Abstract
The present study is focused on the obtaining of the Zn3Nb2O8 nanomaterial using the hydrothermal method and its characterization through different techniques. X-ray diffraction at room temperature revealed that a novel crystalline form of the nanomaterial forms at 1100°C belonging to monoclinic space group C2/c. Field-emission scanning electron microscopy evidenced the columnar morphology of the particle’s agglomeration and the high resolution electron transmission microscopy confirms the measured interplanar distances calculated from the X-ray diffraction experiments. Using the UV-VIS spectrum and Kubelka-Munk equations, the absorbance and the band gap for the Zn3Nb2O8 nanomaterial were calculated. PL spectrum reveals a single peak at 465 nm corresponding to the blue color fluorescence. The novel crystalline nanomaterial might find applications in fluorescence covering of technical devices, due to its capacity to preserve blue fluorescence both in acrylic based paint and after embedding in isopropyl alcohol.
1. Introduction
Considerable attention was gained in the latest years by the light-emitting materials and the lighting technology and the researches were focused to produce low cost inorganic, organic, or hybrid materials capable of exhibiting fluorescence in different colors [1–5].
Zn3Nb2O8 has been reported as being an excellent dielectric [6–8] and a good microwave material due to its high quality factor [9, 10] and it can be used for applications in the high frequency domains [6–8]. Its demanded properties consist of resonance frequency in narrow temperature range [6], a high dielectric quality factor [11], and high dielectric constant [6]. The dielectric properties strongly depend on the microstructure organization and of the chemical composition of the material. The density and the particle sizes of the materials should be taken into account in order to obtain the maximum of the dielectric properties [10].
Zn3Nb2O8 represents one of the three phases of the ZnO-N2O5 system beside ZnN2O6 and Zn2Nb34O87 [6, 7, 12, 13]. The reported literature methods for obtaining Zn3Nb2O8 refer only to solid state mixed oxide method [6, 10, 14–17] using as precursors ZnO and N2O5 and the coprecipitation method performed simultaneously with Eu3+/Dy3+ doping [18]. This pseudobinary oxide was also obtained as a secondary phase of the ZnO phase (when trying to obtain ZnO ceramics doped with Nb) and affected the thermoelectric properties of the material (increasing the electrical conductivity and decreasing the Seebeck coefficient) [1].
The main aim of this paper is to present the results regarding the obtaining of the Zn3Nb2O8 nanomaterial using the hydrothermal method that allows for the first time generating crystalline self-assembled columnar nanomaterials. The structural and optical properties of the nanomaterial were investigated through different techniques and its blue fluorescence was demonstrated to be preserved both in isopropyl alcohol and in acrylic based paint, starting the possibilities of novel applications in fluorescence coatings.
2. Experimental
The Zn3Nb2O8 nanomaterial was obtained by the hydrothermal method. The starting materials used during the synthesis of Zn3Nb2O8 were as follows: niobium (V) oxide, Nb2O5 (99%, Merck), and zinc oxide, ZnO (99%, Merck), in a molar ratio of 1 : 3. The pH of the obtained mixture was adjusted by using sodium hydroxide (NaOH) solution of 10 M concentration. The pH value for the synthesis was fixed at 9. The resulting suspension was transferred into a Teflon-lined stainless steel autoclave and then was introduced in an oven at 240°C for 24 h. The filling degree of the used autoclave was set at 80%. The resulting white precipitate was filtrated and then washed six times with distilled water and, finally, washed for four times with ethylic alcohol. In the next stage, the precipitate was dried in the oven at 100°C for 4 h.
The characterizations using the room temperature and 1100°C have been performed on well-crystallized powder samples of Zn3Nb2O8. The phase identification of the synthesized powders was performed using X-ray diffraction (XRD) with monochromatic Cu Kα (λ = 1.5418 ) incident radiation on an X’pert Pro MPD X-ray diffractometer equipped with the X’celerator detector and a high temperature attachment HTK 2000 MSW chamber. Diffraction data were obtained for the angular range 2θ = 10–80° using a 0.02° step and the counting time was 5 s. After identifying the phases, the mixture of oxides was afterwards thermally treated at 1100°C for 6 h soaking time. The heating and cooling rate of the heated furnace was set at 5°C/min.
The morphology and the particle’s dimension of the sample were investigated by field-emission-scanning electron microscopy (SEM) (Model INSPECT S) and transmission electron microscopy with high resolution transmission microscope (HRTEM) (TITAN G2 80–200). The band gap of the Zn3Nb2O8 materials was calculated by recording the diffuse reflectance spectrum at room temperature, using a UV-VIS-NIR spectrometer Lambda 950. The PL spectra were recorded with an Edinburgh Instruments FLS980 luminescence spectrometer, by using a special holder for powder solid samples.
