Numerous plant extracts have been found capable of reducing metal salts to their corresponding metal oxides. Ullah et al. [
212] harnessed
Bryophyllum pinnatum leaf extract to produce MnO
2 NPs ranging from 4–18 nm in diameter. Meanwhile, Dewi and Yulizar [
213] utilized Euphorbia heterophylla leaf extract to generate MnO
2 NPs with a crystallite dimension of 56 nm. Their reaction mixture was stirred for an hour at 80°C and subsequently heated for two hours at 500°C. Other studies have reported the synthesis of MnO
2 nanomaterials using extracts from
Gardenia resinifera,
Phyllanthus amarus,
Kalopanax pictus,
Origanum vulgare,
Artemisia dracunculus,
Sapindus mukorossi,
Rosmarinus officinalis, and
Vernonia amygdalina [
73,
180,
214,
215,
216,
217]. Additionally, Souri et al. [
180] highlighted the biosynthesis of MnO
2 nanoparticles utilizing
Yucca gloriosa leaf extract. This synthesis was confirmed via XRD, revealing an average particle size of 80 nm as calculated by the Debye-Scherer equation. Another notable precursor is the fruit extract of
Acacia concinna, employed in the sol-gel method for green synthesis of manganese oxide nanomaterials. This natural reducing agent was observed to adjust the nanoparticle size and eradicate detrimental byproducts [
73].
6.1. Lemon Juice and Lemon Peel Extracts
Lemon juice is primarily composed of water. The acidity of lemon juice predominantly stems from citric acid, which accounts for about 5% by volume (or 48 g L
-1). Other contributors to its acidity include malic acid and smaller quantities of ascorbic acid (which provides 40 mg of vitamin C per 100 g of lemon) and tartaric acid [
203]. Citrus fruit peels, like those of lemons, serve as potent antioxidants [
218,
219]. They are abundant in several reducing agents, notably p-coumaric acid, flavonoid glycoside, and β-sitosterol [
220,
221].
Figure 10 showcases the molecular structures of the antioxidative compounds present in both lemon juice and lemon peel [
72].
ZnO nanoparticles were synthesized using
Citrus aurantifolia extract, which is abundant in citric acid and ascorbic acid (vitamin C) [
222]. Polyphenols and ascorbic acid present in citrus fruit fibers have been found to enhance the crystallization of metal oxides, as reported by Ahmad et al. [
223].
The tetragonal α-phase of MnO
2 polymorph has garnered significant attention compared to other polymorphs due to its expansive 2×2 tunnels. These tunnels are conducive for the movement and accommodation of foreign ions [
224], such as K
+ found in cryptomelane-type K
xMn
8O
16 compounds [
225]. Zhang et al. highlighted that K
0.25Mn
2O
4 nanofibers possess remarkable lithium insertion abilities, exhibiting superior charge capacities and a high-rate capability [
226]. Hashem et al. [
72] reported the successful production of MnO
2 NPs using lemon juice and lemon peel (designated as J-MnO
2 and P-MnO
2, respectively) at room temperature without producing harmful waste. This was achieved through a green synthesis approach, using a redox reaction between KMnO
4 and natural reducing agents: specifically, lemon juice for J-MnO
2 and lemon peel extracts for P-MnO
2.
All XRD peaks corresponded accurately to α-MnO
2 (JCPDS card No. 44-0141). The reflection (121) associated with the cryptomelane structure had the most pronounced intensity. The P-MnO
2 NPs exhibited superior crystallinity in comparison to J-MnO
2 NPs. The observed differences in crystallinity between the two compounds can be attributed to the specific type of reducing agent employed. This distinction is evident in the varying patterns depicted in the XRD of J-MnO
2 and P-MnO
2, as shown in
Figure 11 [
72]. It is well-recognized that employing diverse carboxylic acids combined with the chelate-assisted sol-gel technique can yield products with markedly different morphologies and structural defects [
227].
