Magnetization and optical bandgap of Cu-Mn vanadate-oxide mixed phase nanostructures

Copper vanadate (CV) and manganese vanadate (MV) exhibit magnetic and optical properties that have drawn the attention. Due to CV polymorphism and phase multiplicity, CV is common to exist as mixed phases. In this study, nanostructures of mixed-phase CVs mixed with MV were synthesized hydrothermally followed by calcination at 400 °C, with Mn mole fractions 0.0, 0.4, 0.6, 1.0. The uncalcined and calcined Mn-Cu vanadates (MCVs) were investigated by XRD, SEM, TEM, FT-IR, EDX, ICP-AES, TGA, DTA, DSC, BET, XPS, and VSM. XRD analysis shows co-existence of multi-phase CVs with MnV2O6 and V oxides. Electron micrographs show nanostructures of multiple morphologies (rods, cubes, sheets, and irregular). As Mn content increased in the MCVs, their thermal stability increased, optical bandgap (Eg) declined from 2.46 to 1.60 eV, and magnetism diverted from the superparamagnetic-like to paramagnetic (Hc from 1362 to 69 G and Mr/Ms from 0.430 to 0.003). Magnetism parameters of calcined MCVs were more labile to Mn content variation compared to the uncalcined MCV counterparts.

Slight perturbation in the synthesis conditions leads to switching from one CV phase to another. Sometimes obtaining one pure phase can be a challenging task and just mixed-phase CVs originate [13][14][15][16][17]. The resulting mixed phases not only contain copper vanadates, but in some cases vanadium oxide may emerge as a distinct phase amongst the rest of the mixture. Hossain et al. and Keerthana et al. reported co-existence of V 2 O 5 alongside with α-Cu 2 V 2 O 7 as a result of solution-combustion synthesis [3,7].

Hydrothermal synthesis of MCVs
MCVs of this study were synthesized according to a published method with some modifications [34]. Then, 4.8 mmol copper chloride (0.818 g) was dissolved into 80 mL of deionized water. Further, 9.6 mmol ammonium monovanadate (1.123 g) was dissolved into 80 mL deionized water at 60 °C, and then filtrated. Clear ammonium monovanadate solution was added slowly to the copper chloride solution under continuous vigorous stirring for 10 min. After the addition was completed, the resulting suspension was transferred into a 300 mL Teflon-lined stainless autoclave and maintained at 210 °C for 12 h. The autoclave was allowed to cool down to room temperature. The solid powder was collected by centrifugation, washed several times with deionized water and ethanol, and then dried at 60 °C for 4 h. The sample was labeled CV, then calcined at 400 °C for 2 h in the air (calcined sample is denoted as cCV).
Manganese vanadate was prepared by adding manganese chloride instead of copper chloride. The obtained manganese vanadate is labeled MV (before calcination) and cMV (after calcination). Mixed phases MCV4, MCV6, and their corresponding calcined samples cMCV4, cMCV6 were prepared by introducing different copper and manganese chlorides ratios as shown in Table 1.

