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.
Transition metal divanadates (MV2O6 or M(VO3)2, M is a transition metal) have received great interest due to their application in various fields. Specifically, CuV2O6 nanoparticles have been applied in supercapacitors [1], batteries [2], photocatalytic water remediation [3], photoelectrochemical sensing [4], and photoelectrochemical catalysis [5]. Specifically, copper vanadate (CV) exhibits polymorphism and phase multiplicity, and takes the formulas CuV2O6, α-Cu2V2O7 (Blossite), β-Cu2V2O7 (Ziesite), Cu3V2O8 (Pseudolynsite), γ-Cu3V2O8 (McBirneyite), Cu5V2O10, Cu11V6O26 (Fingerite), and Cu3(OH)2V2O7.(H2O)2 (Volborthite) [6]. Interestingly, the different CV phases can be viewed as mixtures of CuO with V2O5 in different ratios and mole fractions, as shown in Fig. 1 [7]. CVs can be synthesized with various approaches including co-precipitation [8, 9], hydrothermal [10, 11], thermal decomposition [6, 12], and solution combustion [7, 13].
Fig. 1
The different copper vanadate phases (CuxVyOz) viewed as mixtures of CuO with V2O5 in different ratios (CuO:V2O5) and CuO mole fractions (nCuO) [7]
×
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 [13‐17]. 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 V2O5 alongside with α-Cu2V2O7 as a result of solution-combustion synthesis [3, 7]. Mixed-phase CVs have been synthesized, studied, and applied in various applications, such as photocatalytic water treatment [3], photoelectrocatalytic water treatment [13, 16], photoelectrocatalytic O2 generation [18], photoelectrocatalytic water splitting [15]. Despite the controversy that may arise from the polymorphism and phase multiplicity of CVs, they have been pivotal in the aforementioned photoactivity-based studies, due to their visible-light absorptivity (Eg 1.80–2.74 eV) [19], and magnetism investigations [20‐26].
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The present work aims to study the magnetization and optical bandgap of mixed-phase nanostructures of CVs with monoclinic Mn vanadate. As well as CVs, MnV2O6 exhibits visible-light photoactivity [27, 28] and magnetic properties [29‐33]. The proposed Mn-Cu vanadate mixed-phase (MCV) was synthesized hydrothermally followed by calcination, with Mn mole fractions \(\left({x}_{\mathrm{Mn}}=\frac{\mathrm{Mn}}{\mathrm{Cu}+\mathrm{Mn}}\right)\) of 0.0, 0.4, 0.6, 1.0. The investigated characteristics of the MCVs included their crystal structure, morphology, thermal stability, surface area, optical bandgap, and magnetization.
Experimental
Materials
Ammonium monovanadate extra pure (NH4VO3) (Oxford, India), manganese chloride tetrahydrate extra pure (MnCl2.4H2O) (Oxford, India), copper chloride dihydrate (CuCl2·2H2O) (Adwic Pharmaceutical and Chemicals Company, Egypt), and ethanol (C2H5OH) 99% (Adwic Pharmaceutical and Chemicals Company, Egypt). All chemicals and solvents were of analytical grade and used without further purification.
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.