The surface morphology of the two green-synthesized compounds was examined using SEM and TEM, as displayed in
Figure 11c [
72]. Distinct morphologies are evident in the figure. For J-MnO
2, a cluster of extremely small particles with sizes less than 10 nm can be seen in image (iii). The diminished size of J-MnO
2's primary particles can be attributed to its limited crystallinity. Conversely, P-MnO
2 exhibits a different morphology. It consists of uniform nanorods with a crystallite size of 17 nm, as illustrated in image (iv). It is evident that the morphology of α-MnO
2 nanoparticles is profoundly affected by the choice of reducing agent. The lemon juice extract, rich in citric and ascorbic acids, yields the tiniest nanoparticles, whereas the more complex reducing agents in lemon peel foster the formation of nanorod structures with a more pronounced crystallite size.
To evaluate the electrochemical properties and discharge performance of J-MnO
2 and P-MnO
2 NPs for potential use as cathodes in lithium-ion batteries, galvanostatic charge-discharge studies were conducted. Cycle tests in the voltage range of 1.5-3.5 V vs. Li
+/Li
0 were carried out at various current densities ranging from 10 to 400 mA g
-1 (with 1C = 260 mA g
-1). As depicted in
Figure 11d and 11e, the superior performance of P-MnO
2 cells is evident across discharge current densities from 10 to 400 mA g
-1 [
72]. The P-MnO
2 cell exhibits a reversible specific capacity of 50 mA h g
-1 at 400 mA g
-1, whereas the specific capacity of the J-MnO
2 cell diminishes, indicating lattice disarray. Both compounds exhibited reduced coulombic efficiency during the first cycle. However, by the second cycle, the cells showcased high rechargeability, achieving efficiency close to 98%. The cycling stability of J-MnO
2 and P-MnO
2 electrodes is highlighted in
Figure 11f. After 50 cycles, the capacity retention of P-MnO
2 was 73%, while J-MnO
2 managed 55%. Reports suggest that the enhanced electrochemical performance of P-MnO
2 stems from its superior crystallinity.
6.2. Black and Green Tea Extracts
Both black and green tea extracts, though structurally distinct, contain flavonoids. These are powerful antioxidants that have been employed to convert KMnO
4 to MnO
2 through redox reactions. The high phenolic content of these teas imparts them with potent antioxidant capabilities [
228].
Green tea is especially rich in Epigallocatechin gallate (EGCG), a renowned antioxidant. Unlike black tea, green tea avoids fermentation and the subsequent oxidation process. This oxidation in black tea transforms catechins into the more complex aflavins and arubigins. Consequently, green tea exhibits enhanced antioxidant activity in comparison to black tea. However, it is important to note that while oxidation alters the type of flavonoids present, it doesn’t affect their quantity or antioxidant potency [
229,
230,
231].
Figure 12 highlights the key flavonoid concentrations in both black and green teas [
75].
In 2018, Abuzeid et al. [
75] developed nanosized MnO
2 NPs using both green and black tea extracts, termed as GT-MnO
2 and BT-MnO
2, respectively.
Figure 13a illustrates their crystal structures, as determined by XRD analysis. Reflections in α-MnO
2 (JCPDS card No. 44-0141) are indexed across both. GT-MnO
2 displays a crystalline nature with a Scherrer crystallite size of roughly 8.0 nm, in contrast to the amorphous or highly disordered BT-MnO
2 which has a crystallite size of about 4.4 nm. This XRD pattern discrepancy between GT-MnO
2 and BT-MnO
2 is attributable to their differing crystallinity. Experimental data suggest that the redox reaction facilitated by black tea is slower compared to that by green tea. This difference, stemming from variances in flavonoid structures and antioxidant power, impacts the MnO
2 structure's crystallinity. The potency of the reducing agent, governing ion extraction from the 2×2 tunnels and K
+ ion movement, also affects the synthesis methods. Raman spectra depicted in
Figure 13b, was used to further explore the MnO
2 NPs' crystal structures, focusing on the influence of the reducing agents [
75]. The Raman spectrum for GT-MnO
2 displays distinct bands at 181, 380, 510, 577, 630, and 754 cm
-1. In contrast, BT-MnO
2's Raman bands are broad and less defined, suggesting a highly disordered structure in the material. Key insights can be drawn from the band positions: the tetragonal 2 × 2 tunnel structure is revealed by the high-frequency signals at 577 and 630 cm
-1 (
Ag modes). The low-frequency band at 181 cm
-1 arises from the translational motion of [MnO
6], and the band at 380 cm
-1 is attributed to O-Mn-O bending vibrations. Furthermore, the band situated at 754 cm
-1 is linked to antisymmetric Mn-O stretching vibrations. These distinctive Raman characteristics align with findings for α-MnO
2 documented in prior research [
232,
233]. Notably, the crystallization process of MnO
2 NPs notably affects the intensity of the ν
577 and ν
630 bands. This consistency between Raman and XRD results underscores GT-MnO
2's superior crystallinity over BT-MnO
2 [
75].