Characterization methods and techniques
Powder x-ray diffraction (XRD) patterns were collected with BRUKER D 8 Advance, Germany. It operated with Cu Kα radiation wavelength 1.54°A, at 40 kV, 40 mA, in the diffraction angle of 2θ from 5° to 80° at a scan rate of 5° min −1 . The Fourier transform infrared (FT-IR) spectra of samples before and after calcination was collected by Perkin Elmer spectrum version 10.5.3 IR spectrometer in the range 4000-400 cm −1 at room temperature. The stoichiometric composition was obtained with inductively coupled plasma-atomic emission spectroscopy (ICP-AES) ICP-OES 5100 VDV, Agilent where 0.01 g of the MCVs after calcination were dissolved in 5 mL concentrated nitric acid then heated and completed with water to total 25 mL. Energy-dispersive X-ray spectroscopy (EDX) was conducted with field emission scanning electron microscope JEOL JSM 7000F with a 15 kV accelerating voltage. A 10 mm working distance was used to confirm the constituent elements to determine the elemental ratios. The size and morphology of the MCVs before calcination were characterized with (JEOL JEM-200CX2100F, Japan) transmission electron microscopy (TEM) with an accelerating voltage of 200 kV. Divanadate TEM samples were prepared by sonicating 0.01 g for 30 min with 5 mL of ethanol in a centrifugal tube. Cu grid 400 mesh with a thin carbon film (Quantifoil) was immersed into the dilute dispersion of samples. The morphology and microstructure of MCVs after calcination were analyzed using scanning electron microscopy (SEM) JSM-IT200. Thermal gravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) were carried out with SDT Q600 V20.9 Build 20; the MCV samples before calcination were put into the crucible and heated in air at a heating rate of 10 °C min −1 from room temperature to final temperature 800 °C. UV-vis diffuse reflectance spectroscopy (DRS) was carried out at room temperature using a PerkinElmer Lamda-900 spectrophotometer in the range of 200-800 nm. Brunauer-Emmett-Teller (BET) determined the specific surface areas of MCV samples after calcination by nitrogen adsorption at 77 K using a Belsorp-mini II (Japan). Before surface area analysis, the powders were degassed at 150 °C for 3 h under vacuum. To study the surface chemistry of MCV samples after calcination, X-ray photoelectron spectroscopy (XPS) was collected with K-ALPHA (Thermo Fisher Scientific, USA) of monochromatic X-ray Al K-alpha radiation (− 10 to 1350 eV spot size 400 µm, at pressure 10-9 mbar, spectrum pass energy 200 eV, and at narrow-spectrum 50 eV). The magnetic properties of MCVs before and after calcination were measured with a vibrating sample magnetometer (VSM, Lake Shore-7410, USA) at room temperature with a magnetic field up to 20 kOe and magnetic moment sensitivity up to 1 m emu.

Results and discussion
Characterization X-ray diffraction Figure 2a and

Fourier transform infrared
FT-IR spectra analysis was performed to investigate the structure and functional groups. Figure 3a shows the FTIR spectra of the uncalcined MCV samples. The bands observed at 500-1010 cm −1 correspond to the metal-oxygen bonds stretching and wagging (Cu-O, Mn-O, and V-O). The band at 899-886 cm −1 arises from the symmetric stretching vibration of ν 1-VO 3 , and that at 751 cm −1 is corresponding to the anti-symmetric stretching vibration of ν 3-VO 3 of CV. The two bands at 530 cm −1 and 509 cm −1 are assigned to the ν 3 symmetric and ν 1 symmetric stretching modes of (V-O-V) of CV [36]. Those at 993-1010 cm −1 are corresponding to ν(V 4+ = O) [37,38]. The band observed at 575 cm −1 for MV is assigned for Mn-O vibration [39]. The bands at 1401-1405 cm −1 are assigned to symmetric stretching of CO 2 due to the absorption of CO 2 from the air [40]. The bands at 1613-1620 cm −1 correspond to the ν-OH stretching vibration and δH 2 O bending vibration of water molecules. The bands at 2700-3000 cm −1 are assigned to organic impurities on the surface of the samples (maybe resulting from handling) [41]. The bands at 3400-3600 cm −1 are assigned to the ν-OH stretching vibration of water molecules [36]. Figure 3b shows the FTIR spectra of the calcined MCVs. It can be noticed that plenty of the bands of the uncalcined MCVs have disappeared upon calcination due to the elimination of H 2 O, CO 2 , and OH groups. The bands at 400-1010 cm −1 correlate to metal-oxygen bonds that have become stronger by calcination. Those at 795-1024 cm −1 are attributed to the vibrational mode of V = O bonds. The strong bands at1016-1024 cm −1 represent the V = O bond in V 2 O 5 as a common product in all calcined samples. This band is very small for cMV due to the dimmish of V 2 O 5 [42]. The presence of the Mn-O bond could be observed at ~ 558 cm −1 , which is closed to that reported in the literature at 564 cm −1 [39,40]. The bands at 1401-1405 cm −1 of CO 2 symmetric stretching disappeared completely after calcination. The intensities of vibration bands at 1613-1620 cm −1 of the ν-OH stretching vibration and δH 2 O bending vibration of water molecules decreased due to water removal. The bands at 3000-2700 cm −1 are assigned to organic impurities decreased also. The bands at 3600-3400 cm −1 of the ν-OH stretching vibration of water molecules also disappeared due to the water elimination during the calcination process.

