Room-Temperature Synthesis and Optical Properties of NdVO4 Nanoneedles

Tetragonal NdVO4 nanoneedles were prepared via a simple room-temperature precipitation method in the absence of any surfactant or template, starting from simple inorganic salts, NdCl3 and Na3VO4, as raw materials. The nanoneedles were characterized by XRPD, SEM, Raman, PL, and lifetime spectroscopy. The particles have a length of about 100 nm and a diameter of 20 nm and grow along <112> direction. The advantages of this method lie in the high yield, non-toxic solvents, mild reaction conditions, and that it can potentially be employed for the preparation of other 1D lanthanide vanadates.


Introduction
The interesting optical properties of lanthanides such as luminescence, up-conversion, wide optical transparency, or large birefringence originate primarily from the multitude of transitions within the 4f n electronic states of the lanthanide ion.2][3][4][5] The efficiency of the 4f n excitations in a lanthanide ion can be enhanced through a charge transfer from a host material with a higher absorption coefficient.The orthovanadate group, VO 4  3− , is a good host for the trivalent ion because it can excite most of the lanthanide ions via the charge transfer transition within the VO 4 3− group, followed by an energy transfer to the emissive lanthanide ion.Choosing a crystal site with a very low symmetry for the lanthanide ion further increases the rate of absorption and emission, which can result in higher quantum yields.
In a tetragonal ABO 4 structure type, the A-site ion has a D 4h symmetry.Thus, a lanthanide ion sitting on this crystal site has a low symmetry which favours the elec-tric dipole transitions resulting in higher radiative rate constants and less quenching processes.Neodymium vanadate, NdVO 4 , is one of the most studied orthovanadate from the lanthanide orthovanadate family with the ABO 4 -type structure.Numerous investigations have been made on optical materials based on NdVO 4 due to their good optical properties. 3,6,7For example, Y-doped NdVO 4 is a well-known laser material with five times higher absorption coefficient at 808 nm (the standard wavelength of the currently available laser diodes) than the Nd:YAG laser diode. 6The catalytic properties of NdVO 4 have also been investigated, i.e. for oxidative dehydrogenation of propane. 2 Additionally, it has been found that NdVO 4 exhibits a photocatalytic activity for degradation of dyes and organic pollutants comparable or even higher than that of the commercial TiO 2 . 8,9Another study indicated that Mo-doping increased the photocatalytic activity of NdVO 4 for the degradation of different dyes (e.g., methylene blue, rhodamine B, remazol brilliant blue). 4t ambient conditions, NdVO 4 adopts a zircon-type structure in the I4 1 /amd (Z = 4) space group with the lanthanide ion located in a polyhedron coordinated by eight oxygen ions.Under an applied pressure of about 6 GPa, Dragomir and Valant: Room-Temperature Synthesis and Optical Properties ... the zircon-type NdVO 4 undergoes a phase transformation to a metastable monazite-type structure with the space group P2 1 /n (Z = 4) where the Nd atoms are located in a eight-coordinate site (with eight unique Nd−O bond distances). 10,11At around 11.4 GPa, NdVO 4 further transforms to a scheelite-type phase and so to a denser packing.All these phase transformations are accompanied by a decrease of the band gap by 0.5 eV (measured on a single crystal). 10ue to large specific surface areas and quantum size effects, nanocrystalline materials exhibit properties that are usually not observed in the bulk.3][14][15][16] Therefore, the design and synthesis of the nanosized NdVO 4 opens up many opportunities for applications.8][19][20][21] Each of these methods has certain drawbacks − the requirement of either thermal treatment at high temperatures, long reaction time (up to several days), expensive equipment or the use of toxic solvents.
A new and simple method to obtain crystalline NdVO 4 nanoparticles at room temperature through a precipitation method, using a cheap and non-toxic solvent is reported in this study.In addition of being a very convenient and fast method, this route also conserves energy because it does not involve any thermal treatment.