Table 1
Amounts of Cu and Mn precursors added during the synthesis of the Cu-Mn vanadate-oxide mixed phase nanostructures. xMn is Mn mole fraction with respect to Cu + Mn
Sample
xMn
CuCl2.2H2O
MnCl2.4H2O
pH of mixed reactants
mMol
Wt (g)
mMol
Wt (g)
CV
0.0
4.80
0.818
0.00
0.00
4.5
MCV4
0.4
2.88
0.491
1.92
0.38
5.2
MCV6
0.6
1.92
0.327
2.88
0.57
5.6
MV
1.0
0.00
0.00
4.80
0.95
6.1
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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 Table 2 of the virgin CV show peaks characteristic for three different phases. Frist, monoclinic volborthite Cu3(OH)2V2O7·2H2O according to JCPDS Card No. 46–1443. Volborthite is a natural divanadate of formula Cu3(OH)2V2O7·2H2O that exhibits interesting crystalline structure with porous framework [35]. As a metastable phase, Volborthite can be used as a precursor of another interesting mixed and layered crystalline phase composed of an octahedral copper-oxygen layer in coordination with tetrahedral vanadium layer. Second, monoclinic copper vanadium oxide Cu0.261V2O5 according to JCPDS Card No. 01–079-0796. Third, a minor trace of monoclinic V3O7 according to JCPDS Card No. 20–1378. Two peaks observed at 2θ: 27.00 and 25.72 cannot be distinguished. In the MCV4, monoclinic MnV2O6 (JCPDS Card No. 35–0139) was formed beside all previous phases; Cu3(OH)2V2O7·2H2O, Cu0.261V2O5, and V3O7. The highest noticed ratio is Cu0.261V2O5 as shown from the highest sharp peak at 2θ: 9.08°. In the MCV6, the peaks characteristic for Cu3(OH)2V2O7·2H2O disappeared, whereas the intensities of the peaks characterized for Cu0.261V2O5 and MnV2O6 increased, besides a weak peak intensity for V3O7 is noticed. Ammonium vanadium oxide hydroxide hydrate, (NH4)8(V19O41(OH)9)(H2O)11, exists as a trace amount with a characteristic peak at 2θ: 5.4° according to JCPDS Card No. 01–078-2016. In MV, the dominant phase is MnV2O6 beside a foreign peak at 2θ: 25.56°, which may be due to the presence of traces of orthorhombic V6O13 according to JCPDS Card No. 778–0983.
Fig. 2
a XRD diffractograms of the Cu-Mn vanadate-oxide mixed phase nanostructures before calcination: (asterisk) Cu3(OH)2V2O7·2H2O, (black circle) Cu0.261V2O5, (black diamond) V3O7, (white circle) MnV2O6, (plus sign) (NH4)8 (V19O41 (OH)9) (H2O)11, (multiplication sign) V6O13. b XRD pattern for Cu-Mn vanadate-oxide mixed phase nanostructures after calcination: (asterisk) CuV2O6, (black circle) Cu3.64(V4O11), (white circle) MnV2O6, (down-pointing triangle) V2O5
Table 2
Based on the XRD results, phases constituting the uncalcined and calcined Cu-Mn vanadate-oxide mixed phase nanostructures, in addition to their average particle sizes estimated with the Scherrer equation. P: present, M: major and T: traces. JCPDS Card numbers: (a) 46–1443, (b) 01–079-0796, (c) 20–1378, (d) 35–0139, (e) 778–983, (f) 01–078-2016, (g) 1- 085–0226, (h) 9–0387 and (i) 45–1054
Phase
CV
MCV4
MCV6
MV
Cu3(OH)2V2O7·2H2O (a)
P
P
–
–
Cu0.261V2O5(b)
P
P
P
–
V3O7(c)
T
P
P
–
MnV2O6(d)
–
P
P
M
V6O13e
–
–
–
T
(NH4)8(V19O41(OH)9)(H2O)11(f)
–
–
T
–
Average DSch (nm)
25
15
19
20
Phase
cCV
cMCV4
cMCV6
cMV
Cu3.64(V4O11) (g)
P
P
P
–
V2O5(h)
P
P
P
T
CuV2O6(i)
P
P
–
–
MnV2O6(d)
–
P
P
M
Average DSch (nm)
13
20
19
27
×
The calcined samples in the air at 400 °C for 2 h, cCV, cMCV4, cMCV6, and cMV, show significant XRD changes than those of the virgin samples in addition to formation of new phases as shown in Fig. 2b and listed in Table 2. The XRD peaks of cCV are indexed to the Cu3.64(V4O11) phase according to JCPDS Card No. 1–085-0226, CuV2O6 phase according to JCPDS Card No. 45–1054 and Shcherbinaite V2O5 phase according to JCPDS Card No. 9–0387. The XRD peaks of cMCV4 refer to the presence of the MnV2O6 phase in addition to all the previously mentioned phases: Cu3.64(V4O11), CuV2O6, and V2O5. The XRD of cMCV6 demonstrates the presence of Cu3.64(V4O11), V2O5, and MnV2O6 phases, while the CuV2O6 phase disappeared. For cMV, all the XRD peaks are indexed to MnV2O6 and V2O5.
