Effect of MnO2 and MnO2-NCNO addition on structural and nonlinear optical properties of CoWO4 ceramics

The nitrogen-doped carbon nano-onions (NCNOs) were prepared by annealing the ultra-dispersed aminated-nanodiamond solution under He gas at 1150 °C followed by calcination at 400°C. The nanostructures of CoWO 4 , MnO 2 , CoWO 4 -MnO 2 , and CoWO 4 -MnO 2 -NCNO were synthesized through the simple precipitation method under ultrasonication followed by calcination at 450°C. The morphology, structure, and optoelectronic properties of the samples were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and Z-scan method. The homogeneous distribution of the tiny aggregated plate-like and spongy particles throughout the nanocomposite created a highly porous nanostructure with a large surface area. The nonlinear absorption (NLA) coefficient and nonlinear refractive (NLR) index were of the order of ( and 8 2 10 ( ) c m W  , respectively. In different incident intensity of laser, all synthesized samples show TPA effect implying the positive sign of NLA coefficient. MnO 2 and NCNO structure have positive NLR index indicating the self-focusing optical nonlinearity. The self-defocusing effect and the negative sign of nonlinearity in the CoWO 4 nanoparticles, MnO 2 -CoWO 4 , and MnO 2 -NCNO-CoWO 4 nanocomposites are vivid. The porous structure of CoWO 4 nanoparticles and trapping the light into CoWO 4 nanoparticles are the main reasons for nonlinearity of this nanoparticles. Porosity is also one of the most important reasons for the nonlinear optical responses of NCNO structure. The NLR index and NLA coefficient of MnO 2 decreased by increasing the incident intensity. Moreover, an increase in the incident intensity nonlinear responses in the rest of samples. The present nonlinear optical results of the synthesized samples can be applied in optical devices.


Introduction
Over the last decade, novel optical features have been discovered in nanoparticles that can not be observed in individual molecules and bulk metals [1]. According to several experimental investigations, the nanoparticle surfaces play critical roles in their linear and nonlinear optical behavior, including emission behavior and third-order optical nonlinearity [2]. Today, several photonic applications like optical power limiting, optical switching, optical modulating, and three-dimensional optical memory systems have been developed based on the nonlinear optical features of different materials. The nonlinear refractive (NLR) index of these structures is of crucial importance in the design of optical devices [3]. Incredible nonlinear optical features can be observed in several nanostructured materials, motivating the design and production of photonic systems [4,5].
Remarkable physical characteristics have been observed by the extensive investigation of the optical nonlinearities in semiconductor nanomaterials. When the dimension of a semiconductor system reduces from bulk to nanoscale, the nonlinearity is enhanced as a result of the quantum size effect as well as other nanoparticle mesoscopic phenomena [6,7]. Furthermore, the properties of semiconductor materials could be tailored by changing their size, shape, and compositions to fit a diverse range of photonic and optoelectronic applications [7,8].
In this way, different types of materials were assessed to maximize the nonlinear optical properties. Due to its fascinating size and structure-dependent optical and electronic features, CoWO4 has attracted much attention as a significant inorganic material [9]. CoWO4 is a p-type semiconductor known for its amazing optical, electrical, and magnetic features. It can be applied in different areas including photovoltaic cells, electrochemical cells, pigment additive, gas sensor, and catalyst [10,11]. CoWO4 nanoparticles have shown wide optical absorption band, photothermal and photodynamic characteristics, and multimodal imaging abilities [12].
Moreover, this material has found applications in scintillation counters, lasers, and optical fibers.
Some of its advantages like large mean refractive index and excellent chemical stability can be attributed to the tungstate as a self-activating phosphor. In this respect, the nonlinear optical properties of Er 3+ -doped ZnWO4 have been explored which indicated the applicability of MWO4 (metal tungstates) laser for designing advanced photonic devices [13]. However, this property may be amplified with the incorporation or doping of other materials especially transition metal oxides.
Alternatively, since transition metal oxides including manganese oxides (MnO2) are important components in solid-state batteries, they have been increasingly interested among researchers. As a significant group of dioxides, MnO2 nanoparticles have over 14 polymorph forms. MnO2 is classified as a transitional substance with versatile physical and chemical features including narrow bandgap, high optical constant, and ferroelectric characteristics [14]. Nano MnO2 has shown outstanding capacitive properties when utilized as a supercapacitor. Its stable redox reaction, cost-effectiveness, and high theoretical capacitance (1370F/g) made it a suitable material for electrochemical capacitors [15]. The development of various MnO2 nanocomposites containing conductive materials such as carbon-based or other transition metals compounds can enhance its electrical conductivity, which may also improve its nonlinear responses. The study of optical constants plays an important role in improving the optical features of MnO2 thin film towards photovoltaic applications. To study the optical features of the MnO2-containing composites, the optical nonlinearity of β-MnO2 nano-thin films was determined which showed its suitability for nonlinear optoelectronic applications [16].
As an electrical double layer capacitor, carbon nano-onions (CNOs) store energy only through electrostatic interactions without any diffusion confinement, presenting the linear chargedischarge profile. They can supply high power density but very low energy density. CNOs possess exciting futures such as extended surface area and high mesoporosity [17]. Nonlinearity can be under the influence of high conductivity, large surface area, and porosity [18]. CNOs have been shown to offer various potential applications [19]. Optical properties of some CNO nanocomposites including the CNO/ZnO [20], CNO [21], CNO C60@C240 [19], and polyethyleneimine-poly(ethylene glycol)-CNO [22] have been studied but there is no report on the optical nonlinearity of pure CNO.
Therefore, various composites including CNO, MnO2, and CoWO4 constituents were generated for enhancing the nonlinearity of components. In the present communication, nitrogen-doped CNOs(NCNO), CoWO4, and MnO2 were prepared in the form of hybrid nanostructures to improve the nonlinear optical behavior.

