Figure 1 shows the two nanostructures as observed by electron microscopy. The average diameter of HRMA's is 5 ± 0.88 µm, as shown in Figures a and b. In contrast, the nanorods exhibit a diameter of 100 ± 20 nm and a length of 1.3 ± 0.17 µm, as illustrated in Figures d and e. TEM images also prove that the petals and bars have minuscule crystals forming the primary bars. Each individual nanorod has a diameter of around 9 nm, as shown in Figures c or f. It is evident that the petals are composed of several uniform radial nanorods. Both structures exhibit a pristine and uniform surface devoid of any nanoparticles or agglomerates.
The X-ray diffraction (XRD) patterns of the β − MnO2 powders are displayed in Fig. 2a-b. The peak intensities indexation provided evidence supporting the classification of rutile-phase β-MnO2 ((JCPDS card-numbers: 24–0735) as belonging to the tetragonal phase and space group P42n, P42/mnm. The profile's R-factor values were less than 10%. The lattice constants examination revealed an increase in the cell unit's volume (V) for the dandelion structure due to the corresponding increases in the cell dimensions a, b, and c.
The octahedral array investigation showed that the square base of the dandelion's bipyramid experienced a contraction. This contraction was mainly due to a decrease in the bond length between manganese (Mn) and oxygen (O) atoms at position 1 (Mn–O(1)), whereas the bond length between Mn and O atoms at position 2 (Mn–O(2)) remained relatively unchanged. Figure SI-1 illustrates the correlation between the experimental (Ycalc) and theoretical (Yobs) peak profiles.
The WPPF analysis application yielded valuable insights into the dandelion sample's crystallite size (Dv) and strain (τ). Specifically, the analysis indicated a decrease in Dv along the (110) direction, while an increase was observed along the (001) direction. These changes in Dv were shown to be associated with a strain increment (τ).
Figure 3 displays the nitrogen isotherms recorded at a temperature of 77 K. As per the IUPAC classification [20], the isotherm can be classified as type IV based on the shape of the hysteresis loop within a relative pressure (P/P0) range of 0.3-1.0. By analyzing the parallel sorption boundary curves shown in the figure, we can observe significant differences in the pore structure, which can be attributed to the extensive connectivity of pores in dandelions [21]. The inset of Fig. 3 compares the narrow pore-size distribution. The variation in average pore diameter (Dpore) has resulted in a modification of the total pore volume (Vp), leading to a substantial increase of up to five times in the specific surface area (SSA) of nanocrystalline β − MnO2.
X-ray photoelectron spectroscopy (XPS) was used to investigate the nanostructure's chemical composition and the oxidation states of the metal. Three elements were observed in the full-scan spectrum of powders, as shown in Figure SI-2. The Mn 2p spectrum is shown in Fig. 4, where Lorentzian peak-deconvolution analysis was performed. The study focused on two aspects: (1) the splitting of the Mn 2p spin-orbit doublet into Mn 2p3/2 (642 eV) and Mn 2p1/2 (653 eV) [22], and (2) the examination of the O 1s area, as depicted in Fig. 4a and c, spanning from 526 eV to 538 eV [23]. The analysis of the Mn 2p3/2 core-level spectra revealed the two distinct initial states: Mn3+ [3d3] and Mn4+ [3d4], localized at energy levels of 641 eV (Mn3+) and 643 eV (Mn4+), respectively [24]. The energy difference between the two peaks was 11.9 electron volts (eV), which aligns with previous findings [25–27].
The O 1s spectrum, shown in Figs. 4b and 4d, exhibits three distinct peaks with intensities at 529 eV, 531 eV, and 533 eV. The peak at 529 eV corresponds to lattice oxygen (Olatt), while the peak at 531 eV is associated with oxide defects related to adsorbed-oxygen species (Oads), such as O2−, O−, and OH·. Lastly, the peak at 533 eV is attributed to adsorbed water (Ow) on the surface [28]. The variable φ is defined as the quotient of Mn4+/Mn3+, while the variable ϕ represents the ratio Oads/Olatt. The dandelion shape exhibited a notable disparity in the various shapes, with an φ value increase of up to 1.5 times and an ϕ value increase of up to 45% compared to rods. Table 1 provides a comprehensive summary of the physicochemical properties of the powders.
Table 1
Physicochemical and electrochemical parameters for β-MnO2 shapes
| Dandelions | Nanorods | Commercial MnO2 |
Physicochemical: | | | |
dW (nm) | 9 | 100(20) | -- |
dL (nm) | 5000(880) | 1300(170) | -- |
f (dimensionless) | 555 | 13 | ≈ 1.0 |
SSA (m2/g) | 28.50 | 31.96 | 5.30 |
Dpore (nm) | 28.5 | 21.3 | 20.5 |
Vp (cm3/g) | 0.12 | 0.10 | 0.16 |
Dv (110) | 74(7) | 84(10) | 273(22) |
e (10− 3) | 7.1(1) | 6.2(1) | 1.3(1) |
a (nm) | 0.4408(2) | 0.4402(3) | 0.4396(1) |
c (nm) | 0.2879(1) | 0.2872(1) | 0.2871(0) |
V (nm3) | 0.0559 | 0.0556 | 0.0554 |
Mn–O(1) (Å) | 1.9 (4) | 1.9 (4) | 1.9 (3) |
Mn–O(2) (Å) | 1.9 (4) | 1.9 (5) | 1.8 (4) |
φ | 1.52 | 0.60 | -- |
ϕ | 1.51 | 1.03 | -- |
Electrochemical: |
N | 4 | 4 | 3 |
Jlim (mA·cm-2) | -7.0231 | -5.2465 | -2.2951 |
E0 (V vs. RHE) | 0.8700 | 0.7600 | 0.7400 |
E1/2 (V vs. RHE) | 0.7400 | 0.6300 | 0.6300 |
We aimed to evaluate ORR's catalytic activities in an electrolyte containing oxygen-saturated 0.1 M NaOH. We employed the rotating disk and ring-disk electrode method for this purpose. Figure 5 shows the linear sweep voltammograms of β-MnO2 nanorods and dandelions and the results obtained using the Pt/C catalyst, commercial MnO2, and CNT for comparison. Our findings demonstrate that the electrocatalytic performance of nanorods and dandelions is superior to that of MnO2 and MWCNT in the context of ORR. Notably, dandelions exhibit better catalytic activity than rods. Moreover, our results reveal that only Pt/C exhibits a superior half-wave potential among the tested catalysts.