Figure 13c-d present the galvanostatic discharge-charge profiles of MnO
2//Li cells using GT-MnO
2 and BT-MnO
2 as the positive electrode materials. The profiles of these half-cells were obtained at a consistent current density of 10 mA g
-1 (C/26) and within the potential window of 1.5-3.5 V vs. Li
+/Li
0 [
75]. The discharge cell potential steadily decreases throughout the entire discharge span, revealing two pseudo-plateaus. Each plateau is characterized by an "S-shaped" curve, indicative of a topotactic reaction during lithium insertion into the electrodes. However, in the case of BT-MnO
2, the voltage drop is markedly steeper, a characteristic typically observed in disordered electrode materials [
234]. This electrochemical behavior aligns with the structural observations made earlier. The highly disordered BT-MnO
2 exhibits an initial specific capacity of approximately 236 mA h g
-1, while the well-crystallized GT-MnO
2 sits at around 198 mA h g
-1. The expansive tunnel (4.6 Å) accommodates a significant quantity of electrochemically inactive K
+ ions, which remain lodged at the 4
e sites, leading to the reduced starting capacity. For GT-MnO
2, potassium occupies more than half of these 4e sites, whereas for BT-MnO
2, it's just above a quarter. This inert cation might hinder the ingress of Li ions into the tunnel and also obstruct the ion movement during the discharge phase [
235]. GT-MnO
2's Coulombic efficiency showcases impressive rechargeability, approaching 99%, even at a steady current density of 200 mA g
-1 (= 0.75 C). This remains consistent except for the initial two cycles. The cycling behavior over 54 cycles for MnO
2//Li cells in lithium-ion batteries is depicted in
Figure 13e [
75].
GT-MnO
2 demonstrates superior capacity retention compared to BT-MnO
2. After the third cycle, GT-K
yMnO
2 exhibits a discharging capacity of 161 mA h g
-1, which gently reduces to 141 mA h g
-1 by the 54th cycle at a C/10 rate. The capacity degradation for GT-K
yMnO
2 averages 0.25% per cycle, while it is 0.58% for BT-K
yMnO
2. Both materials experience some capacity loss post the initial cycle. GT-MnO
2's irreversible capacity caps at 30 mA h g
-1, retaining 70% of its inaugural capacity after 20 cycles. In contrast, BT-MnO
2 holds onto roughly 62% of its capacity. This diminished initial capacity suggests that certain lithium ions became ensnared within the cell's internal voids during the cell fabrication [
236].
6.3. Broccoli Vegetable Extract
Broccoli, akin to cabbage and cauliflower, boasts antioxidant, antibacterial, and anticancer attributes [
237,
238,
239,
240]. It is rich in polyphenols, particularly flavonoids [
238]. Various parts of broccoli, including its leaves, flowers, and other tissues, house these flavonoids and phenolic acids [
237].
Figure 14 depicts the antioxidant constituents in broccoli extract, namely α-lipoic acid, sulforaphane, and coenzyme Q10 [
240].
Figure 15a presents the XRD pattern of K
yMnO
2 nanoparticles (NPs) synthesized using broccoli extract [
240]. The most prominent peak at 2
θ = 37° corresponds to the (211) plane of the tetragonal α-MnO
2 phase, with no detectable defects. The subdued intensity indicates low crystallinity, while the peak broadening implies that the α-K
yMnO
2 NPs are of nanoscale dimensions. Using the half-width at half-maximum of the (211) diffraction line, observed at 2
θ ≈ 37.6, the calculated particle size is approximately 4.4 nm. This highlights the polycrystalline character of the α-K