Reagents and Materials
The commercially available powder of the aminated-nanodiamond (AND, 4 nm ≤ crystal sizes ≤ 6 nm, purity more than 97 %) was purchased from Carbodeon μDiamond®Molto, Vantaa, Finland. All other utilized chemical and compounds were of analytical grade and were purchased from Merck, and used as received without further treatment.

Synthesis of NCNO
The ultra-dispersed AND solution was employed for preparation of NCNO. The procedure was according to a process developed by Kuznetsov with some modification [23]. The ultra-dispersed AND solution was annealed under He gas (1.1 MPa). The temperature was raised to 1150 °C and let the reaction to be done for 60 min. Then, the temperature was slowly decreased to ambient condition during 60 min. After that, resulted product was annealed at 400 °C in an open air furnace to eliminate the amorphous carbon.

Synthesis of CoWO 4 Nanostructure
One liter of 0.4 M Na2WO4.2H2O aqueous solution was prepared by dissolving 131.9 mg of Na2WO4. 2H2O salt in 1000 mL D.I. water, which was named as solution A. The 0.4 M Co (II) solution was prepared by dissolving Co(NO3)2.6H2O in D.I. water. Then, the solution A was slowly added dropwise into the solution B in ratio of 1:1 in room temperature under vigorous stirring. The following reaction was took place: After that, the precipitated product was thoroughly washed with D.I water and filtered. The precipitate was dried at 100 o C for 24 h. Finally, the dried product was grounded to a fine powder using an agate mortar which further calcinated at 400 o C for 4 h.

Synthesis of CoWO 4 -MnO 2 and MnO 2 nanostructures
1.0 g of the prepared CoWO4 was dispersed in 100 mL of D.I water and 2.4 g Mn(Cl)2 was dissolved into it. The pH of the solution was adjusted to 10 by adding adequate amount of NaOH and then stirred during 2 h. Then, the solution was ultrasonicated for 20 min after that was dried at 150 o C. The MnO2 nanoparticles were prepared in the same manner without addition of CoWO4.

