Simple Methods to Synthesize YVO4 Nanocrystals or Microcrystals without Any Templates or Surfactants

YVO4 crystals with different sizes and shapes were produced through hydrothermal treatment and sonicationmethod without any surfactants or templates. X-ray powder diffraction (XRD), transmission electronmicroscopy (TEM), and photoluminescence (PL) were used to characterize the obtained products. YVO4 nanocrystals with spindle-like shape had been produced through the two different treating methods. YVO4 crystals, which are bipyramid-capped and micrometer-sized, had been obtained through a simple hydrothermal treatment. Uniform microsized cuboids had been produced through hydrothermal treatment with the final pH value� 2.5. ,e effects of different final pH values on the shape and crystallinity of products were studied. To determine photoluminescence performances, Eu(5%)-doped YVO4 nanocrystals had been synthesized through different methods in various environments and it had been confirmed that crystallinity would affect photoluminescence intensity.

Wu et al. used a hydrothermal method to synthesize rodlike, olive-like, pineapple-like, and particle-like nanocrystals of the YVO 4 :(5%) Eu, respectively, and they confirmed that the olive-like nanocrystals have enhanced luminescence intensity compared with nanocrystals with other shapes [23]. Spindle-like YVO 4 :(5%) Eu 3+ nanocrystals have been synthesized with (Y, Eu) (NO 3 ) 3 and NH 4 VO 3 as starting materials through sonication method by Zhu et al. ey claimed that the use of ultrasonic irradiation has a remarkable effect on the morphology of the particles produced [24]. Li et al. demonstrated a hydrothermal treatment for obtaining YVO 4 :(5%) Eu 3+ nanobelts and polyhedron microcrystals with (Y, Eu) (NO 3 ) 3 and NH 4 VO 3 as reacting reagents. It has been proved that the influence of pH values on the morphology of the products is essential [25].
In this paper, we report two simple routes to synthesize spindle-like YVO 4 nanocrystals with Y (NO 3 ) 3 and Na 3 VO 4 as starting materials. YVO 4 microcrystals of bipyramidcapped shape and cuboids have been produced through hydrothermal treatment with different final pH values. It has been found that pH value is a key factor for influencing the morphology of products, which is consistent with the results gained by previous works. And it is also found that different morphologies of products will bring different PL intensities.

Experimental Section
2.1. Synthesis. All the starting materials used in this work were of analytical grade. In a typical synthesis procedure, 0.0016 mol Y (NO 3 ) 3 ·6H 2 O and 0.0016 mol Na 3 VO 4 ·12H 2 O were mixed with an appropriate amount of distilled water and immediately a white precipitate appeared. en, 3 M HNO 3 was dripped into the obtained suspension to dissolve the white precipitate and a transparent golden yellow solution was obtained. e proper amount of 1 M NaOH was added to the solution to make the final pH at a designed value and stirred for 10 min to obtain solution precursor. en, the solution precursor was transferred into a 100 ml Teflon-lined stainless steel autoclave for hydrothermal treatment. When the solution had been hydrothermally treated at a determined temperature for a needed time, the autoclave was cooled to room temperature naturally. And the precipitate was separated by centrifugation, washed with distilled water and absolute ethanol, and dried at room temperature for further characterization. When the samples of YVO 4 were obtained through the sonication method, the solution precursor was produced through the same process. And the solution mixture was exposed to ultrasonic irradiation in ambient air for 2 h.

Characterization.
e X-ray diffraction (XRD) patterns of the samples were examined on a Japan Rigaku D/Max-c X-ray diffractometer equipped with a rotation anode and graphite monochromatized CuKα radiation (λ �1.54178Å).
e transmission electron microscopy (TEM) images of the as-synthesized samples were obtained on a JEOL JEM-100 CXII operated at an accelerating voltage of 100 kV. e highresolution transmission electron microscopy (HRTEM) images were obtained on a Philips Tecnai F30 high-resolution field-emission transmission electron microscope operated at 300 kV. e samples to be measured were dispersed in absolute ethanol under vibrating in the ultrasonic pool, and then, the solutions were dropped onto a copper grid coated with amorphous carbon films and dried in air before performance. Fluorescence spectra were recorded on a Hitachi F-4500 spectrophotometer equipped with a 150 W Xe arc lamp at room temperature; the emission spectra were measured at a fixed band-pass of 0.2 nm with the same instrument parameters (5.0 nm for excitation split, 1.0 nm for emission split, and 700 V for PMT voltage).