Table 2 summarizes the kinetic characteristics of β-MnO2 nanorods and dandelions compared to commercially available MnO2. The commercial MnO2 has a half-wave potential (E1/2) of -0.410 V and a limiting current density (Jlim) of -1.29 mA/cm2 at -1.0 V. When different morphologies are used, E1/2 values shift towards more positive values, and there is an increase in Jlim. The experimental results show that nanorods have a half-wave potential (E1/2) of -0.304 V and a limiting current density (Jlim) of -3.85 mA/cm2. During the evaluation of dandelion blossoms, the measured current density (Jlim) increased from − 3.85 mA/cm2 to -4.54 mA/cm2. They used a specified morphology for MnO2, resulting in a threefold enhancement of Jlim and a shift of E1/2 by over 100 mV towards more positive values.
Equation (1) was used to calculate exchange current density (Jo) values for β-MnO2 nanorods, dandelions, and MnO2 commercial at |Fη| "RT, as shown in Table 2.
$$\frac{\eta }{J}=\frac{RT}{{J}_{o}F}$$
1
In the micropolarization region of oxygen reduction and evolution reaction (ORR and OER), the overpotential is denoted by η. The current density is represented by J, while R is the universal gas constant (8.314 J mol− 1K− 1). T refers to the temperature of 298.15 K, and F is the constant Faraday, which has a value of 96,485 C mol− 1. Figure SI-3a illustrates the micro-polarization zone for ORR/OER. The value of Jo for the MnO2 commercial and β-MnO2 nanorods were similar, with the former measuring 1.40x10− 6 A/cm2 and the latter measuring 1.92x10− 6 A/cm2. The B-MnO2 dandelions, on the other hand, achieved a superior value of 3.75x10− 5 A/cm2.
The Tafel plots are displayed in Figure SI-3b, depicting the Tafel slopes for ORR in two different β-MnO2 morphologies. The Tafel slope for the dandelion-shaped β-MnO2 is 61.0 mV/dec, whereas the nanorod-shaped β-MnO2 has a Tafel slope of 73.9 mV/dec. These values suggest that the rate-determining step of the ORR is the transfer of the first electron, accompanied by less-than-ideal oxygen adsorption on the electrocatalyst, known as Temkin adsorption. The Tafel slopes increase to 149.5 mV/dec when using commercial MnO2, indicating alterations in the rate-determining phase of the reaction, specifically towards the optimum adsorption of oxygen on the electrocatalyst, known as Langmuir adsorption. Cabot et al. (2012) [29] documented similar alterations in the Tafel slope.
ORR is a complex process that involves the transfer of multiple electrons. It consists of several primary steps and the formation of various reaction intermediates. To determine the production rates of H2O and H2O2, we can use the RRDE approach. This involves setting the ring's potential at 1.1 V, a value that limits the oxidation of H2O2 generated by the reduction of O2 on the disc due to diffusion. Figure 6 displays the currents present in both the ring and the disk.
Figures SI-4a and SI-4b exemplify the creation of hydroperoxide in alkaline environments and the measurement of transferred electrons (n) during ORR. The production of H2O2 was calculated using Eq. 2. In this context, Ir represents the ring current, Id represents the disk current, "N" represents the collection efficiency (with a specific value of N = 0.25), and the number of transferred electrons was determined by using Eq. 3.
$${H}_{2}{O}_{2}\text{%}=\frac{100*\frac{2*{I}_{r}}{N}}{{I}_{d}+\frac{{I}_{r}}{N}}$$
2
$$n=\frac{4*{I}_{d}}{{I}_{d}+\frac{{I}_{r}}{N}}$$
3
β-MnO2 dandelions produce less than 20% H2O2, and n is close to four. With β-MnO2 nanorods, 50% of the oxygen is converted to H2O2, so n is approximately three.
To summarize, the results from the physicochemical analysis indicate that the flower-like hierarchical 3D-nanoarchitecture and rod create structural changes as the Mn-O(1) bond weakens. This alteration leads to the distortion of the irregular octahedral array and increased mobility for both the O atom and the electron density around the Mn atom in the unit cell. The radial contraction of the nanocrystal and the elongation axial also increase the number of defects and modify the specific pore structure. These changes are likely linked to the increase in adsorbed-oxygen species, which suggests that the highest OVs sites on the β-MnO2 surface experience an increase in the occurrence of Mn3+. As a result, the catalyst's performance is likely to improve due to the increase in catalytic sites through the restructuring of the β − MnO2 shape. This adjustment can modify the ORR mechanism to 4e−, which requires less energy for the reaction to occur and can avoid the generation of H2O2.