1. Synthesis
The precipitation procedure for the synthesis of NdVO 4 nanoparticles employed in this study is summarized in the scheme depicted in Fig. 1.
In this method, NdCl 3 • 6H 2 O (99.9%, Alfa Aesar) and Na 3 VO 4 (99.9%,Alfa Aesar), were used as precursors and NH 3 (aq) (25%) was the precipitating agent.Firstly, a NdCl 3 aqueous solution was prepared by adding 0.05 mol NdCl 3 • 6H 2 O to 2 mL of distilled H 2 O, while a Na 3 VO 4 solution was prepared by dissolving 0.05 mol Na 3 VO 4 in 3 mL of distilled H 2 O.The pH of the final solution was adjusted to < 1 with a few mL of HCl(aq) (32%).Secondly, the two solutions were mixed slowly until a clear yellow solution resulted.NH 3 (aq) (25%) was then added fast and under vigorous stirring to the above solution until the pH reached a value of ~11.A blueish-green precipitate formed.Then the obtained mixture was stirred for five more minutes before the precipitate was filtered, washed thoroughly with NH 3 (aq) and then dried at room temperature over-night.The reaction that leads to the formation of NdVO 4 can be summarized as follows: (1)

Characterisation
The phase composition was analysed by X-ray powder diffraction (XRPD) using a PANalytical X'Pert PRO diffractometer with Cu Kα 1 radiation (λ = 1.54056Å).The X-ray powder diffraction pattern was collected over the 2θ range 5-80° with a step size of 0.017°.A structure refinement was conducted using Topas (version 6, Bruker, AXS, Karlsruhe, Germany).A fundamental parameters approach was used for the profile fitting. 22A profile refinement was conducted in which the background (6 th order Chebychev polynomial), the unit cell parameters, the scale factor, the crystallite size, the sample displacement, and preferred orientation were stepwise refined to obtain a calculated diffraction profile that best fit the experimental pattern.All the occupancies were fixed at nominal composition and kept constant during refinement.Finally, the quality of the fit was assessed from the fit parameters such as R wp , R p and χ 2 .
The morphology of the NdVO 4 nanopowders was examined with a Scanning Electron Microscope (SEM) model JEOL JSM 7100F, operating at an accelerating voltage of 10 kV (in secondary electron mode).The samples were first dispersed in ethanol, then few drops of this dispersion were added onto a Si wafer and air dried.The Si wafer was fixed on the SEM sample holder using a carbon tape.
Raman spectroscopy was used for identification and structural characterisation of the NdVO 4 nanoparticles.Room temperature Raman spectra were collected in a 180° backscattering geometry, with a microprobe Raman system type Horiba Jobin-Yvon Lab RAM HR spectrometer equipped with a holographic notch filter and a CCD detector, using a 632.81 nm excitation line of a 25 mW He-Ne laser.The samples were placed and oriented on an Olympus BX 40 microscope equipped with 50× objective and the spectra were recorded in the 50-1000 cm −1 range with a resolution of 1 μm.To test the phase purity, the a spot resolution of were recorded on different regions of the sample.
Diffuse reflectance spectroscopy (DRS) measurements were performed to obtain the band gap energies.The DRS spectra were recorded in the 250−800 nm range, with a UV-Vis spectrophotometer (Perkin Elmer, model λ 650S) equipped with a 150 mm integrated sphere and using Spectralon as a reference material.The DRS data were converted to absorbance coefficients according to Kubelka-Munk method where NdVO 4 was considered a direct band gap semiconductor. 23The details of the determination of band gap energies by using the Kubelka-Munk theory are described elsewhere. 24The photoluminescence (PL) emission spectra were collected with an Edinburgh Instruments Spectrometer (model FLS920) using a steady state 450 W xenon arc lamp.The experimental setup was equipped with a blue-sensitive high speed photomultiplier (Hamamatsu H5773-03 detector) tube.The emission spectra were collected at room temperature, in a 400-700 nm range, using an excitation wavelength of 371 nm (λ em = 524 nm).
Information on the electron relaxation and recombination mechanisms were obtained by monitoring the PL intensity at a specific wavelength as a function of time delay after an exciting laser pulse.The time-resolved PL spectra were recorded at room temperature on a pico-second diode laser EPL 375 with an excitation wavelength of 371 nm, in the time range 0 to 50 ns.The analysis of the fluorescence decays was performed using the F900 analysis software..This method fits the sample response to the data over the rising edge, to match the theoretical sample response, R(t).The Reconvolution Fit procedure extracts the raw data (fluorescence decay) and eliminates both the noise and the effects of the exciting light pulse.