ICP and EDX elemental analysis
The EDX (Fig. 4) and ICP analysis show that the metals ratios (Cu, Mn, and V) are in good match with the starting concentrations of Cu 2+ , Mn 2+ , and VO 3 − added during the synthesis as shown in Table 3. Detailed raw EDX scan results are given in Figs. S1, S2, S3, and S4 in the supplementary information for samples cCV, cMCV4, cMCV6, and cMV, respectively.    Figure 6 shows SEM micrographs of cMCVs. cCV consists of nano-rods beside irregular shapes. This may be attributed to the presence of muli-phases of copper vanadates and V 2 O 5 . In cMCV4, shapes are more irregular, while in cMCV6, sheets the abundance with fewer rods than cCV. In cMV, there are no rods, and the sample is all nanosheets either stacked or separate.  Thermogravimetric analysis of copper and manganese vanadates (TGA, DTA, and DSC) Figure 7 and Table 4 show the thermal analyses of the uncalcined MCVs carried out under an air atmosphere at a heating rate of 10 °C/min from the ambient temperature to 800 °C. The weight losses observed in the TG thermograms (Fig. 7a) are generally due to the removal of adsorbed water, crystallization water, and hydroxyl groups. The results are summarized in Table 4. For CV, the loss in weight takes place at four stages: the first stage starts from ambient temperature to 175 °C, the weight loss is 5.5% which may be due to the removal of adsorbed water and some crystallization water of the Volborthite phase. In the second stage, starting from 175 to 260 °C, the weight loss is 4.8% which is attributed to the complete removal of crystalline water of volborthite. In the third stage, starting from 260 to 400 °C, the weight loss is 3.9% due to removing two hydroxyl groups of Volborthite [43,44]. The TG curve also shows another weight loss started at about 635 °C, which corresponds to the melting point of CuV 2 O 6 in the CV sample. An endothermic peak accompanying this step appeared in each DTA and DSC thermogram (Fig. 7b and c). In the MCV4 sample, the weight loss of the melting step started at about 698 °C due to the melting point of V 2 O 5 [45]. With decreasing the ratio of copper in the investigated samples, the weight loss also decreases since the samples contain fewer phases of crystallization water and hydroxyl groups (as pointed out in the XRD analysis and Table 2). MV exhibits minimal weight loss, which may be due to its phase stability and it shows only phase of MnV 2 O 6 all over the thermal treatment.

Optical bandgap by diffuse reflectance spectroscopy
The optical spectra of the MCVs, before and after calcination, are shown in Fig. 8a and b. All MCVs show two absorption peaks; a small one at the UV range and broadband in the visible region. The results are summarized in Table 5.
The bands that appeared in UV and visible regions are attributed to several phases of copper vanadates accompanying MnV 2 O 6 and V 2 O 5 . The optical bandgaps of the MCVs in the visible region were determined using Tauc equation as shown in Fig. 8c and d.
where hυ is the photon energy (E photon ), α is the absorption coefficient, is a proportionality constant, and "n" depends on the type of transition, n = 1/2 for direct transmission, and n = 2 for indirect transmission [46]. The bandgap energy (E g ) was determined by extrapolating the linear part of (αhν) 2 vs. hν plot to intercept the horizontal E photon axis as shown in Fig. 8c and d. The calculated E g values for calcined and uncalcined MCVs are given in Table 5 and plotted versus x Mn in Fig. 8e. In both uncalcined and calcined MCVs, E g declined with the increase of x Mn , whereas the uncalcined ones were more responsive.

