Anisotropic morphology, formation mechanisms, and fluorescence properties of zirconia nanocrystals

ZrO2 nanocrystals with spheres and elongated platelets were systemically prepared through a simple hydrothermal method by the use of ZrOCl2·8H2O and CH3COOK as raw materials. The anisotropic morphology and formation mechanism of the monoclinic and/or tetragonal ZrO2 were investigated by X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscope, and high-resolution transmission electron microscope techniques. The uniform elongated platelets and star-like structures were composed of short nanorods with a diameter of approximately 5 nm and a length of approximately 10 nm. The different morphologies were formed due to the different contents of CH3COO− and Cl− and their synergy. The fluorescence band position and the band shape remained about the same for excitation wavelengths below 290 nm and the different morphologies of the nanocrystals.

Material characterization. X-ray diffraction (XRD) patterns for the samples heat-treated at different temperatures were recorded on a Bruker AXSD8 Advance X-ray diffractometer with Cu Kα radiation using a graphite monochromator. Intensities of the diffraction peaks were recorded in the 10°-80° (2θ) range with a step size of 0.02°. The Fourier-transform infrared (FTIR) spectrum of the nanocrystals was recorded on a Nicolet 200sx FTIR spectrometer in the 4,000-400 cm −1 region using the KBr pellet method. The Raman spectrum was obtained using an NXR FT-Raman spectrometer with InGaAs as a detector at room temperature. The morphologies and microstructures of the nanocrystals were observed by scanning electron microscopy (SEM) using a Hitachi S-4800 instrument and high-resolution transmission electron microscopy (HRTEM) using a JEM-200CX instrument. The sample was placed in conductive adhesives and sprayed gold at 40 s in the process of SEM sample preparation and the powders were ultrasonic dispersed on the copper network for preparing HRTEM samples. The surface elemental composition and valent state of the nanocrystals were investigated by X-ray photoelectron spectroscopy (XPS) using a VG Scientific spectrophotometer with an X-ray source of Al Kα radiation at 1,486.6 eV. The base pressure was approximately 10 -8 -10 -10 Pa. The calibration of the E b scale and the corrections for the E b shift due to a steady-state charging effect were made by assuming that the C 1s line lies at 284.6 eV. The ZrO 2 was ground into powder and then compressed into a pellet. The absorption spectrum was measured ultraviolet (UV) spectroscopy using a UV solution-U-3501 spectrophotometer. The steady-state fluorescence spectrum was obtained on an Edinburgh FLS920 fluorescence spectrometer equipped with a 450-W Xe lamp. The Brunauer-Emmett-Teller (BET) surface areas of the fibers were measured by N 2 adsorption at 77 K with a Quadrasorb SI instrument. Fibers weighing approximately 40 mg were used for the measurements. Before the BET measurements, the samples were evacuated at 200ºC for 8 h in vacuum. The pore-size distribution was calculated by the density-functional-theory (DFT) method. The samples were pressed into a self-supporting thin wafer and put into the sample holder. The wafer was degassed in dynamic vacuum (10 −2 Pa) at 573 K for 3 h, and then the background spectrum was recorded. After the equilibrium of adsorption pyridine vapor for 1 h, the spectrum was recorded after degassing the wafer in vacuum at 393 K for 2 h.