Results and Discussion
XRD patterns of the produced samples are shown in Figure 1. Diffraction peaks of all the samples can be indexed to tetragonal phased YVO 4 , so it can be resulted that pure tetragonal phased YVO 4 can be produced in a wide pH range through hydrothermal route. Stronger diffraction peaks appear in the products obtained in higher final pH value, which demonstrates that products of higher crystallinity are produced in higher pH surroundings. Figure 2 provides the TEM images of YVO 4 crystals produced at pH � 2.5. It can be seen from Figure 2 that micrometer-sized cuboids have been obtained. e uniform cuboids have lengths of about 3 µm and widths of about 2 µm. Figure 3 presents TEM images of YVO 4 nanocrystals which were produced in weakly acidic and basic environment with hydrothermal treatment at 180°C for 24 h, respectively. Figure 3 shows that uniform nanocrystals are produced in both surroundings and it can be seen that the nanocrystals have a spindle-like morphology. e spindlelike particles with an equatorial diameter of 50-70 nm and a length of 120-200 nm are produced at pH � 6.0. And when treated at pH � 9.0, spindle-like particles with a little bigger size are obtained. It is observed that the nanocrystals produced at pH � 6.0 are accumulated of many small nanorods; however, nanocrystals produced at pH � 9.0, which have higher crystallinity, are accumulated of some bigger nanorods.
TEM images of YVO 4 crystals produced at pH � 12.0 and 13.0 are presented in Figure 4. It is obvious that the pH value of the synthesis solution plays an important role in shape and size of the obtained crystals. It is shown that crystals of different shapes with bigger size are obtained in solution of higher final pH value. And it can also be observed that the    Journal of Chemistry shape of crystals produced at pH � 13.0 is similar to that produced at pH � 12.0; however, the size of crystals produced at pH � 13.0 is bigger. Figure 4(c) presents HRTEM image of crystals produced at pH � 13.0. It can be known from Figure 4(c) that YVO 4 of tetragonal phase is obtained, which is consistent with the result of XRD, and the obtained crystals have a high crystallinity. e selected area electron diffraction (SAED) pattern indicates that products obtained at pH � 13.0 are single crystalline. YVO 4 nanocrystals are also obtained through the ultrasonic irradiation method with the same pretreatment of raw materials; however, it can be observed from Figure 5 that poor crystallinity was obtained when final pH value is increased to 12.0, which is different from the result of hydrothermal treatment. Typical TEM images are presented in Figures 6(c) and 6(d). Similarly to hydrothermal treatment, nanocrystals produced with the ultrasonic irradiation method are spindle-like and nanocrystals produced in the two different environments have the same size, which have equatorial diameter and length in the range of 40-50 nm and 70-120 nm, respectively. Nanocrystals produced in the two different environments are accumulated of small nanorods,  which is also similar to the result of hydrothermal treatment. So it can be presumed that the formation mechanism of YVO 4 nanocrystals obtained through the two different treatments is the same. However, the size of products obtained through the sonication method is smaller than that obtained through hydrothermal treatment, and the crystallinity of the latter is higher than the former. YVO 4 . In other words, a dissolution-reprecipitation process is provided. Here, Y 3+ ions existing as mobile cations are gradually incorporated into the polyvanadate anions and then lead to the formation of uniform YVO 4 crystals [9]. It is obvious that the pH value plays a key role in shape selection. e [010] face is rich in Y 3+ , so it is believed that the (010) growth direction can attract positive ions (Y 3+ ) and exclude negative ions. e (010) growth direction will be disturbed and even inhibited by OH − when its concentration is high. When the final pH value is in the range 4.0-10, the existence of OH − disturbs the (010) growth direction, and nanocrystals produced in this pH range show a spindle-like shape. However, when the concentration of OH − increased and pH value achieves 13, the (010) growth direction is more inhibited and crystals produced in this environment are bipyramid-capped. When the final pH value is fixed at 2.5, the enhancement of H 3 O + makes the face (010) and other faces grow well, and micrometer-sized cuboids are obtained. YVO 4 :(5%) Eu 3+ products are produced through different treatments in different environments to study optical properties. It can be observed from Figure 7 that PL intensity of products obtained in basic solution is stronger than that obtained in acidic solution and PL intensity becomes weaker with decreased final pH value. Figure 8 shows the excitation spectrum of YVO 4 :(5%) Eu 3+ obtained through the hydrothermal method with pH � 11 and the sonication method with pH � 12. e excitation spectrum is obtained under the emission of 618 nm and it can be known that the biggest excitation peak is at 280 nm. It can be observed from Figure 9 that higher final pH value will bring stronger intensity at pH � 10. Weaker intensity is obtained when the final pH value is increased to 12 because of the absence of YVO 4 . Emission spectra shown in Figure 9 are under 280 nm with 600 V for PMT voltage, which is higher than that used for products obtained through hydrothermal treatment. So it can be realized that PL intensity of products obtained through hydrothermal treatment is greatly stronger than that obtained through the sonication method. It can be presumed that the shape of products will influence optical properties and bipyramid-capped products appear greater PL intensity. Higher crystallinity will bring better photoluminescence performance, which is consistent with the previous literature.

Ultrasonic Route.
ree shapes have been obtained through sonication method: spindle-like shape, rod-like shape, and nanoparticles. e crystallinity of products obtained through hydrothermal treatment is higher than that produced through the sonication method. e pH value of the solution will affect the shape and crystallinity of products remarkably. Higher pH value will result in products of higher crystallinity.

Photoluminescence.
A variety of Eu 3+ (5%)-doped YVO 4 products have been synthesized to study their photoluminescence performances. It has been confirmed that higher pH end value will bring stronger PL intensity when the produced host material is YVO 4 ; however, when the pH end value is controlled at 12.0 through ultrasonic irradiation, products exhibit remarkably lower PL intensity due to the low crystallinity.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.