yMnO
2 NPs.
Nitrogen adsorption/desorption at 77 K was conducted over a relative pressure range of
P/
P0 = 0.0 - 1.0, where
P and
P0 denote equilibrium and saturation pressures, respectively, to determine the Brunauer–Emmett–Teller (BET) specific surface area. As the P/P
0 value increases, the volume of N
2 adsorbed on the isotherm curve also grows. The emergence of a hysteresis loop signifies the hierarchical mesoporous structure of α-K
yMnO
2. At
P/
P0 = 0.97, the volume of N
2 adsorbed is estimated to be 450 cm
3 g
-1. The inset in
Figure 15b [
240] showcases the mesoporous nature of the green-synthesized α-K
yMnO
2, determined via the Barrett-Joyner-Halenda (BJH) method [
241].
Suib et al. [
242] suggested that mesoporosity arises from the aggregation of MnO
2 nanomaterials, either in the form of nanorods or nanoneedles, along their lateral facets. The mesopore size distribution, spanning 1-20 nm, primarily displays a single peak at 10.4 nm, accompanied by a cumulative pore volume of 0.950 cm
3 g
-1. Notably, the BET surface area of MnO
2 NPs biosynthesized using broccoli extract registers at 161 m
2g
-1. This is superior to MnO
2 structures derived from alternative methods such as the microemulsion technique (123 m
2 g
-1), hydrothermal synthesis (150 m
2 g
-1), silica templating coupled with the ion-exchange approach (142 m
2 g
-1), mild reactions [
243], and exfoliation [
244]. Thus, green synthesis emerges as a potent strategy for fabricating mesoporous, pure α-phase K
yMnO
2 characterized by minute particle dimensions and an expansive surface area.
The potassium concentration inside the 2 × 2 tunnels, which is considered to be positive to stabilize the tetragonal structure, was determined by thermogravimetry (TG).
Figure 15c presents the differential weight d
w/d
T corresponding to the rapid weight loss in the region 400-600 °C, which depends on the concentration of tunneled foreign ions (potassium or ammonium) in the cryptomelane K
yMn
8O
16 structure. According to decomposition temperature of 504 °C shown in
Figure 15c, a concentration of potassium is estimated to be
y = 0.035, which is close to the value obtained from ICP measurements.
Hashem et al. [
240] investigate the galvanostatic discharge-charge curves of α-K
0.03MnO
2//Li cells over 50 cycles at a consistent current density of 30 mA g
-1. The MnO
2 structure, which possesses two distinct coordination sites for Li
+, exhibits a topotactic behavior for Li
+ insertion, characterized by a gradual voltage decline marked by two pseudo-plateaus and an S-shaped profile. As subsequent cycles commence, these plateaus shift to higher potentials. Over four consecutive cycles, the material's capacity diminishes from 211 to 198 mAhg
−1. The pronounced alterations in the electrochemical profile during the second cycle have been highlighted in various studies [
245,
246,
247].
Figure 15 (d-f) underscores the commendable rate capability and cycle stability of α-K
0.03MnO
2 when employed as a cathode in lithium-ion batteries, operating within voltages of 1.5 to 4.0 V and current densities spanning 0.1C to 10C [
240]. As the current density increases, there is a decline in specific capacity; however, the charge and discharge profiles remain largely unaltered, as depicted in
Figure 15d. Throughout the assessed C-rate spectrum, the characteristic S-shaped profile persists. The modified Peukert plot, which plots discharge capacity against C-rate, exhibits a near semi-logarithmic trend, as illustrated in
Figure 15e. At 10C, the α-K
0.03MnO
2 electrode delivers a specific capacity of 32 mAh g
-1. The α-K
0.03MnO
2 electrode showcases commendable reversibility, as seen in
Figure 15f. This is highlighted by its efficiency, which remains an impressive 98.8% at 0.1C rate over 100 cycles. Given this robust electrochemical stability, it suggests that the cationic exchange (Li
+ vs. K
+) during Li
+ integration into the α-K
0.03MnO
2 structure is minimal.
