Prepared by Solution Combustion Synthesis

'e gadolinium vanadate doped with samarium (GdVO4:Sm) nanopowder was prepared by the solution combustion synthesis (SCS) method. After synthesis, in order to achieve the full crystallinity, the material was annealed in air atmosphere at 900°C. Phase identification in the postannealed powder samples was performed by X-ray diffraction, andmorphology was investigated by high-resolution scanning electron microscopy (SEM). Photoluminescence characterization of the emission spectrum and timeresolved analysis have been performed using the tunable laser optical parametric oscillator excitation and the streak camera. Several strong emission bands in the Sm emission spectrum were observed, located at 567 nm (G5/2–H5/2), 604 nm (G5/2–H7/2), and 646 (654) nm (G5/2–H9/2), respectively. 'e weak emission bands at 533 nm (F3/2–H5/2) and 706 nm (G5/2–H11/2) and a weak broad luminescence emission band of VO4 were also observed by the detection system. We analyzed the possibility of using the host luminescence for two-color temperature sensing. 'e proposed method is improved by introducing the temporal dependence in the line intensity ratio measurements.


Introduction
Many investigations have been devoted to rare earth orthovanadates RVO 4 (R � Sc, Y, La, Gd, or Lu) (see [1][2][3][4] and references therein).Gadolinium vanadate (GdVO 4 ) is a very important host for the luminescence of rare earth activators which find applications in the high-power solid state lasers, X-ray medical radiography, energy-saving fluorescent lamps, artificial production of light, other display devices [5][6][7][8][9][10][11], and temperature sensing [4].Phosphors based on gadolinium compounds play an important role because the Gd 3+ ion (4f 7 ) has its lowest excited level at relatively high energy, which is due to the stability of the half-filled shell ground state [6].
e GdVO 4 :Sm nanopowder is an efficient orange-reddish light emitting material due to a strong absorption of ultraviolet light by GdVO 4 and efficient energy transfer from vanadate groups (VO 4 3− ) to dopants (Sm 3+ ).
In this paper, we present the results of experimental investigation of Sm 3+ -doped GdVO 4 nanopowders, prepared by the solution combustion synthesis (SCS) method [12,13].
Simplicity and low cost are the main characteristics of this process.Phase identification in the postannealed powder samples was performed by X-ray diffraction, and morphology was investigated by high-resolution scanning electron microscopy (SEM).e main aim of this study is time-resolved analysis of luminescence properties of GdVO 4 : Sm 3+ nanopowders.e possibility for GdVO 4 :Sm 3+ usage in phosphor thermometry was analyzed in [4], where temperature determination of sensing calibration curves was based on intensity ratios of luminescence of samarium lines.Here, we have taken a different approach.First, we use intensity ratio of the host and samarium line luminescence emissions (two-color thermometry).is new approach to the ratiometric luminescence thermometry was proposed recently, using TiO 2 nanopowders doped with Eu 3+ [14] and Sm 3+ [15], Zn 2 SiO 4 doped with Mn 2+ [16], and Eu 3+ Gd 2 Ti 2 O 7 doped with Eu 3+ [17].So, this concept, used in our study, provides high relative sensitivities by itself.However, the method presented here is further improved by introducing the temporal dependence in the luminescence intensity ratio measurements, as proposed in [18], providing even more increased sensitivity.
e prepared starting reagents were combusted with the ame burner at approximately 500 °C, yielding a voluminous foamy powder in an intensive exothermic reaction.After the solution combustion synthesis, the nanopowder was annealed for 2 hours, in air atmosphere, at 900 °C.Annealing has an e ect on increasing the grain size of the nanopowders, and it is widely used to achieve the higher emission intensity.