Specific surface area by BET
The N 2 adsorption-desorption isotherms, as shown in Fig. 9, are identified as V type, which exhibits low adsorption at low gas pressure, indicating relatively weak affinity between gas and solid [47]. The sharply upward hysteresis loops of the samples close to P/P 0 = 1 suggest that the adsorption occurs in micropores and/or mesopores, where the interactions between the adsorbent and the adsorbate are relatively weak. The surface data of the specimens investigated are listed in Table 3. The surface area increases in the sequence cMCV4 > cMCV6 > cCV > cMV. The minimal surface area of cMV is attributed to the nanosheet structure, which is subject to stacking. One the other hand, cMCV4 and cMCV6 act show maximal surface area, probably due to the multiplicity of their phase composition. The pore size exhibits a broad range within 52-93 nm. These results further confirm the presence of macropores [48].

X-ray photoelectron spectroscopy
The XPS spectra of cCV, cMCV4, cMCV6, and cMV, and their analysis are given in Fig. 10 and Table 6, respectively. The figure shows the presence of Cu, Mn, V, and O, and absence of any other foreign elements referring to the purity of the samples, which is further confirmed via XRD, FTIR, ICP, and EDX techniques. Due to the spin-orbit coupling, there are peaks for 2p 1/2 at lower energy than those of 2p 3/2 , for each of Cu, Mn, V. Each of the three elements exist in the samples in two oxidation states, one higher than the other, which result in two peaks under each of 2p 1/2 and 2p 3/2 . However, the high and low valency peaks tend to overlap and hide under the parent peaks of 2p 1/2 and 2p 3/2 , and in order to show them in high resolution, those peaks are deconvoluted by the XPS peak differentiation-imitation method. The peak at ~ 284.8 eV is characterized by C 1 s (called adventitious carbon) and originates from the adventitious contamination layer [49]. C 1 s peak is used as a reference to obtain the exact peaks values [50,51]. The intensity of the carbon peak increased as the amount of time spent inside the instrument increased [52].  [53,54]. The binding energy difference between 2p 3/2 and 2p 1/2 is 19.95 eV, which is close to the literature slandered value [55,56]. The molar ratio between Cu 1+ and Cu 2+ are nearly given Table 6, based on 2p 1/2 peaks analysis. In the manner of Cu analysis, Mn shows 2 main peaks (2p 3/2 and 2p 1/2 ) and each of them is deconvoluted to two sub-peaks for Mn 2+ and Mn 3+ [57,58]. The table also shows the molar ratio between Mn 2+ and Mn 3+ according to the 2p 1/2 peaks analysis. The peaks appearing around 516.3-516.78 eV and 521.99-524.32 eV are assigned to the V 2p 3/2 and 2p 1/2 . Due to the multivalency of V (V 3+ , V 4+ , and V 5+ ), multiple sub-peaks appear in the deconvolution of the high resolution [59][60][61][62]. The second highest-binding-energy peak is found at 531.53 eV (perhaps a third peak at 533.00 eV). The O 1 s XPS signals are divided into three peaks for cCV, cMCV6, cMV, and four peaks for cMCV4. The prominent peak in all samples has binding energy in the range of 529.09-529.93 eV, which is usually for lattice O of several spinel 3d metal oxides [63]. However, the exact assignment of the higher binding energy peaks is somewhat complex and controversial as numerous factors like surface defects, contaminants, impurities, or chemisorbed oxygen species could result in the appearance of shoulder peaks [64,65]. Figure 11 shows the M-H magnetization of the uncalcined and calcined MCVs under the influence of an external magnetic field at room temperature, and Table 7 summarizes their magnetism parameters. Figure 12 represents the impact of x Mn on the various magnetism parameters, including coercivity field (H c ), magnetization saturation (M s ), remanent magnetization (M r ), and remanence ratio (M r /M s ).