1. X-ray and SEM Studies
Fig. 2 shows the XRPD patterns of the as-obtained NdVO 4 sample, which was indexed as a tetragonal NdVO 4 phase with the space group I4 1 /amd (ICSD code 78077).No impurities were detected.Additionally, the Rietveld refinement indicated that the (112) reflection appears with higher intensity due to the preferred directional growth of the nanoneedles along <112>.The unit cell parameters obtained after the Rietveld refinement are presented in Table 1.The agreement factors were: R p = 6.59,R wp = 7.78, and χ 2 = 1.26.As it can be seen, the refined cell parameters are in good agreement with the literature reported values.
The particle size and morphology were examined by SEM.From Fig. 3 it can be seen that the NdVO 4 particles prepared in this study have a needle-like shape with a length of about 100 nm and a diameter of about 20 nm.A schematic representation showing the NdVO 4 nanoneedles grown along <112> is depicted in Fig. 4. The SEM study also showed that the nanoparticles tend to agglomerate leading to formation of larger clusters.

Unit cell parameters
This study Yuvaraj et al. 26 Fuess et al. 27 Panchal et al.

2. Raman Analysis
Raman analysis was performed on the as-obtained NdVO 4 nanopowders to study finer structural details (Fig. 5).As the XRPD analysis already suggested, the NdVO 4 synthesised in this study is adopting the zircon-type structure.From a group theory consideration NdVO 4 adopting this structure has 12 Raman active modes: 2A 1g , 4B 1g , B 2g , and 5E g . 28From these 12 modes, 7 are internal modes associated with vibrations in the VO 4 structural unit (2A 1g , 2B 1g , 1B 2g , 2E g ), and 5 are external vibrations (3E g , 2B 1g ).So far, the Raman spectra of NdVO 4 have been measured on single crystals and polycrystalline samples by several   research groups.A comparison of our results with the literature reports is shown in Table 2.
It can be seen that the observed Raman modes in this study are in good agreement with the literature data.The only modes that were not observed are the modes located at 113, 225, and 373 cm −1 (these modes are rarely observed).In the 100-1000 cm −1 region, the Raman spectrum of NdVO 4 shows 9 modes that are separated in two regions.The high frequency region includes the internal modes in the VO 4 units, whereas the external modes occur at lower frequency and correspond to motions of the Nd−O bonds in the NdO 8 polyhedron.The symmetry annotations were made in accordance with the previous assignments reported in the literature. 10,11,27