Results and discussion
Morphology, crystallinity and formation mechanism. Raman spectroscopy is a nondestructive experimental technique for probing the vibrational and structural properties of materials. It is also recognized as a powerful tool for identifying different polymorphs of metal oxides 28 . According to group theory, the monoclinic (m-ZrO 2 ), tetragonal (t-ZrO 2 ), and cubic (c-ZrO 2 ) phases of ZrO 2 are expected to have 18 (9A g + 9B g ), six (1A 1g + 2B 1g + 3E g ) and one T 2g Raman-active modes, respectively. Figure 2 shows the Raman spectra as-obtained products at 200ºC, using a laser with wavelength 532 nm. According to   (11) and (111) reflections gave an average crystalline size of 5.3 ± 0.2 and 10.0 ± 0.5 nm, respectively, indicating the small crystalline size and anisotropic morphology of the products, which were similar to the results of SEM and TEM, and all of which could be seen from the following results.
The morphology and microstructure were characterized by SEM and HRTEM. The SEM images ( Fig. 4) showed that the nanoparticles exhibited different structures. The ZrO 2 spheres were observed for Z1.0 and Z2.0, but for that of Z1.5, the anisotropic star-like ZrO 2 structures with a wide size distribution from 40 to 100 nm. Another interesting phenomenon was that the average crystal size 1 was decreased with the temperature increasing from 200ºC to 250ºC.
The HRTEM images (Figs. 5 and 6) revealed that the nanostructures possessing high crystallinity. As can be seen from Figs. 4c, 5b, the uniform elongated platelets and star-like structures were composed of short nanorods with a diameter of approximately 5 nm and length of approximately 10 nm. This confirmed that the nanocrystals composed of star-like ZrO 2 nanostructures were anisotropic. For the star-like nanostructures, the two particles were super-positioned with a special angle like the butterfly, which were different from the previous report 8 . Furthermore, some nanopores existed at the interface, which should be caused by the particles' aggregation. From  Fig. 5d, the lattices were clear, and the measured results showed that the lattice spacing 3.16 Å, which matched perfectly with the ( 1 11) of m-ZrO 2 . In addition, there are some lattice defects between the lattices. Figure 6a,b showed the spherical agglomerated ZrO 2 particles in the size range of 5-6 nm. The corresponding electron powder-diffraction pattern (Fig. 6c) was presented at least four diffraction rings, which match the ( 111)  www.nature.com/scientificreports/ is consistent with the Raman spectra. In the Fig. 6d, multiple sets of lattice fringes can be seen. Each set of lattice fringes is parallel to each other, and the interplanar spacing is 3.16 Å and 2.84 Å, respectively, corresponding to the ( 111) and (111) plane of m-ZrO 2 . Same as the sample Z1.5, the sample Z2.0 also have some lattice defects. The N 2 adsorption and desorption isotherms of the ZrO 2 nanoparticles are shown in Fig. 7. Through calculation of the BET equation, the resultant BET specific surface areas were 18.18, 20.08, and 67.31 m 2 /g for Z1.0, Z1.5, and Z2.0, respectively. The specific surface of Z2.0 is larger than Z1.0, which was caused by the particle size of Z2.0 is smaller than Z1.0. The result was consistent with the previous analysis.
In Fig. 8, the absorptions at 3,432 and 1633 cm −1 were ascribed to the -OH vibrations of the adsorbed water or the surface hydroxyls. For the samples, the vibration at 1,550 cm −1 was attributed to the symmetric vibration absorption of COO − , and those at 1,462 and 1,379 cm −1 were due to the asymmetric vibration absorptions of COO − , indicating the existence of acetate on the surface of the product. The position and separation (Δ) of COO − bands in the 1,300-1,700-cm −1 region could be used to deduce the carboxylate coordination mode 29    www.nature.com/scientificreports/ absorptions at 734, 589, 507, and 455 cm −1 were the vibrations of Zr-O. It could be concluded from the IR spectrum that absorbed water, hydroxyl, and acetate groups were on the surface of the products. The difference was that the absorption peaks in the region 1,300-1,600 cm −1 of the sample Z2.0 are very weak or are barely even visible.
The XPS measurements technique is often used for observing the variation in surfaces applied for catalysis, including the oxidation of Zr. The XPS spectra of O 1s in Fig. 9 are wide and asymmetric, with the left side wider than the right, which can be seen from the figure, indicating that at least two kinds of O species were present at the surface, which could be recognized by resolving the XPS curves. The dominant peak at approximately 529.6 eV is a characteristic of lattice O in samples, while the signal at 532.6 eV can be associated with surface   Table 1. It is worth noting that the content of CH 3 COO − species could effectively enhance the surface hydroxyl group on the surface of the ZrO 2 nanocrystals. With the increasing addition of CH 3 COO − species, the amount of surface hydroxyl groups increased. The schematic formation of ZrO 2 nanoparticles with different morphologies is illustrated in Fig. 10. ZrOCl 2 •8H 2 O forms a tetramer complex in methanol solution with a structure in which four Zr atoms are arranged in a square, and each Zr atom is coordinated by four bridging -OH groups and four H 2 O molecules or hydroxyl ligands, and then goes through a polymerization process by a dehydration reaction to form polymeric species 31 . When the concentration of polymeric species reaches the critical supersaturation level, ZrO 2 crystal nuclei form spontaneously in the aqueous solution, and then evolve into primary ZrO 2 crystals. The grain growth is affected by the conditions, such as pH value, anions, etc., very obviously. The different reaction equations between ZrOCl 2 •8H 2 O and CH 3 COOK with different molar ratios are expressed as follows: According to the IR results, each Zr atom was coordinated by one CH 3 COO − in the structure of the precursor. For Z1.0, besides CH 3 COO − , the same molar amount of Cl − existed in the aqueous solutions, and only CH 3 COO − was contained in the Z2.0 system. The anions can be considered as "modifiers" that control the morphology and crystal sizes of the ZrO 2 nanoparticles. In contrast, the volume and viscosity of the CH 3 COO − were larger than those of the Cl − , which may have more resistance for grain growth. Therefore, the smaller grain size was observed for the Z2.0 system compared with that of Z1.0. When the two anions existed in one system simultaneously for Z1.5, the primary ZrO 2 nanoparticles coalesced with each other to form the rod-like ZrO 2 through oriented attachment (OA) due to the synergy of the anions, which has been already proved to be an important mechanism for the anisotropic growth of nanostructures. optical properties. Fluorescence spectra were measured with several wavelengths between 245 and 290 nm, while the fluorescence intensity changed to some extent with excitation wavelength. The fluorescence www.nature.com/scientificreports/ band position and the band shape remained approximately the same for excitation wavelengths below 290 nm and the different morphologies of the nanoparticles. Figure 11 shows the representative fluorescence emission spectra excited at 260 nm for Z1.0 and Z1.5 heat-treated at 200ºC for 5 h, which featured a broad fluorescence band centered at 400 nm. This broad band (~ 3.1 eV) and the substantial redshift of the band maximum compared to the band gap (~ 5.6 eV) of the bulk material 32 strongly indicated that the fluorescence involved extrinsic states. Because the particle-size distribution was very narrow, the broad fluorescence band seemed to be mostly caused by the small particle size, which led to an inhomogeneous broadening from a distribution of the surface or defect states. The fluorescence intensity of Z1.5 is stronger than Z1.0, which could be ascribed to the increasing of the crystallinity as shown in the XRD analysis in Fig. 3.

Conclusions
ZrO 2 nanocrystals with different morphologies were prepared by the hydrothermal method. The morphology, crystallinity, and optical property of as-synthesized nanoparticles were characterized using SEM, HRTEM, XRD, Raman spectroscopy, PL spectroscopy, and BET measurements. Both CH 3 COO − and Cl − greatly affected the crystal size, phases, and morphologies of the ZrO 2 nanoparticles. Under the reaction conditions used in this work, the morphology and crystallinity of the resulting ZrO 2 nanoparticles could be adjusted. The nanoparticles    www.nature.com/scientificreports/ Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creat iveco mmons .org/licen ses/by/4.0/.