Instruments and Measurements.
e structure of the nanopowder was veri ed by X-ray di raction analysis, using a Philips PW 1050 instrument, with Ni ltered Cu • K α1,2 radiation (λ 0.15405 nm).X-ray di raction measurements were done at room temperature over the 2θ range of 10-90 °with a scanning step width of 0.05 °and a counting time of 8 s per step.e morphology of nanopowders and the size of crystallites were determined by high-resolution scanning electron microscopy (SEM) equipped with a highbrightness Schottky eld emission gun (FEGSEM, TESCAN) operating at 4 kV.Photoluminescence (PL) studies reported in this work were performed using an optical parametric oscillator (Vibrant OPO), as described in [12,13].e output of the OPO can be continuously tuned over a spectral range from 320 nm to 475 nm.Time-resolved streak images of the emission spectrum excited by the OPO system were collected by using a spectrograph (SpectraPro 2300i) and recorded with a Hamamatsu streak camera (model C4334).All streak camera operations were controlled by the HPD-TA (High Performance Digital Temporal Analyzer) software.We used a homemade temperature control system for luminescence measurements presented here.

XRD and SEM Study.
In order to know the structural properties and di erences in the phase purities of the prepared GdVO 4 :Sm nanopowder, XRD analysis was recorded and is presented in Figure 1.XRD con rmed the successful formation of the pure-phase GdVO 4 powder with the I4 1 /amd space group (JCPDS Card no.86-0996).Ionic radius of the Sm 3+ ion (0.964 Å) is a slightly larger than that of Gd 3+ ion (0.938 Å), which indicates that Sm 3+ could be successfully incorporated into the GdVO 4 host lattice by substituting Gd 3+ without changing the tetragonal zircon type structure of GdVO 4 [9].
e particle size and morphology of the GdVO 4 :Sm nanopowder annealed at 900 °C were characterized by SEM (Figure 2).Some particles are agglomerated as clusters; however, individual sphericalshaped particles are also visible and labeled in Figure 2.
e average grain size, D, was estimated by the Scherer equation, D Kλ/β cos θ, where K is a constant related to

2
Journal of Spectroscopy the shape of the crystallite and is approximately equal to unity, we used K 0.89 as in [19], λ is the X-ray wavelength (0.15405 nm), and θ and β are the di raction angle and full width at half maximum (FWHM, in radians) of the corresponding peak, respectively.e strongest peaks (2θ) from XRD were used to calculate the average crystallite size (D) in the GdVO 4 :Sm nanopowder.e estimated particle size is about 43 nm.e SEM image (Figure 2) reveals that sizes of individual particles of nanopowders are between 30 nm and 105 nm, which is in agreement with the calculated averaged result from XRD.

