Structural, optical and giant dielectric constant properties of pure ktenasite and schulenbergite/CuO minerals

In this study, schulenbergite [(Cu,Zn)7(SO4)2(OH)10·3H2O)]/CuO and pure ktenasite [(Cu,Zn)5(SO4)2(OH)6·6H2O)] minerals were simply synthesized via addition of sodium sulfide (Na2S) to a mixture of nanosized CuO powder dispersed into Zn(NO3)2·6H2O solution. The X-ray diffraction patterns illustrate the formation of schulenbergite/CuO mineral with ratio of 69:31% and 87/13% owing to additions of 0.1 and 0.2 mol L−1 Na2S, respectively. The addition of 0.4 mol L−1 Na2S substance lead to formation of pure ktenasite [(Cu,Zn)5(SO4)2(OH)6·6H2O)] mineral. The absorption vibration modes based on Fourier-transform infrared (FTIR) analysis verified the formation of schulenbergite/CuO and pure ktenasite compositions. The scanning electron microscope micrographs of schulenbergite/CuO and pure ktenasite samples reveal the formation of mixed grains with needle, sheets, cotton and wool shapes. The selected area electron diffraction images of the synthesized powders show strong dot-rings, indicating polycrystalline nature. Optically, all samples possess a high absorption ability for infrared-visible light wavelengths. At lower frequencies, the pellet of pure ktenasite sample exhibits giant dielectric constant characteristics. Exactly, pure ktenasite sample reveals a dielectric constant value of ~ 1.3 × 106 at frequency of 42 Hz. For schulenbergite/CuO (87/13%) sample, a large dielectric constant value of ~ 5311 was measured at frequency of 42 Hz. The colossal and variable relative permittivity values make the pure ktenasite [(Cu,Zn)5(SO4)2(OH)6·6H2O)] mineral is a suggested material for energy storage applications.


Introduction
Design and synthesis of new materials and compositions can help in solving of many problems in different fields including biological, medical, environmental, magnetic targeting and separators, and energy storage applications [1][2][3][4][5][6][7][8]. Besides, the explore of the unique features and functions of the materials increases the knowledge of the characteristics of the material world in natural sciences and leads to various industrial and technological applications [9][10][11][12][13][14]. Discovering of nanomaterials with advanced optical and morphological properties have a great benefit in the photodegradation of industrial organic pollutants which discharge into water resources [15][16][17][18][19][20][21]. On the other hand, nanocomposites such as DyMn 2 O 5 /Ba 3 Mn 2 O 8 were synthesized via the hydrothermal technique as potential hydrogen storage materials [22]. In the same context, study of colossal relative permittivity (dielectric constant) compositions is very important for technological and capacitive energy storage applications [23][24][25]. The colossal permittivity structures can be defined as the materials that have relative permittivity (e r ) value usually greater than 10 3 and also reveal extreme polarization in an applied electric field [26]. These materials have promising applications in electronic, multilayer ceramic capacitors, memory and energy storage devices [26]. The colossal dielectric constant characteristics have been reported for numerous metal oxides and perovskites structures [26][27][28]. With respect to mineral materials, Zheng et al. studied the dielectric characteristics of the hard rock minerals (Calcite, Diopside, Albite, Hornblende or Olivine) and implications for microwave-assisted rock fracturing [29]. Very rare studies have been carried out on the preparation and characterization of new copper-zinc (Cu,Zn) sulfate minerals such as hodgesmithite [(Cu,Zn) 6 Zn(SO4) 2 (OH) 10 [30][31][32][33][34][35]. Besides that, the optical, electrical and dielectric characteristics of these minerals are not studied until now. In Germany, schulenbergite was first described in 1984 by Hoderberg et al. [31]. With respect to ktenasite mineral as a basic copper zinc sulfate, it was first reported by Kokkoros in 1950 (Greece) [32]. Only, in 2004, Mei et al. reported the synthesis of pure ktenasite mineral using ZnO powder with CuSO 4 solution at room temperature [32]. According to our knowledge, there is no detailed results in the published literature about the optical, ac electrical conductivity and dielectric characteristics of schulenbergite and pure ktenasite minerals were found. In this study, we reported a new room temperature synthesis route based on coprecipitation method for schulenbergite and pure ktenasite minerals using nanosized CuO powder-Zn(NO 3 ) 2 Á6H 2 O solution and sodium sulfide (Na 2 S) substance. The structural, morphological and optical properties of the synthesized schulenbergite and pure ktenasite minerals were investigated by using X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM) and diffuse reflectance techniques. Additionally, the ac electrical conductivity and dielectric characteristics of the fabricated schulenbergite and ktenasite pellets were measured and discussed. Remarkably, the pellet of the pure ktenasite powder has shown giant dielectric constant (relative permittivity) characteristics with a dielectric constant value of * 1.3 9 10 6 at frequency of 42 Hz. The giant relative permittivity values make the pure ktenasite [(Cu,Zn) 5 (SO 4 ) 2 (-OH) 6 Á6H 2 O)] mineral is a promising material for energy storage applications.