6.4. Orange Juice and Orange Peel Extracts
Millions of tons of oranges are produced globally, with a significant portion dedicated to industrial orange juice extraction. This industry generates large amounts of byproducts, including orange peels and segments. The orange peel, accounting for 50%–65% of the fruit's weight, is rich in 7.1% protein, 12.79% crude fiber, and bioflavonoids. These bioflavonoids possess antioxidant properties, making them suitable for NP production [
73]. Addressing the vast amounts of orange peel waste is essential to avert potential environmental harm and other adverse effects [
248,
249].
Orange juices and peels are rich in ascorbic acid, flavonoids, phenolic compounds, and pectin. The primary components of orange juice are organic acids, sugars, and phenolic compounds, including sucrose, glucose, fructose, and citric, malic, and ascorbic acids. Additionally, orange juice contains phenolic substances such as flavanones, hydroxybenzoic acids, hydroxycinnamic acids, ferulic acid, hesperidin, and narirutin [
250].
Orange peels are primarily composed of polyphenolic and flavonoid compounds. Prominent among these flavonoids are hesperidin, narirutin, naringin, and eriocitrin [
251]. The glycosides hesperidin and naringin endow orange peel extracts with their antioxidant activity. Furthermore, orange peel molasses contains coniferin and phlorin, which aid in radical scavenging and support the sustainable recycling of orange peels [
252]. Skiba et al. [
253] reported the use of orange peel extract in the fabrication of silver NPs using a plasma chemical extraction process, along with the degradation of methylene blue in sunlight. Abuzeid et al. [
73] employed orange peel extract for the green synthesis of MnO
2 nanomaterials. These were then utilized as electrodes for supercapacitors, representing an innovative approach to repurpose the vast residue from orange production. Notably, MnO
2 is widely used as an electrode in both electrochemical supercapacitors and batteries [
254].
Supercapacitors, offer high power in short time spans. These devices are vital for high-power applications due to their cost-effectiveness, low maintenance, safety, rapid charging, and extended cycle life [
255,
256,
257]. While supercapacitors may have a lower energy density than lithium-ion batteries, they bridge the gap, providing a balance between the high energy density of batteries and the power density of electrochemical capacitors [
258,
259,
260].
X-ray powder diffraction patterns of synthesized OP-MnO
2 ad OJ-MnO
2 using orange peel and orange juice, respectively are presented in
Figure 16a and 16b. The prominent peaks characteristic of α-MnO
2 were identified in alignment with the reference (JCPDS No. 44-0141) as documented in a previous study [
261]. It has been highlighted that the potency of the reducing agent can influence the amount of K
+ integrated within the 2×2 tunnels of α-MnO
2. The presence of potassium plays a pivotal role in fortifying the α-MnO
2 structure. Materials with a lesser degree of crystallization tend to have reduced K
+ concentrations within the α-MnO
2 framework, which can be attributed to the utilization of a less potent reducing agent [
72,
75,
261].
The mesoporous nature of the prepared OJ-MnO2 and OP-MnO2 samples was confirmed from BET experiments. The pore size was estimated to be 7.25 and 6.75 nm for OJ-MnO2 and OP-MnO2, respectively. Calculated BET surface area according to Barrett-Joyner-Halenda method are 5.63 and 8.40 m2 g−1 for OJ-MnO2 and OP-MnO2, respectively
Figure 16d and 16f present the charge and discharge data (CD) utilized to compute the specific capacitance (SC) of both OJ-MnO
2 and OP-MnO
2 NPs based on the following equation [
73]:
where
m is the total mass of materials coated on the glassy carbon electrode,
I is the discharging current (A),
Δt is the discharging time (s), and
ΔV is the voltage range.
At current densities of 15, 5, 2, and 0.5 A g
-1, the specific capacitances for OJ-MnO
2 NPs are 18.5, 25, 33, and 50 F g
-1, respectively. In contrast, at the same current densities, the specific capacitance values for OP-MnO
2 are 61, 85, 107, and 139 F g
-1. Notably, the specific capacitances for OP-MnO
2 are approximately two and a half times those of OJ-MnO
2. This significant difference can be explained by two primary factors. Firstly, OP-MnO
2 possesses a larger surface area and smaller particle size, as evident from the BET surface area data [
73]. Secondly, OJ-MnO
2 has a substantial concentration of K
+ ions lodged within its 2×2 tunnel. This increased presence of K
+ ions in the 2×2 tunnel restricts the easy insertion and extraction of H
+ ions. Moreover, it is important to note that these K
+ ions act as inert components, thereby reducing the overall capacitance value, as detailed in the study [
73].