Photoluminescence and Lifetime
Analysis.e streak image of the time-resolved photoluminescence spectrum of the GdVO 4 :Sm 3+ nanopowder using the 330 nm excitation is presented in Figure 3. Horizontal scale of the streak image corresponds to wavelength, and the vertical scale shows development of spectra in time.Images are presented in pseudocolor, where di erent colors mean di erent optical intensities.
Spectral characteristics of luminescence emission intensities of the synthesized GdVO 4 :Sm 3+ (1 at.%) nanopowder sample are shown in Figure 4. e spectrum was obtained by integrating in time the spectral image acquired by the streak camera in the photon counting mode, with the time scale of 5 ms, at the excitation of 330 nm.
It could be seen in Figure 4 that the GdVO 4 :Sm nanopowder sample have comparable luminescence emission intensities in green, orange, and red regions.All those emission bands correspond to the transitions from excited energy level 4 G 5/2 of Sm 3+ ion to 6 H 5/2 (∼567 nm), 6 H 7/2 (602 nm), 6 H 9/2 (646 and 654 nm), and 6 H 11/2 (∼706 nm) level, respectively.e strong emission of Sm 3+ is due to e cient energy transfer from the VO 4 3− group to Sm 3+ ion in the GdVO 4 :Sm 3+ sample.However, the deep red emission of these samples is almost on the end of the region of human eye color sensitivity, so the small in uence of this emission on the color chromaticity coordinates of GdVO 4 :Sm 3+ is expected.e transitions at 604 and 646 nm have relatively higher emission intensities over the other transitions causing an orange-reddish emission from the Sm 3+ .Fluorescence lifetime analysis for the GdVO 4 :Sm 3+ nanopowder has also been performed, and the obtained result is presented in Figure 5.We present the uorescence lifetime analysis for the most intense emission peak ( 4 G 5/2 -6 H 7/2 ) in Sm 3+ ion.Luminescence decay curve is well tted using a double-exponential function.e average luminescence lifetime can be determined by the following formula [20][21][22][23][24]: τ where A 1 and A 2 denote the amplitudes of respective decay components and τ 1 and τ 2 are uorescence lifetime components contributing to the average lifetime.e obtained result for average lifetime for Sm 3+ ion is 0.726 ms.It is found that our result for lifetime of Sm 3+ ion in the GdVO 4 nanopowder is longer in comparison with 0.66 ms [20], 0.55 ms [25], and 0.42 ms [6].It is well known that the luminescent lifetime of rare earth ion is in uenced by the structure of the host, the rare earth located sites (on the surface or bulk) of the host, defects, and impurity [26].e defect and impurity may act as quenching centers and reduce luminescent lifetime.e double-exponential decay behavior of the activator is frequently observed when the excitation energy is transferred from the donor [6].e energy transfer is not the main cause of the deviation from the single-exponential behavior of the decay curve since the energy transfer from the VO 4 3− groups to Sm 3+ ions mainly in uences the rise time of the decay curve [27].e streak image of GdVO 4 : Sm 3+ nanopowder luminescence with a time scale of 100 µs is shown in Figure 6.In this time scale, the host luminescence and weak samarium line at 533 nm ( 4 F 3/2 -6 H 5/2 ), barely discernible and not denoted in Figures 3 and 4, are easily identi ed.In our measurements, all luminescence bellow 500 nm is cut o by the optical lter used for blocking the OPO excitation.e spectral shape of host luminescence, detected by us only above 500 nm, is similar to spectra presented in [1,9,28].e GdVO 4 luminescence is ascribed to the VO 4 3− group.Calculated lifetime of host luminescence of 2.82 µs agrees well with the time-resolved analysis provided in [28].Moreover, as we have shown in the next subsection, this luminescence is gradually quenched by raising the temperature, which means that it is not measuring error caused by stray light of laser excitation.
e estimated rise time of samarium luminescence shown in Figure 6 is negligible for e ects of interest.For used time scale, it is determined mainly by the instrumental response.So, the multiexponential behavior should be ascribed to the absorbed impurities, which lead to the defects and the quenching centers [27,29].Decay kinetics behavior depends also on the number of di erent luminescent centers [20,27].If rare earth ions occupy several di erent sites in host lattices, they can generate several di erent luminescent centers which lead to a multiexponential behavior [29].Tian et al. [30] explained that, in Dy 3+ -doped bulk GdVO 4 :Dy phosphors, the energy transfers mainly happen between Dy 3+ ions [31,32].In the case of nanoparticles, the energy transfer starting from the luminescent level can be more complicated, since the defects, which can act as quenching centers, in nanoparticles are more plenteous than those in bulk phosphors [29,31,33].Due to the defects produced during the preparation process and trace impurities contained in the raw materials, the samples necessarily have quenching centers (traps) with very low concentrations.When the excited luminescent center is in the vicinity of the trap, the excited energy could be transferred easily to the trap from which it is lost nonradiatively [33].As a result, decay time should become short.In our case, longer lifetime of 0.726 ms may be due to the better crystallization and less defects, which reduces the nonradiative probability and results in the longer lifetimes.