Synthesis of CuO nanopowder
CuO nanoparticles was synthesized through using the coprecipitation method. Such method is a simple way which attracts significant interest in industries owing to low energy and temperature, inexpensive and cost-effective approach for a large scale production. Exactly, 10  The obtained results illustrated that the concentration of Na 2 S substance plays an important role in the ratio of the schulenbergite/CuO mineral formed or also the formation of pure ktenasite mineral.

Characterization and measurements
The X-ray diffraction (XRD) measurements for the synthesized Schulenbergite/CuO pure ktenasite were performed by using PANalytical X-ray diffraction equipment model X 0 Pert PRO (CuKa radiation = 1.5406 Å ). The results were obtained at room temperature in the 2h range between 5°and 80°. The lattice constant (a, b, c) and the unit cell volume (V) were refined thru the least square method using FullProf software. To investigate the vibration absorption modes, Fourier-transform infrared (FTIR) spectra were measured via the KBr technique using JASCO model 4600 spectrometer. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images were achieved by model Quanta 250 FEG and JEOL JEM-2100, respectively. The energy-dispersive X-ray (EDX) spectroscopy was carried out to examine the elemental composition. The diffuse reflectance analysis of the synthesized powders was carried out by using a double beam spectrophotometer-JASCO (model V-570 UV-Vis-NIR). At room temperature, the ac electrical conductivity, dielectric constant (relative permittivity) and dielectric loss tangent (tan d) properties were measured using LCR meter (Hitester, model Hioki 3532-50, made in Japan). For electrical measurements, the synthesized powders were converted to pellets form with diameter of 1 cm and thickness of 2.5 mm.
3 Results and discussion 3.1 XRD study  (15). The ratio of hexagonal schulenbergite mineral to monoclinic CuO which formed due to addition of 0.1 mol L -1 Na 2 S was 69-31%, respectively. Whereas, the obtained ratio of hexagonal schulenbergite mineral to monoclinic CuO after addition of 0.2 mol L -1 Na 2 S was 87-13%, respectively.
As illustrated in Fig. 1d, the X-ray diffraction pattern of the powder produced by the addition of 0.4 mol L -1 Na 2 S substance to CuO dispersed into Zn(NO 3 where b represents the full width at half maximum, h signifies to peak angle (2h/2), K is a constant, k indicates the wavelength of the incident XRD-ray (0.15406 nm), D point to the crystallite size and e represents the microstrain of the samples. As demonstrated in Fig. 2, the plot of bcosh against 4sinh yields the microstrain from the slope and the crystallite size from the intercept (D = Kk/intercept). In case of schulenbergite/CuO (69/31%) sample the crystallize size (D) and the microstrain (e) were estimated to be 36 nm and 0.000025, respectively. For schulenbergite/CuO (87/13%) sample, the crystallize size and the microstrain were valued to be 29 nm and 0.00076, respectively. with respect to pure ktenasite powder, the identified crystallize size and microstrain were 93 nm and 0.003, respectively.  two weak absorption bands detected at 1552 and 1387 cm -1 can be attributed to the adsorbed CO 2 and/or the residual nitrate group (from starting zinc nitrate material) linked to surface of the synthesized powder, respectively [40,41]. The absorption bands sited at 1062 and 1042 cm -1 and the shoulder band at 960 cm -1 were attributed to the vibration of SO 4 group [30][31][32]. In the low wavenumber region, the various absorption bands which situated at 827, 595, 485 and 439 cm -1 can be ascribed to the vibration of (Cu,Zn) 7 octahedra of schulenbergite mineral and monoclinic Cu-O bonds vibrations [30-32, 42, 43]. With respect to schulenbergite/CuO mineral (87/ 13%), Fig. 3b, similar absorption bands to the previous sample were detected, but the intensities of the absorption bands related to CuO component were weak. In case of pure ktenasite sample, Fig. 3c Figure 4 shows the scanning electron microscope (SEM) micrographs as well as the parallel 3D view of the synthesized schulenbergite/CuO (69/31%), schulenbergite/CuO (87/13%) and pure ktenasite powders. As illustrated in Fig. 4a, the image of the synthesized schulenbergite/CuO (69/31%) powder displays the presence of little particles have needle and sheets shapes with major particles have cottonlike structure. The SEM micrograph of the schulenbergite/CuO (87/13%) powder exhibits sheets or potato chips structure with different size agglomerated together in large masses. In case of pure ktenasite mineral powder, the particles have cotton or wool shapes. Figure 5