Magnetic properties
According to Fig. 12, all four parameters are more responsive to the Mn/Cu ratio in the calcined MCVs than in their uncalcined counterparts. Furthermore, with the increase of Mn, both H c and M s seem to change linearly in the uncalcined samples, while the former declines and the later rises. M r does not show a remarkable response to the variation in x Mn in the uncalcined MCVs. In the calcined MCVs, all of H c , M r , and M r /M s decrease remarkably with the increase of x Mn , unlike M s , which increases. At x Mn > 0.6 in the calcined MCVs, none of H c , M r , or M r /M s is responsive to the changes in x Mn . As x Mn increased, the MCVs tend to divert from the superparamagnetic-like to paramagnetic (especially the calcined), where the uncalcined H c declined from 396 to 118 G and M r /M s from 0.110 to 0.006 and for the calcined from 1362 to 69 G and from 0.430 to 0.003, respectively.  The magnetic characteristics of the studied materials depend on the magnetic interaction (superexchange interaction) between metal ions with magnetic moments in the crystal lattice. Thus, VSM studies were performed at room temperature within − 20 kOe to + 20 kOe. As shown in Fig. 11 and 12, the uncalcined MCVs show weak hysteresis with a very small magnetic coercivity at a low magnetic field, indicating superparamagnetic-like behavior. The value of magnetic coercivity of the uncalcined MCVs follows the order: CV > MCV4 > MCV6 > MV, as shown in Table 7 and Fig. 12.
It is noted that all the studied MCVs did not reach complete saturation even under a high magnetic field of 20 kOe. Several authors have reported the reduction of magnetization in NPs and proposed mechanisms to explain the no-saturation behavior in a high magnetic field. It can be attributed to the presence of a spin disordered surface layer, which requires a larger magnetic field to reach saturation magnetization [66]. The saturation magnetization (estimated from the linear extrapolation of M vs. 1/H plot) of all samples is listed in Table 7. The chemical composition can significantly influence the magnetization behavior because of changes in the distribution of cations and the particle size. The reduction of the magnitude of magnetization is ascribed to the noncollinear spin arrangement at the particle surface and the difference in the magnetization characteristic of two sub-lattices due to cation redistribution [67]. The disordered or misaligned surface spins weaken the total magnetization of the material NPs, with small retentivity and coercivity values. These values indicate that the thermal variations are enough to dominate the anisotropic energy barrier of the studied samples and reverse the magnetization direction spontaneously. As seen in Table 7 and Fig. 12, the retentivity (M r ) and coercivity (H c ) values show variation concerning sample composition, which can be attributed to the interaction among the oxygen and metal ions in the sublattice of the crystals.

Conclusion
Due to CV polymorphism and phase multiplicity, the Cu, Mn, and V stoichiometry in the MCVs did not follow the metal divanadate formula MV 2 O 6 (M = Cu or Mn) nor did it follow the precursor Mn mole fraction x Mn = Mn Cu+Mn = 0.0, 0.4, 0.6, 1.0. The V Cu+Mn ratio reached as high as 3.0-11.5, while it was expected to  6 . When there was no copper, vanadium oxide was a trace phase accompanying the major phase of MnV 2 O 6 . Phase multiplicity of the MCVs clearly appears in the electron micrographs, where particles morphology in each MCV varies between nanowires, bundles of nanowires, nanocubes, nanosheets, and irregular structures. The uncalcined MCVs tend to divert from the superparamagnetic-like to be paramagnetic with the increase of Mn content, since both H c and M s seem to linearly decline and rise, respectively. This behavior is more evident in the calcined MCVs, where all of H c , M r , and M r /M s decrease remarkably with the increase of x Mn , unlike M s , which increases.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
Prime novelty statement All data and work included in this paper are original and have not been published or

Competing interests
The authors declare no competing interests.
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 licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence 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 licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.