Optical Studies
Fig. 6 shows the UV-Vis diffuse reflectance spectrum of the NdVO 4 nanoneedles prepared in this study.
The light absorption was observed at a wavelength of about 770 nm, followed by another absorption at about 615 nm.A third absorption peak started at about 353 nm, increased sharply and reached a maximum at about 280 nm (associated with the O −2 −V +5 charge transfer within the VO 4 3− group). 32,33The sharp increase at about 353 nm corresponds to the band gap transition in NdVO 4 .The additional absorption peaks observed in the UV-Vis spectrum of NdVO 4 at about 615 and 770 nm have been described in the literature and they are summarized in Table 3.
From the plot of the absorbance versus the energy (Fig. 7) the band gap of the NdVO 4 nanopowders was calculated to be 3.50 eV, which is in the UV region of the electromagnetic spectrum.This value falls well in the range of values reported by other research groups. 8,10he electronic structure of zircon-type NdVO 4 has also been investigated by Panchal et al. 10 They observed   Under UV excitation (Fig. 8) the NdVO 4 nanoneedles show green, yellow, and orange emissions.The NdVO 4 spectrum consists of a broad peak centred at about 500 nm with three small shoulders at about 523, 545, and 600 nm.Several researchers have reported the photoluminescence spectra of nanosized NdVO 4 (excited with UV light).Wu et al. reported the PL emission spectrum of single crystalline nanorods (of ~200 nm in diameter and 400-700 nm in length). 37The spectrum shows a strong emission around 490 nm followed by two less intense peaks at ~525 and 550 nm and a triplet at about 600-615 nm.
The NdVO 4 nanoneedles prepared in this study show similar peaks.Similar results were also obtained for 1 μm NdVO 4 particles prepared by a solid-state method and described by Dragomir et al. 24 Briefly, the emission at about 500 nm can be assigned to the 4 G 11/2 → 4 I 11/2 transition, whereas the shoulders at ~525, 545, and 600 nm can be attributed to the 4 G 7/2 → 4 I 9/2 , 4 G 7/2 → 4 I 9/2 , and 4 G 5/2 → 4 I 9/2 transitions, respectively.Fig. 9 shows the time-resolved photoluminescence spectrum of the NdVO 4 nanoneedles excited by a 371 nm laser wavelength and monitored at 524 nm.This decay curve could only be simulated by a third-order exponential function with the three lifetimes: τ 1 = 0.16 ns, τ 2 = 1.70     4.These results suggest that the PL decay processes were dominated by third order kinetics.To the authors' knowledge, this is the first report on the dynamics of photo-excited carriers in NdVO 4 .

Conclusions
A facile room-temperature precipitation method was employed for the synthesis of NdVO 4 nanoneedles.This approach utilizes aqueous solutions of NdCl 3 , Na 3 VO 4 and NH 3 (aq) as a precipitating agent.The synthesis yielded impurity-free, crystalline NdVO 4 nanoneedles with a tetragonal structure, space group I4 1 /amd.The Rietveld refinement study indicated a preferential growth of the nanoparticles along the <112> direction.The Raman analysis further supported the fact that the nanoparticles are single phase NdVO 4 with a zircon-type structure, while SEM analysis showed that the as-synthesised particles have a needle-like morphology with a length of about 100 nm and a width of about 20 nm.The UV-Vis absorption spectrum showed an absorption band located at 353 nm (3.5 eV) which corresponds to the band gap transition.The room-temperature PL spectrum of the NdVO 4 nanoneedles shows green, yellow, and orange emissions.The lifetimes of the nanoneedles were monitored for the 524 nm emission and were found to be 4.74 nanoseconds.
In addition to its simplicity, the synthetic method employed in this study also conserves energy since it does not require any thermal treatment.This method could potentially be tailored for the facile preparation of other 1D lanthanide vanadate structures.

Fig. 1 .
Fig. 1.A schematic representation of the synthetic procedure to obtain NdVO 4 nanoparticles.

Fig. 2 .
Fig. 2. X-ray powder diffraction pattern of the NdVO 4 nanopowder.The black full circles represent raw data and, red solid line is the Rietveld fit, the black vertical bars are the Bragg reflections, while the grey line shown below is the difference between observed and calculated intensity.

Fig. 9 .
Fig. 9. Photoluminescence lifetime measured on the NdVO 4 nanoneedles, λ em = 524 nm.The black dots represent the time-domain intensity decay.The red line is the fitting curve, while the grey line is the response of the detector.The green line below represents the difference between the fitted curve and the measured data.The goodness of fit, χ 2 , is 1.418.

Table 1 .
The unit cell parameters obtained after Rietveld refinement of the NdVO 4 nanoneedles and a comparison with the literature-reported values.

Table 2 .
The Raman modes of NdVO 4 reported in the literature and the Raman modes observed in this study.

Table 3 .
Characteristic peaks observed in the 500-800 nm range in the UV-Vis absorption spectra of NdVO 4 .