The cycle stability is a pivotal aspect for electrochemical supercapacitors [
262]. An investigation was conducted on the cycle stability of OP-MnO
2 electrodes over 500 cycles, using a current density of 3 A g
-1 and employing the charge/discharge method within voltage parameters spanning from -0.2 to 1.2 V. As depicted in
Figure 16g [
73], there's a noticeable uptrend in capacitance retention for the first 450 cycles relative to the inaugural cycle. Interestingly, by the 500th cycle, the electrode's capacitance had reverted to its initial value. Specifically, the electrode began with 119 F g
-1 during the 1st cycle, peaked to 137 F g
-1 by the 350th cycle (indicating a capacitance retention of 115%), and then circled back to 119 F g
-1 on the 500th cycle, showcasing a complete 100% capacitance recovery.
6.5. Moringa and Cinnamon Herbs Extracts
Moringa oleifera is renowned for its myriad benefits, encompassing health, nutrition, commercial, and clinical attributes, primarily due to its potent antioxidant properties. This plant is a rich source of various vitamins, minerals, amino acids, fatty acids, glucosinolates, and phenolic compounds. Specifically, Moringa oleifera leaves are enriched with vitamin C, amino acids, and beta-carotene. Delving deeper, compounds present in this plant include flavonoids, L-ascorbic acid, retinol, niacin, thiamine, chlorogenic acid, tocopherol, caffeic acid, O-coumaric acid, gallic acid, and riboflavin, all of which possess remarkable reducing properties.
Cinnamon, a potent spice, has held medicinal significance for millennia. In modern medicine, cinnamon is recognized for its ability to reduce blood glucose, cholesterol, and blood pressure levels. This spice boasts antiparasitic, antibacterial, antioxidant, and free-radical scavenging properties. Key natural antioxidants found in cinnamon, such as cinnamaldehyde, eugenol, borneol, cinnamyl acetate, cinnamic acid, and coumarin, are responsible for its therapeutic benefits [
263,
264].
Moringa and cinnamon extracts are recognized for their potent antioxidant properties and their excellent capacity to reduce KMnO
4 to α-MnO
2, as recently explored by Abuzeid et al. [
177].
Figure 17a presents the XRD characteristics of the biosynthesized MnO
2 nanoparticles M-MnO
2 and C-MnO
2 using moringa and cinnamon extracts, respectively. Based on the JCPDS data (card no. 44 0141), the characteristic peaks for both compounds can be attributed to the tetragonal α-MnO
2, with no additional defects observed. Both compounds exhibit low crystallinity, suggesting a nanosized structure. This is evident from the reduced intensity and broadening of these peaks. Electronic properties of C-MnO
2 and M-MnO
2 have been investigated by UV-Vis diffuse reflectance spectroscopy.
C-MnO
2 exhibits a higher reflectance intensity compared to M-MnO
2. This difference can be attributed to the formation of defect-induced energy levels in the nanoparticles during the synthesis process. The intermediate optical response to visible light results in the absorption band at 282 nm [
265]. The band gap (
Eg) value of the green-synthesized C-MnO
2 and M-MnO
2 compounds was determined using the Kubelka-Munk equation, as shown in
Figure 17b [
177]. C-MnO
2 and M-MnO
2 possess band gap values of 1.42 and 1.39 eV, respectively. The variation in the band gap energy between these compounds can be attributed to disparities in their internal electronic structures and particle sizes [
266,
267]. Owing to their narrow band gap (1-2 eV), manganese oxides can act as photocatalysts in the visible light spectrum [
268,
269]. MnO
2 is considered a potent catalyst because of its porous nature, available lattice oxygen, and the presence of multiple valence states of manganese ions, such as Mn
4+/Mn
3+ and Mn
3+/Mn
2+. In addition to its notable catalytic properties, the widespread availability and affordability of MnO
2 make it a preferred choice for organic dye removal. Moreover, MnO
2 displays a diverse range of crystal structures and morphologies, enhancing its suitability for photocatalytic applications [
270].