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 ν1-VO3, and that at 751 cm−1 is corresponding to the anti-symmetric stretching vibration of ν3-VO3 of CV. The two bands at 530 cm−1 and 509 cm−1 are assigned to the ν3 symmetric and ν1 symmetric stretching modes of (V–O–V) of CV [36]. Those at 993–1010 cm−1 are corresponding to ν(V4+ = 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 CO2 due to the absorption of CO2 from the air [40]. The bands at 1613–1620 cm−1 correspond to the ν-OH stretching vibration and δH2O 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].
Fig. 3
FTIR spectra of Cu-Mn vanadate-oxide mixed phase nanostructures before a and after b calcination
×
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 H2O, CO2, 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 V2O5 as a common product in all calcined samples. This band is very small for cMV due to the dimmish of V2O5 [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 CO2 symmetric stretching disappeared completely after calcination. The intensities of vibration bands at 1613–1620 cm−1 of the ν-OH stretching vibration and δH2O 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 Cu2+, Mn2+, and VO3− 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.
Fig. 4
EDX qualitative and quantitative detection of Cu, Mn, V, and O in the calcined Cu-Mn vanadate-oxide mixed phase nanostructures
Table 3
The Cu, Mn, and V elemental ratios according to the ICP and XPS techniques, in addition to the specific surface area, average pore volume, and pore diameter according to the BET technique, all for the calcined Cu-Mn vanadate-oxide mixed phase nanostructures. (*) The ideal stoichiometric ratio of V/(Cu + Mn) based on the expected formula MV2O6 should be 2, as shown in the brackets. (#) According to the added ratios during the synthesis, x was expected to change as 0.00, 0.40, 0.60, and 1.00 for samples cCV, cMCV4, cMCV6, and cMV, respectively, as shown in the brackets
cCV
cMCV4
cMCV6
cMV
ICP
Cu
0.27
0.13
0.06
0.00
Mn
0.00
0.08
0.19
0.34
V
0.73
0.79
0.75
0.66
V/(Cu + Mn) *
2.70 (2.00)
3.76 (2.00)
3.00 (2.00)
1.94 (2.00)
x = Mn/(Mn + Cu) #
0.00 (0.00)
0.38 (0.40)
0.76 (0.60)
1.00 (1.00)
XPS
Cu
0.25
0.06
0.04
0.00
Mn
0.00
0.04
0.04
0.16
V
0.75
0.90
0.92
0.84
V/(Cu + Mn) *
3.00 (2.00)
9.00 (2.00)
11.50 (2.00)
5.25 (2.00)
x = Mn/(Mn + Cu) #
0.00 (0.00)
0.40 (0.40)
0.50 (0.60)
1.00 (1.00)
BET
SBET (m2/g)
4.7
8.6
5.4
1
Pore volume (cm3/g) × 10−2
6.13
11.60
7.96
2.30
Main Pore diameter (nm)
52.3
53.9
59.4
92.7
×
Morphology of the uncalcined and calcined copper/manganese vanadates by scanning and transmission electron microscopy
Figure 5 is the TEM micrographs of the synthesized MCVs before calcination. CV shows nanowire bundles with several micrometers in length and cubic nanostructures of an average size of 100–300 nm. MCV4 shows nanowires with a slight increase in diameter (around 50 nm), cubic structures, and nanosheets (around 500 nm size). Since MV TEM images show all nanosheets only, these nanosheets are assumed to be of the MnV2O6 phase. MCV6 shows an increase in the abundance of MnV2O6 nanosheets and the thickness of nanowires, while absence of the cubic structures. MV shows MnV2O6 nanosheets of different sizes.
Fig. 5
TEM images of the uncalcined Cu-Mn vanadate-oxide mixed phase nanostructures
×
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 V2O5. 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.