Transmission electron microscopy
The transmission electron microscope (SEM) images and the corresponding selected area electron diffraction (SAED) patterns of the synthesized schulenbergite/CuO (69/31%), schulenbergite/CuO (87/ 13%) and pure ktenasite powders were illustrated in Fig. 6. In case of schulenbergite/CuO (69/31%) sample, it can be seen the formation of wrapped threads and nearly sheets particles agglomerated together, Fig. 6a. With respect to schulenbergite/ CuO (87/13%) sample, elongated rods overlapped with sheets particles were noticed as shown in Fig. 6b. The TEM image of the pure ktenasite powder, Fig. 6c, displays the presence of elongated particles and cotton-like agglomerated particles. The corresponding SAED patterns of all the synthesized samples obviously show the existence of strong dotsrings (consecutive circles), indicating a high crystalline nature.

Optical characteristics
The optical properties of the synthesized schulenbergite/CuO (69/31%), schulenbergite/CuO (87/ 13%) and pure ktenasite powders were studied by diffuse reflectance technique within wavelength of 200-1800 nm as shown in Fig. 7. In the infrared (700-1800 nm) and visible light (400-700 nm) region, the synthesized compositions exhibit a low diffuse    Table 1. The dielectric constant behavior of the synthesized samples with frequency are similar to those observed for many metal oxides and can be described by using Maxwell-Wagner model (interfacial polarization) which in agreement with Koop's phenomenological theory [48][49][50][51][52][53]. Numerous kinds of polarization including ionic polarization, electronic polarization, space charges polarization, molecular orientation, chain relaxation or free counter ions Numerous of these dipole moments can switch under the effect of the applied electric field. In this case, at the grain boundaries the rotation-direction polarization takes place which finally contribute to the relative permittivity of pure ktenasite sample. Figure 8b shows the variation of the dielectric loss tangent (tan d, dissipation factor) which represents the convert of the electrical energy to heat of the dielectric material. The tan d values were decreased with increasing the applied frequency for all samples. The schulenbergite/CuO (87/13%) sample exhibits the highest value of the dielectric loss factor. Although ktenasite material reveals huge dielectric constant but it shows a low dielectric loss factor (tan d) compared to schulenbergite/CuO (87/13%) sample. The lowest tan d values was achieved for the schulenbergite/CuO (69/ 31%) sample. In the same context, the performance of the tan d can be explained by using Maxwell-Wagner and Koop's phenomenological theories [48][49][50][51][52][53]. At the low frequency, the big value of tan (d) can be ascribed to the large resistivity of grain boundaries which are more effective than the grains. Figure 9 demonstrates the frequency dependence of the ac conductivity of schulenbergite/CuO (69/31%), schulenbergite/CuO (87/ 13%) and pure ktenasite samples. The schulenbergite/ CuO (69/31%) sample shows the lowest ac electrical conductivity. On contrast, schulenbergite/CuO (87/ 13%) and pure ktenasite samples reveal a high ac electrical conductivity. With increasing the frequency, the enhancements of r ac can be explained by using Maxwell-Wagner interfacial theory [48][49][50][51][52][53]. With increasing of the applied frequency, the charge carriers achieve the adequate energies to move over the potential barrier which yields the high progress in the r ac . On contrast, at small frequencies, the low progress in the r ac can be attributed to the fewer electric charges that can tunnel over the potential barrier. In this work, the dielectric measurements point out that the pure ktenasite mineral has giant and variable relative permittivity values with low dielectric loss tangent which make ktenasite sturcture as a promising material for energy capacitive storage applications.

Conclusions
In this study, the electrical and dielectric properties of schulenbergite/CuO (69/31%), schulenbergite/CuO (87/13%) and pure ktenasite samples were studied for energy capacitive storage applications. The structural analysis confirmed that the Na 2 S concentration has a main role on the schulenbergite [(Cu,Zn) 7 (SO 4 ) 31%) and schulenbergite/CuO (87/13%) composites were formed owing to addition of 0.1 and 0.2 mol L -1 Na 2 S, respectively. Pure ktenasite mineral was formed after increasing the Na 2 S concentration to 0.4 mol L -1 . Morphologically, schulenbergite and ktenasite powders reveals mixture grains of needle, sheets and wool shapes. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) observation show the formation of particles of different shapes with obvious crystalline nature. The results of the dielectric constant verified that ktenasite sample exhibits colossal relative permittivity at lower frequencies with recorded value of * 1.3 9 10 6 at frequency of 42 Hz. The giant dielectric constant values of pure ktenasite mineral is suitable for energy capacitive storage applications.

Author contributions
All authors have equal contributions (study conception, design, material preparation, data collection and written of the final manuscript).

Funding
Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). The authors have not disclosed any funding.

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.