Influence of Fe doping on the structural, optical and thermal properties of α-MnO₂ nanowires

The phase purity and crystallinity of the obtained nanowires was investigated by x-ray Diffraction over a 2θ range of 10°–80° at a scan rate of 0.02° per second. Figure 1 displays the diffraction pattern of pure and different concentrations (5, 10 and 15 mol %) of iron doped α-MnO₂ nanowires. The diffraction peaks at 2θ = 12.69°, 18.04°, 25.67° 28.76°, 36.49°, 37.52°, 38.78°, 41.94°, 47.23° 49.77°, 56.01°, 60.18°, 65.31°, 69.56°, 72.92° were assigned to the reflection of (110), (200), (220), (310), (400), (211), (330), (301), (510), (411), (600), (521), (002), (541) and (312) plane of tetragonal phase α-MnO₂ (JCPDS No: 44-0141, space group 14/m 87). The sharp diffraction peaks demonstrate good crystallinity of the as-prepared nanowires. No peaks related to iron oxides or other phases of manganese oxides were detected, indicating the high purity of the obtained α-MnO₂ nanowires. The standard lattice constant values for this system is a = b = 9.785 Å and c = 2.863 Å. Our calculated (table 1) lattice constants for pure α-MnO₂ are in good agreement with these values.

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However, for doped samples, an increase in the lattice constants was observed due to incorporation of Fe³⁺ in α-MnO₂ lattice. The radius of Fe³⁺ (0.55 Å) is comparable to that of Mn⁴⁺ (0.53 Å), therefore, iron can either substitute Mn⁴⁺ or occupy the large tunnel (4.6 Å) of α-MnO₂, producing lattice defects and lowering the degree of crystallinity [28]. Crystallinity index (CI) is calculated using the method reported by Pardo et al [29].

$$\begin{eqnarray}&&CI=\displaystyle \frac{{A}_{c}}{{A}_{c}+{A}_{a}}\end{eqnarray} \tag{ 1 }$$

Where A_c is the area under crystalline peaks and A_a is the area of amorphous hollows. The crystallinity index of Fe-doped α-MnO₂ nanowires decreased from 75.5 to 61.1% by increasing the amount of iron added to α-MnO₂ (table 1). The sample S4 (doped with 15 mol % of iron) has higher concentration of iron, therefore, exhibiting comparatively lower degree of crystallinity [30, 31]. The average crystallites sizes of the obtained nanowires were calculated using the Scherrer equation [32].

$$\begin{eqnarray}&&{\rm{D}}=\displaystyle \frac{K\lambda }{\beta \,\cos \,\theta }\end{eqnarray} \tag{ 2 }$$

Here in this equation, 'D' is the crystallites size, 'K' is the Scherer constant, 'λ' is the wavelength of the x-ray sources, 'β' is the FWHM and 'θ' is the peak position. The average crystallites size calculated for S1, S2, S3 and S4 was found to be 17.36 nm, 14.59 nm, 14.93 nm and 13.32 nm respectively (table 1). Strain and dislocation density of the material are other important factors that can be calculated from XRD data [33, 34]. The values of strain (ε) and dislocation density (δ) for all the samples were calculated from (2 1 1) diffraction using the following relations and are tabulated in table 1 [35].

$$\begin{eqnarray}&&\varepsilon =\displaystyle \frac{\beta \,\times \,\cos \,\theta }{4}\end{eqnarray} \tag{ 3 }$$

$$\begin{eqnarray}&&\delta =\displaystyle \frac{15\,\times \,\varepsilon }{a\,\times D}\end{eqnarray} \tag{ 4 }$$

Where 'ε' is the strain, 'δ' is the dislocation density, 'β' is the FWHM, 'θ' is the peak position, 'D' is the crystallites size and 'a' is lattice constant. The value of strain and dislocation density was observed to increase by increasing the dopant concentration (table 1). Same phenomena was observed by Muthukumaran et al [36]. According to their report, decrease in crystallite size causes an increase in lattice strain and vice versa.

The chemical composition of α-MnO₂ and Fe-doped α-MnO₂ nanowires were carried out by EDX analysis. Figures 2(a), (b) show the EDX spectrum of S1 and S2, in both samples, the higher concentration of manganese (Mn) and oxygen (O) indicates that the main components of hydrothermally synthesized α-MnO₂ nanowires are Mn and O.