Temperature Sensing Using GdVO 4 :Sm Nanophosphor.
Recently, the new concept of using the host luminescence for the uorescence intensity ratio method was introduced (see [ [14][15][16][17] and references therein).In our study, the method is   Journal of Spectroscopy improved by introducing the temporal dependence in the intensity ratio measurements, as proposed in [18].Namely, it is possible to increase the sensitivity of the curve of intensity ratio between the host and samarium luminescence if an appropriately selected part of temporal evolution is used in calculation.We used the streak camera to prove the concept.e real application of this method will be based on using the gated CCD cameras and appropriate bandwidth lters for selecting the emission region of interest.
e luminescence spectra of the GdVO 4 :Sm nanopowder were measured at various temperatures, using the OPO excitation at 330 nm and the streak camera.For calculation of intensity ratio, the narrow bands (5 nm) of host luminescence around 520 nm and the samarium line at 602 nm were used and integrated in time from their beginning.However, we varied gating times for end of signal integration.In order to apply the intensity ratio method in thermometry, it is required to t a calibration function of analyzed thermophosphor.Based on considerations in [34,35], we decided to use the simple empirical equation for tting the calculated intensity ratios of experimental data: IR(T) A + C • e −T/α [34,35], where T is the temperature in K and empirical A, C, and α are the constants obtained through tting of measured data.e results are shown in Figure 7.
It could be seen in Figure 7 that the intensity ratio between the host luminescence band centered at 520 nm and the line at 602 nm increases with decreasing the gate time of the signal.However, decreasing the gate time decreases the integrated signal intensity, so we did not use delays smaller than 30 µs. e absolute thermal sensitivity, S a , of the intensity ratio method is de ned as the rate at which IR changes with the temperature: e relative thermal sensitivity, S r , of the intensity ratio method Sr is determined using the following formula:  ( Absolute and relative temperature sensitivities of GdVO 4 :Sm 3+ nanophosphor are shown in Figure 8. e best sensitivity is obtained by the gate time of 30 µs.We estimate that this kind of temperature sensing is useful up to 500 K, where the calculated intensity ratio values stop increasing and the curve attens, resulting in small sensitivity, even for the optimal gate time of 30 µs (see Figures 7 and 8).e proposed method provides better sensitivity than that reported in [4] for the same nanophosphor.
e CIE (Commission Internationale de I'Eclairage, 1931) chromaticity coordinates of GdVO 4 :Sm 3+ are presented in Figure 9.It could be seen that the GdVO 4 :Sm 3+ sample shows the orange-reddish luminescence color, and the chromaticity coordinates at room temperature are x 0.5963 and y 0.3989.Denoted points correspond to the temperature range from 300 K up to 673 K. e diagram shows that the luminescence intensities corresponding to the last two points, 623 K and 673 K (350 °C and 400 °C), are obviously measured with high uncertainty, and they were not included in tting the temperature sensing calibration curves of GdVO 4 :Sm 3+ .e host luminescence was also identi ed.Lifetime of host luminescence is 2.82 µs. e rise time of samarium luminescence is estimated as negligible for e ects that are here of interest.

Conclusion
We have shown that, for the analyzed GdVO 4 :Sm 3+ material, the temperature sensing based on ratio of intensities of the host luminescence and the samarium line is useful up to 500 K. e used method was improved by introducing the temporal dependence in the ratio measurements.Our analysis shows that the gating time of 30 µs is optimal for acquiring the integrated luminescence intensity.
By using the CIE chromaticity diagram of emission spectra, it has been shown that this GdVO4:Sm 3+ nanophosphor material (chromaticity coordinates x 0.596, y 0.398) can be used for development of orange-reddish light emitting  6 Journal of Spectroscopy optical devices.We also plotted the temperature dependency of CIE chromaticity coordinates for this nanophosphor.In summary, results of all our analyses prove that the Sm 3+ -doped GdVO 4 nanopowder prepared by a simple and low-cost solution combustion synthesis method is an appropriate material for light-emitting optoelectronic devices and remote temperature sensing.

Figure 1 :
Figure 1: e XRD patterns of the GdVO 4 :Sm nanopowder with respective Miller indices.

Figure 6 :Figure 7 :
Figure 6: e streak image of the GdVO 4 :Sm nanopowder.e time scale of 100 µs was adjusted for analysis of rise time of samarium lines and detection of fast host luminescence emission (OPO excitation at 330 nm).
e time-resolved analysis of GdVO 4 :Sm 3+ nanophosphor luminescence was conducted.e estimated lifetime of the most prominent optical emission of samarium from the 4 G 5/2 level is 0.726 ms.