70% 1.30% 1.32% 1.19% 1.18% selected shortest contacts
sample space group calculated cell parameters
Rw Rwnb Rall Rnb σ Y|Nd−Nd|Y Y|Nd−O Y|Nd−V V−O Y|Nd−O−Nd|Y Wyckoff atom positions Y1 V1 O1 Nd1
4a 4b 16h 4a
3.8918(3) 2.4374(2) 3.1465(0) 1.5539(1) 58.006(1)°
Å Å Å Å
Figure 5. Representative reflectance absorption (blue line), excitation spectra (red line), and emission spectra of 10 mol % Nd3+:YVO4 prepared at 700 °C.
x
y
z
Biso
0.0 0.0 0.0 0.0
0.75 0.25 0.4173 0.75
0.125 0.375 0.2164 0.125
0.23 (9) 0.12 (9) 0.92 (1) 0.022 (8)
Occ. (