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The potassium (K⁺) ions comes from the precursor used for the synthesis of α-MnO₂ nanowires. Besides this, no other element as an impurity is detected. In figure 2(b), the presence of Fe confirms successful doping of iron in as-prepared Fe-doped α-MnO₂ nanowires.

Hollandite type α-MnO₂ is well famous due to its large (2 × 2) tunnel structure, therefore, it is extensively investigated among all the other phases [37]. The size of the tunnel is favourable for the accommodation of K⁺ ion. Therefore, during synthesis K⁺ ions occupy the tunnels of α-MnO₂ (figure 3). Feng et al studied the alkali metal ions (Li⁺, Na⁺, K⁺) insertion/extraction mechanism in α-MnO₂ and reported that, K⁺ can be extracted from α-MnO₂ by treating it with highly concentrated nitric acid solution [38]. In contrast, Luo et al reported that acidic treatment can reduce the amount of K⁺ ions in α-MnO₂ but it cannot be completely removed, because it helps in the formation and stabilization of tunnels [39]. Recently, rietveld refinement study of α-MnO₂ was carried out by John et al [40]. According to their report, the tunnel structure of α-MnO₂ is completely disfigured and collapses without the presence of K⁺ ions.

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The morphology of pure and Fe-doped α-MnO₂ were examine by SEM operating at 20.0 KV as shown in figures 4(a)–(c). The SEM images confirm the preparation of nanowires with average diameter of 38 nm, 28 nm and 64 nm for S1, S2 and S4 respectively. The diameters of nanowires were determined by particle size distribution histogram calculated through SEM images using ImageJ software as shown in figures 4(d)–(f). Among all the samples, S2 (figure 4(b)) has the smallest diameter and ultra-smooth morphology. Similarly, S4 (figure 4(c)) which is prepared with highest doping concentration has largest diameter along with agglomerated nanoparticles.

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Fourier Transform Infrared Spectroscopy (FTIR) was used for the analysis of functional group present in a sample. For our synthesized nanowire, FTIR spectroscopy was carried out over a spectral range of 500 cm⁻¹ to 4000 cm⁻¹. Figure 5 shows the typical FTIR spectrum of S1, S2 S3 and S4. The absorption band at 3228 cm⁻¹, 1636 cm⁻¹ and 1113 cm⁻¹ belongs to hydroxyl group and could be assigned to the stretching, bending and vibration of O–H bond [41, 42]. The absorption band at 1415 cm⁻¹ could be assigned to the bending vibration of Mn–O–H bond [43]. The spectrum obtained for S4 displays dominant absorption bands at various location corresponding to adsorbed water, indicating that, during the synthesis, Fe-doped α-MnO₂ accrue large amount of water [41].

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The presence of adsorbed water was further confirmed by thermal gravimetric analysis study (figure 6) and was found that significant amount of water is accumulated in the tunnel of Fe-doped α-MnO₂ [44]. The strong absorption bands at 501 cm⁻¹ and 699 cm⁻¹ correspond to the bending and stretching vibration of Mn–O bond [45].

Thermal stability of the pure and Fe-doped α-MnO₂ nanowires was studied in the temperature range of 50 °C to 1000 °C through Thermal Gravimetric Analysis (TGA). Temperature of the TG analyser was allowed to increase at the rate of 10 °C per minute in air. Figures 6(a), (b) shows the TGA and differential TG curve of the pure (S1) and highest concentration Fe-Doped α-MnO₂ (S4) nanowires. In figure 6(a), the observed weight loss from 50 °C to 388 °C for both samples and it's corresponding broad endothermic peaks in figure 6(b) is due to evaporation and dehydration of the surface and trapped water inside the (2 × 2) tunnels [46]. The weight loss due to release of water was estimated to be 0.46% for S1 and 2.31% for S4. The sample S4 shows comparatively large amount of weight loss due to dehydration, this is analogy to the dominant hydroxyl peaks of S4 in FTIR results. The next 4.08% weight loss of S1 and 6.12% weight loss of S4 in TGA curve is due to the phase transformation from α-MnO₂ to Mn₂O₃ by release of oxygen. This phase transformation can be observed in differential TG curve at 588 °C for S1 and at 598 °C for S4 [23]. The shift towards higher temperature region of S4 indicates that the doping of ferric ion enhances thermal stability of α-MnO₂ nanowires in contrast to the previous work reported by Kingondu et al [47]. Reason behind thermal stability is that, the K⁺ work as stabilizing agent for the tunnel of α-MnO₂ [48]. On the other hand, during the synthesis of α-MnO₂ the introduction of transition metals increases the amount of K⁺ ions in the tunnels, which results in improved tunnel stability of α-MnO₂ [49, 50]. Our EDX results also confirmed that, the amount of K⁺ increased in Fe-doped α-MnO₂ compared to pure α-MnO₂ (figure 2(b)).