Fig. 6
SEM images of the calcined Cu-Mn vanadate-oxide mixed phase nanostructures. The scale bar is equivalent to 1 µm
×
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 CuV2O6 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 V2O5 [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 MnV2O6 all over the thermal treatment.
Fig. 7
Plots of TG (a), TDA (b), and DSC (c) of the uncalcined Cu-Mn vanadate-oxide mixed phase nanostructures
Table 4
Thermogravimetric data of uncalcined Cu-Mn vanadate-oxide mixed phase nanostructures
Temperature (°C)
Weight loss (%)
Significance
CV
27–175
175–260
260–400
635–800
5.5
4.8
3.9
1.3
Removing adsorbed water, besides the elimination of crystallized water molecules
MCV4
27–170
170–390
390–600
600–698
698–760
3.4
5.1
0.7
0.9
1.7
Removing adsorbed water, besides the elimination of crystallized water molecules
MCV6
27–175
175–410
2.3
2.7
Removing adsorbed molecules
MV
27–550
2.5
Removing adsorbed water
×
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 MnV2O6 and V2O5. 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 (Ephoton), 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 (Eg) was determined by extrapolating the linear part of (αhν)2 vs. hν plot to intercept the horizontal Ephoton axis as shown in Fig. 8c and d. The calculated Eg values for calcined and uncalcined MCVs are given in Table 5 and plotted versus xMn in Fig. 8e. In both uncalcined and calcined MCVs, Eg declined with the increase of xMn, whereas the uncalcined ones were more responsive.
Fig. 8
UV–vis absorption spectra of the Cu-Mn vanadate-oxide mixed phase nanostructures, before (a) and after (b) calcination. The Tauc plots [(αhν)2 vs. hυ] before (c) and after (d) calcination. Dependence of bandgap of the uncalcined MCVs (blue curve in inset e) and calcined MCVs (red curve in inset e) on xMn
Table 5
Absorption peaks wavelengths in the UV and visible spectra as well as Eg values of the uncalcined and calcined Cu-Mn vanadate-oxide mixed phase nanostructures
CV
MCV4
MCV6
MV
UV peaks (nm)
215
240
234
238
Visible peaks (nm)
380
382
380
516
Band gap (Eg) (eV)
2.46
1.98
1.94
1.60
cCV
cMCV4
cMCV6
cMV
UV peaks (nm)
232
240
235
237
Visible peaks (nm)
380
348
380
545
Band gap (Eg) (eV)
2.17
2.13
2.10
1.63
×
Specific surface area by BET
The N2 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/P0 = 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].
Fig. 9
N2 adsorption/desorption BET isotherms of the calcined Cu-Mn vanadate-oxide mixed phase nanostructures
×
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 2p1/2 at lower energy than those of 2p3/2, 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 2p1/2 and 2p3/2. However, the high and low valency peaks tend to overlap and hide under the parent peaks of 2p1/2 and 2p3/2, 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].
Fig. 10
XPS survey scans for the calcined Cu-Mn vanadate-oxide mixed phase nanostructures (top panel) and their corresponding Cu, Mn, V, and O high-resolution peaks (the bottom 10 panels)
Table 6
XPS-based core-electrons binding energies (eV) of Cu, Mn, V, and O constituents in the calcined Cu-Mn vanadate-oxide mixed phase nanostructures
XPS spectra of Cu2+ have characteristic satellite peaks, which are prominent at 940.7, 943.3, 961.7 eV; these satellite peaks are characteristic of materials having ground-state d9 configuration, i.e., Cu2+ [53, 54]. The binding energy difference between 2p3/2 and 2p1/2 is 19.95 eV, which is close to the literature slandered value [55, 56]. The molar ratio between Cu1+ and Cu2+ are nearly given Table 6, based on 2p1/2 peaks analysis. In the manner of Cu analysis, Mn shows 2 main peaks (2p3/2 and 2p1/2) and each of them is deconvoluted to two sub-peaks for Mn2+ and Mn3+ [57, 58]. The table also shows the molar ratio between Mn2+ and Mn3+ according to the 2p1/2 peaks analysis. The peaks appearing around 516.3–516.78 eV and 521.99–524.32 eV are assigned to the V 2p3/2 and 2p1/2. Due to the multivalency of V (V3+, V4+, and V5+), multiple sub-peaks appear in the deconvolution of the high resolution [59‐62]. 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].