The photocatalytic efficiency of green-synthesized MnO
2 nanoparticles using
Y. gloriosa leaf and curcumin extracts for degrading acid orange as an organic contaminant was previously studied, with decomposition results reported [
145]. Green-synthesized M-MnO
2 and C-MnO
2 nanoparticles have been examined for their potential in photocatalytically degrading methylene blue and Congo red under visible sunlight. For the tests, 100 ml of each of the Congo red and methylene blue dye solutions (10 ppm) was combined with 0.05 g of the respective nanoparticles. The mixtures of M-MnO
2 or C-MnO
2 with the dyes were stirred in the dark for 30 minutes before being exposed to visible sunlight [
177]. The absorbance of Congo red and methylene blue at 464 nm was determined using a JASCO V630 UV-Vis spectrophotometer to gauge the degradation efficiency. The following equation was employed to calculate the photodegradation efficiency (
Ph in %):
in which
Co and
Ao represent the initial concentration and absorbance of MB and Congo red prior to radiation and
Ci and
Ai represent the concentration and absorbance of MB and Congo red, respectively, after a specific period of time of exposure.
Figure 17c illustrate the alteration of the methylene blue (MB) highest absorbance peak (664 nm) of C-MnO
2 and M-MnO
2 after 140 min of exposure to sunlight [
177]. After 140 min of exposure to visible light, there was a notable reduction in the absorbance intensity of MB. The photodegradation efficiency for MB dye using M-MnO
2 reached 96%. Meanwhile, C-MnO
2 displayed a slightly reduced photodegradation efficiency of 89% over the same exposure duration.
Figure 17d and 17e display the photodegradation percentages of MB and CR dyes over time under visible light exposure. Both C-MnO
2 and M-MnO
2 exhibit commendable photocatalytic activity in degrading MB and CR. Among the two, M-MnO
2 consistently outperforms C-MnO
2 across all time intervals. Specifically, M-MnO
2 achieves a photocatalytic activity of 96% for MB and 93% for CR, while C-MnO
2 registers 89% for MB and 91% for CR. The superior photocatalytic performance of M-MnO
2 can be attributed to its narrower band gap. Studies have noted that both C-MnO
2 and M-MnO
2 nanoparticles exhibit urchin-like morphologies, composed of interconnected nanowires with particle sizes ranging from 4–10 nm. This structure significantly amplifies their photocatalytic activity [
177].
Figure 17f elucidates the mechanism of the photodegradation process, showing how sunlight activates M-MnO
2 and C-MnO
2 to produce highly reactive radicals such as OH-, O2–, and H
2O molecules.
The bandgap values for M-MnO
2 and C-MnO
2 nanoparticles suggest that both compounds can be effectively activated by sunlight's visible spectrum. Upon activation, electrons (e-) are excited from the valence band to the conduction band in both M-MnO
2 and C-MnO
2, creating electron-hole pairs. These photo-excited electrons in the conduction band can subsequently interact with the dissolved oxygen (O
2) adsorbed on the surfaces of M-MnO
2 or C-MnO
2, yielding superoxide anion radicals (O
2-). Simultaneously, the holes in the valence band can react with hydroxide ions (OH
-) and water molecules (H
2O) to produce hydroxyl radicals (OH-). These radicals, O
2- and OH
-, being highly reactive, are instrumental in breaking down organic molecular pollutants. The described mechanism provides insights into the photodegradation process of dyes, specifically methylene blue (MB) and Congo red (CR) [
271].
The superior photocatalytic activity of M-MnO
2 over C-MnO
2 can be attributed to its narrower band gap and reduced light scattering, enhancing its ability to absorb more light, as evident from the diffuse reflectance measurements. These findings suggest the potential for creating high-performing, cost-effective photocatalysts tailored for environmental and water treatment applications. While prior studies on MnO
2 have demonstrated rapid degradation, they often relied on UV radiation as the light source, which comes with higher operational costs [
177]. In this study, the photocatalytic experiment was conducted using direct visible sunlight on an immediately prepared dye solution, presenting a practical and cost-effective approach. Given the escalating production of industrial wastewater, particularly from the textile industry, there is an urgent need to commercialize this environmentally-friendly method. Further market research and exploration are crucial for scaling up this innovative approach to address the increasing environmental challenges.