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Third and last observed weight lost in TGA curve is 1.46% for S1 and 0.69% for S4. The weight loss in this region of the curve is due to phase transformation from Mn₂O₃ to Mn₃O₄ by release of oxygen [51]. The endothermic peaks in differential TG curve appear at 780 °C for S1 and at 793 °C for S4 belongs to this phase transformation. Similar to previous phase transformation, at this stage Fe-doped α-MnO₂ again exhibit comparatively higher thermal stability due to combine effect of ferric and potassium ions. Thus, from the TGA studies we confirmed that, doping of iron in α-MnO₂ enhances its tunnel stability.

The optical absorption response of the as-prepared α-MnO₂ nanowires was investigated in Ultraviolet-Visible (UV–vis) region of the electromagnetic spectrum. Figure 7(a) displays the absorption spectra obtained from pure and Fe-doped α-MnO₂ nanowires.

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A broad absorption band ranging from 250 nm to 600 nm with peak position at ∼450 nm was observed for S1 and broad absorption band ranging from 250 nm to 700 nm having peak position at ∼470, ∼464 and ∼490 nm was observed for S2, S3 and S4 respectively. The absorption in the visible region of the spectrum by α-MnO₂ nanowires is due to the d-d transition between lower t_2g and higher e_g of the Mn ion [52]. The optical bandgap energy of α-MnO₂ nanowires corresponds to the energy difference between these two states and can be estimated by using the Tauc relation [53].

$$\begin{eqnarray}&&{\left(\alpha {\rm{h}}\upsilon \right)}^{1/2}={\rm{K}}\left({\rm{h}}\upsilon -{{\rm{E}}}_{{\rm{g}}}\right)\end{eqnarray} \tag{ 5 }$$

Where K is an energy independent constant, hυ is the incident photon energy, α is the absorption coefficient and E_g is the optical bandgap energy of α-MnO₂ nanowires. The exponent on the left-hand side of equation determines the nature of transition, it is 2 for allowed direct transition and ½ for allowed indirect transition [54]. Figure 7(b) shows a plotted graph of (αhυ)ⁿ verses hυ. The best fit for the curve at n = ½ was observed, which corresponds to an indirect optical bandgap energy of 0.90 eV, 0.30 eV, 0.29 eV and 0.18 eV for S1, S2, S3 and S4 respectively. MnO₂ is an unusual semiconductor at normal conditions, sometimes it displays a bandgap as wide as 2.53 eV and sometimes as narrow as 0.2 eV [55, 56]. Some previously reported bandgap values are tabulated in table 2 in comparison with the values obtained in present work.

Our estimated bandgap values (0.90 eV) for pure α-MnO₂ nanowires is in close approximation to the value (0.909 eV ) reported by Liu et al [61]. The reduction in energy level for Fe-doped α-MnO₂ (S2, S3 and S4) can be attributed to the impurity level produced by Fe ion. Li et al [28] reported that, Al used as a dopant in MnO₂ introduces impurity level resulting in the shortening of bandgap values. Similar reduction in the bandgap value was recently observed by AlFaify et al for Hg doped PbI₂ nanostructure [64]. To the best of our knowledge, so far we have not found any report on the indirect bandgap energy of Fe-doped α-MnO₂ nanowires. Herein, we report for the first time an indirect optical bandgap energy of 0.30 eV, 0.29 eV and 0.18 eV for 5 mol % Fe-doped α-MnO₂, 10 mol % Fe-doped α-MnO₂ and 15 mol % Fe-doped α-MnO₂ nanowires respectively. Due to narrow and tuneable bandgap energy, Fe-doped α-MnO₂ can be used in infrared detectors and could replace toxic lead and mercury detectors [65].