Besides all these investigations, the Zn3Nb2O8 nanomaterial was embedded in an acrylic based paint and also in isopropyl alcohol in order to prove its blue fluorescence. The suspensions were allowed to dry in air and were then illuminated with an UV light lamp (, nm).
3. Results and Discussion
3.1. Structural Characterization
The XRD patterns of Zn3Nb2O8 (Figure 1) revealed that the novel crystalline phase of the nanomaterial starts forming around 800°C and the maximum of this phase is set around 1100°C. The spectra were compared and indexed with the 01-079-1164 JCPDS. Zn3Nb2O8 is monoclinic with the space group C2/c (space group number 15), with the following lattice parameters: Å, Å, Å, and ° and the interplanar distance Å. The mean crystallite size of the Zn3Nb2O8 was calculated using Scherrer’s formula [19]. The average crystallite size determined from XRD line broadening was found to be 32 nm.
In the XRD spectrum, the most intense peak appears at 2θ = 31.25° (-511). In the right upper corner of Figure 1 is presented the magnified image of the most intense peak of the XRD pattern.
3.2. Morphological Characterization
In Figure 2(a), the morphology of the Zn3Nb2O8 material can be observed. The agglomerates formed by the Zn3Nb2O8 nanomaterial particles are crystallizing as multifacet columns which is a novelty.
(a)
(b)
Using the high resolution electron transmission microscopy (HRTEM), in Figure 2(b), the forming of the crystallographic planes of the Zn3Nb2O8 nanomaterial can be observed. The crystalline plane with Miller indices (-511) indicates the perfect orientation of the lattice. The measured interplanar distance is about 2,68 Å and corresponds to the measurements determined by the X-ray diffraction.
3.3. Optical Properties
The optical absorption spectra (Figure 3) of the Zn3Nb2O8 nanomaterial were performed in the 240–480 nm range, at ambient conditions. The absorbance was calculated from the reflectance using Kubelka-Munk equation [20, 21]. From each of the absorption spectra, the versus was plotted (Figure 3, right upper corner), where denotes the absorption coefficient, “’’ is the scattering coefficient, and is the photon energy.
The band gap was calculated from Figure 3 (absorbance) and its value was found to be 4.3 eV for Zn3Nb2O8.
The PL spectra recorded in the 220–700 nm range for Zn3Nb2O8 are presented in Figure 4. The PL spectra show only a single peak at 465 nm for Zn3Nb2O8 which is corresponding to the blue color fluorescence.
To point out the blue emitting color of the Zn3Nb2O8, in relation to our further interest to exploit this property, a sample of the nanomaterial was embedded in an acrylic based paint and another sample was embedded in isopropyl alcohol. After drying, these two samples were exposed to a UV light lamp (λ = 254 nm). In Figure 5 are presented the images for these two samples that both preserved the initial blue fluorescence of the nanomaterial.
(a)
(b)
4. Conclusions
In this paper are presented the results regarding the obtaining of the Zn3Nb2O8 using the hydrothermal method, in a novel crystalline phase (multifacet columnar crystals) and the characterization of the nanomaterial. The powder structure was established by XRD diffraction at room temperature which revealed the forming of the novel structured material at 1100°C.
Field-emission scanning electron microscopy evidenced the columnar morphology of the particle’s agglomeration. High resolution electron transmission microscopy confirms the measured interplanar distances calculated from the XRD. Using the UV-VIS spectra and Kubelka-Munk equations, the absorbance and the band gap for the Zn3Nb2O8 nanomaterial were calculated. PL spectra revealed a single peak at 465 nm corresponding to the blue color fluorescence that is preserved also in acrylic based paint and after embedding in isopropyl alcohol.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This paper is partially supported by the Sectoral Operational Programme Human Resources Development (SOP HRD), financed from the European Social Fund and by the Romanian Government under Project no. POSDRU/159/1.5/S/134378. This work was partially supported by the strategic Grant POSDRU/159/1.5/S/137070 (2014) of the Ministry of National Education, Romania, cofinanced by the European Social Fund-Investing in People, within the Sectoral Operational Programme Human Resources Development 2007–2013. Author Eugenia Fagadar-Cosma is acknowledging Romanian Academy for financial support in the frame of Programme 3 of ICT-AR.