Synthesis of CoWO 4 -MnO 2 -NCNO
1.0 g of the prepared CoWO4 was dispersed in 100 mL of D.I water and 2.4 g Mn(Cl)2 was dissolved into it. After adjusting the solution pH to 10, it was stirred for 2 h. Then, 100 mg NCNO powder was dispersed in the solution with the aid of 2 h magnetic stirring and 20 min ultrasonication. The obtained precipitate was dried at 60 o C. Finally, the dried product was grounded to a fine powder using an agate mortar and further calcinated at 400 o C for 4 h. Fig. 1 indicated the whole synthesized procedure of CoWO4-MnO2-NCNO nanostructure.

Material characterization
Fourier transform infrared (FTIR) spectra were obtained using a Bruker FTIR spectrometer

Characterization of synthesized nanostructures
The functional groups in nanostructures of synthesized (NCNO, MnO2, CoWO4, CoWO4-MnO2, and CoWO4-MnO2-NCNO) were identified using FTIR analysis as depicted in Fig Table 1. NCNO (Fig.3c) showed a very weak and broad peak at 2Ө~43° originated from remained nanodiamond (ND) precursor utilized for the synthesis of NCNO. The peak that appeared at 2Ө~23° approved the successful conversion of the sp 3 hybridization of the ND structure to the sp 2 in graphitic carbon with a crystal plane of (002) [27][28][29][30]. The XRD pattern of CoWO4-MnO2-NCNO (Fig.3e)  indicating that the addition of NCNOs to the composite structure did not influence the structure of the product. Note that all synthesized materials showed homogeneous distribution of their components.

Z-scan measurements
Among the techniques used to analyze nonlinear optical features, the Z-scan measurement has been widely applied for the analysis of NLR indices and the nonlinear absorption (NLA) coefficients owing to its great sensitivity as well as simplified architecture [31].
Where in P and out P are the input and output power of the laser beam after passing through the sample, respectively.
The CA Z-scan data divided by the data of the OA Z-scan present the NLR, n2, given that both Z-scans perform at the identical incident intensity. In the CA Z-scan configuration, the normalized data was put in the following equation to calculate the NLR index [31]: Where 0  depends on 2 n according to the following equation [31]: Where  is the wavelength of the incident laser beam. For calculation of the nonlinear absorption coefficients, β, Eq.(2) was used for fitting the normalized transmittance acquired from the OA Z-scan setup. Fig. 6 shows the scattered data acquired from the OA Z-scan setup, with a solid line indicating the theoretically fitted experimental data using Eq. (2).
For determining the amount of n2, ΔT (distance between peak and valley) could be calculated at various intensities using the graphs in the CA Z-scan setup. Then the amount of n2 for the evaluated intensities was computed through Eqs. (5) and (6). To retrieve the NLR indices of the synthesized samples, the normalized transmittances were fitted to Eq.(5).
According to Sheik-Bahae et al., the signs of NLR index are positive (or negative) if there is a valley to peak (or a peak to valley) in the experimental data of CA Z-scan setup [33]. The positive and negative NLR indices refer to the self-focusing and self-defocusing effect of laser in the materials, respectively [33]. The CA Z-scan curves are shown in Fig.7.

Nonlinearity responses of MnO 2
According to Fig.6(a), the approximately symmetrical curves with a minimum value at z=0 indicate the positive value of β. This nonlinearity could be ascribed to the two-photon absorption.
The normalized curves for the CA Z-scan of MnO2 nanoparticles (at the concentration of 0.1g/L) are shown in Fig.7(a) for altered incident intensities for the aperture linear transmission of S=0.27. As a peak appears after the valley, the NLR index is positive [34]; implying the selffocusing optical nonlinearity of the material. The amount of n2 and β are listed in Table 2. The NLR index and NLA coefficient decreased by increasing the incident intensity and input power.
Similar studies on MnO2 nanoparticles have revealed the presence of important nonlinear aspects in lower incident intensities. For example, the magnitude and order of nonlinear response in this study are in line with the results of a study on MnO2 nanoparticles using a CW laser beam with 532 nm wavelength [35]. The decrease of nonlinearity response by increasing the incident intensity was also confirmed [35]. Fig.6(b) depicted the nonlinear responses of CoWO4 nanoparticles. According to OA Z-scan data, the sign of NLA of CoWO4 is positive due to the two-photon absorption process. In the CA Z-scan data, the curve in Fig.7(b) shows the peak-valley shape, implying a negative sign of nonlinearity in the CoWO4 nanoparticles due to a self-defocusing effect, which can be assigned to the local variation in the refractive index with the temperature. An increase in incident intensity attenuated the nonlinear responses of CoWO4 nanoparticles. All linear and nonlinear optical parameters are tabulated in Table 2. The porous structure of CoWO4 nanoparticles and trapping the light into CoWO4 nanoparticles are the main reasons for nonlinearity [36].