Magnetic properties
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 xMn on the various magnetism parameters, including coercivity field (Hc), magnetization saturation (Ms), remanent magnetization (Mr), and remanence ratio (Mr/Ms).
Fig. 11
M–H magnetization plots of the uncalcined Cu-Mn vanadate-oxide mixed phase nanostructures (insets a–h) and the calcined ones (insets i–p). Plots b, d, f, h, j, l, n, and p are zoomed-in M-H plots for clarification of the hysteresis behavior
Table 7
Magnetism parameters of the uncalcined and calcined Cu-Mn vanadate-oxide mixed phase nanostructures
CV
MCV4
MCV6
MV
Hc (G)
396
272
249
118
Ms (emu/g)
0.07
0.42
0.77
1.24
Mr (emu/g × 10−3)
8.0
6.9
13.0
7.6
Mr/Ms (× 10−3)
110
17
15
6
cCV
cMCV4
cMCV6
cMV
Hc (G)
1362
1277
140
69
Ms (emu/g)
0.51
0.65
0.80
1.23
Mr (emu/g × 10−3)
220
110
7
4
Mr/Ms (×10−3)
430
170
9
3
Fig. 12
Plots of various magnetism parameters Hci(a), Ms(b), Mr(c), and Mr/Ms(d) versus Mn molar ratio in the synthesis precursors added during the synthesis of the Cu-Mn vanadate-oxide mixed phase nanostructures. The blue and red curves represent the uncalcined and calcined samples, respectively
×
×
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 Hc and Ms seem to change linearly in the uncalcined samples, while the former declines and the later rises. Mr does not show a remarkable response to the variation in xMn in the uncalcined MCVs. In the calcined MCVs, all of Hc, Mr, and Mr/Ms decrease remarkably with the increase of xMn, unlike Ms, which increases. At xMn > 0.6 in the calcined MCVs, none of Hc, Mr, or Mr/Ms is responsive to the changes in xMn. As xMn increased, the MCVs tend to divert from the superparamagnetic-like to paramagnetic (especially the calcined), where the uncalcined Hc declined from 396 to 118 G and Mr/Ms 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 (Mr) and coercivity (Hc) 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 MV2O6 (M = Cu or Mn) nor did it follow the precursor Mn mole fraction \({x}_{\mathrm{Mn}}=\frac{\mathrm{Mn}}{\mathrm{Cu}+\mathrm{Mn}}=\) 0.0, 0.4, 0.6, 1.0. The \(\frac{\mathrm{V}}{\mathrm{Cu}+\mathrm{Mn}}\) ratio reached as high as 3.0–11.5, while it was expected to be only 2 if it followed the divanadate formula MV2O6.
Regardless of the precursor Cu and Mn mole fractions and regardless of the post-synthesis thermal treatment, each MCV sample consisted of 2–4 different vanadate-oxide phases of (NH4)8(V19O41(OH)9)(H2O)11, V2O5, V3O7, V6O13, Cu3(OH)2V2O7·2H2O, Cu0.261V2O5, Cu3.64(V4O11), CuV2O6, or MnV2O6. When there was no copper, vanadium oxide was a trace phase accompanying the major phase of MnV2O6. 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.
As Mn content increased, the uncalcined MCVs showed more thermal stability and their optical bandgap remarkably declined. The calcined MCVs showed less bandgap response to the Mn-content variation than the uncalcined ones, although they all lay within 1.60–2.46 eV (all visible).
The uncalcined MCVs tend to divert from the superparamagnetic-like to be paramagnetic with the increase of Mn content, since both Hc and Ms seem to linearly decline and rise, respectively. This behavior is more evident in the calcined MCVs, where all of Hc, Mr, and Mr/Ms decrease remarkably with the increase of xMn, unlike Ms, which increases.
Acknowledgements
Not applicable.
Declarations
Competing interests
The authors declare no competing interests.
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