Nonlinearity of CoWO 4
Furthermore, the presence of the double bond also led to nonlinear optical responses, although it is not very effective compared to porosity.
Similar results of nonlinearity of the tungstate family were reported earlier (such as NiWO4). The order of n2 and β are in good agreement with the nonlinear responses of NiWO4 nanoparticles.
Furthermore, in lower incident intensity, the NLA amount of CoWO4 was higher than NiWO4 [37]. At higher intensities, no difference was recognized between CoWO4 and NiWO4 nonlinearity responses.

Nonlinearity responses of NCNO
Nonlinear optical parameters of NCNO were investigated by OA and CA Z-scan techniques as presented in Figs.6(c) and 7(c). A dip in the transmittance curve at the focus position of OA Zscan data of NCNO pointed to positive signs of NLA, β, and two-photon absorption process.
Furthermore, the valley-peak configuration of CA of NCNO Z-scan data presented the positive sign of NLR index, n2, and self-focusing optical nonlinearity of NCNO structure. According to recorded nonlinearity data listed in Table 2, an increase in the incident intensity and input power incremented the NLR index and NLA coefficient.
Consequently, different parameters (including particle size, surface passivation, and nature of the employed organic moieties along with the electronic band structure) should be considered to take the NLO responses of these carbon-based nanomaterials into account [38]. Porosity is one of the most important reasons for the nonlinear optical responses of CNO [36]. Moreover, nonlinear scattering could be another factor for recorded nonlinearity responses, especially at higher intensities [38].
Previous investigations comparing the nonlinear optical responses of certain nanodiamonds and onion-like nano-carbons also reported similar observations in which the onion-like nano-carbons showed greater nonlinear optical responses and optical limiting due to having more sp 2 carbons [31]. The onion-like carbon nanoparticles were found to exhibit weaker nonlinear optical responses in comparison with the present findings, in which the corresponding NLA coefficients β was 3~30×10 -11 m/W [38,39].

Nonlinearity responses of CoWO 4 -MnO 2
The NLA of CoWO4-MnO2 nanocomposite was evaluated by OA Z-scan curves in different incident intensities as depicted in Fig.6(d). According to the OA Z-scan setup, the positive sign of β is related to the valley in the focus position of the transmittance curve. The two-photon absorption effect can lead to a dip in the center of the transmittance curve versus the z-axis. On the other hand, in the CA Z-scan setup, the valley is followed by a peak (Fig.7(d)). Thus, the sign of the NLR index is negative and the self-defocusing effect was dominant. The NLR and NLA are calculated and listed in Table 2.
As mentioned, the porosity plays a decisive role in the nonlinear optical responses of CoWO4-MnO2 nanocomposite. The CoWO4-MnO2 nanoparticles are larger than the CoWO4 and MnO2 nanoparticles. These bigger particles can convey high irradiation energy to the environment, resulting in density inhomogeneities that can scatter the light [40]. The CoWO4-MnO2 nanoparticles also exhibited porous structure. The impact of porosity is greater than the size of CoWO4-MnO2 nanocomposite, so the CoWO4-MnO2 nanocomposite enhanced the nonlinear responses compared to its constituent components. Moreover, an increase in the incident intensity improved the nonlinear responses.
The solid curves in Fig.7(e) are the best theoretical fit for the experimental data. The calculated nonlinear refractive indices of CoWO4-MnO2-NCNO nanocomposite are also listed in Table 2.
By increasing the incident intensity, the NLA and NLR grew. As previously mentioned, the porous structure of CoWO4-MnO2-NCNO nanocomposite trapped the laser light and improved the nonlinearity.