Experimental and Computational Study of Zr and CNC-Doped MnO2 Nanorods for Photocatalytic and Antibacterial Activity.

Cellulose nanocrystals (CNC), MnO2, CNC-doped MnO2, and Zr/CNC-doped MnO2 were prepared with a hydrothermal method to assess their photocatalytic and antibacterial properties. Various characterizations were undertaken to determine the phase composition, the existence of functional units, optical characteristics, elemental analysis, surface topography, and microstructure of the prepared materials. Sample crystallinity was improved, whereas a decrease in crystallite size was observed with increasing amounts of dopants. Incorporation of dopants (CNC and Zr) into MnO2 instigated a transformation in morphology from nanoclusters to nanorods with different diameters. Furthermore, photocatalytic activity experiments indicated a more effective degradation of methylene blue (MB) dye with CNC-doped MnO2 and Zr/CNC-codoped MnO2 while enhancing the bacterial efficacy for both G +ve and G -ve. Density functional theory was utilized to model the structures and elucidate their bonding and charge transfer mechanisms. The Zr/CNC-MnO2 system showed charge depletion around Mn atoms, while charges were observed to accumulate around oxygen atoms.

Introduction

Rapid industrialization and population growth have led to increased environmental degradation during the past century, affecting the quality of the environment, which is vital for life on Earth.[1] Currently, many efforts are being expanded into finding effective solutions for combating environmental pollutants to make way for a clean and wholesome ecosystem.[2] Polluted water outflowing from industries has become a serious environmental issue.[3] Thousands of various forms of dyes are produced and widely used in paper, textiles, food, printing, plastic, cosmetics, and leather industries.[4] Approximately 10–15% of methylene blue (MB) is dumped directly into water bodies, endangering humans, plants, and wildlife.[5] In humans, it can manifest as cancer, skin irritation, allergies, and malfunctioning of kidneys and the reproductive system.[5] Purification of water polluted with harmful dyes is necessary to ensure the ongoing availability of this critical resource for all forms of life.[6] For degradation of dyes, various techniques including adsorption, biochemical treatment, photocatalytic degradation, ultrafiltration, and reverse osmosis are employed which serve to extract hazardous toxins.[7,8] Among these techniques, photocatalysis is accepted as a promising and cost-effective method in decontaminating wastewater or polluted air.[9] The degree of photocatalytic activity is controlled by the size, morphology, and surface area of the photocatalysts. Therefore, by controlling these factors, higher photocatalytic performance can be attained. For efficient photocatalytic reactions, one-dimensional (1-D) nanomaterials provide large surface area, which is likened to zero-dimensional (0-D) nanomaterials, and thus show superior photocatalytic behavior.[10] In this process, light with energy higher than the semiconductor’s band gap (hν ≥ Eg) is allowed to fall on the surface of the semiconductor material. Generation of electrons and holes (e––h+) takes place upon interaction between the light and semiconductor. Water molecules interact with the h+ ion to form hydroxyl radicals (•OH) along with the reduction of oxygen molecules to H2O2 and superoxide anion (•O2–). H2O2 produces •OH radicals, which convert hazardous chemicals into harmless materials.[11,12]

Common semiconducting photocatalysts include TiO2, CuO, and MnO2. TiO2 is a highly attractive semiconductor because of its chemical inertness and harmlessness to the environment. However, it is not suitable for photocatalytic activity as it has a wide band gap energy of ∼3.2 eV.[13,14] CuO is a p-type semiconductor with narrow band gap (1.3–2.1 eV) and exhibits numerous interesting properties including high optical absorption coefficient and nontoxicity. These characteristics make them potential candidates for various applications such as catalysis, semiconductor equipment, gas sensing, antimicrobial materials, and luminescence source fields.[15,16] CuO-based materials also possess photocatalytic or photovoltaic properties and have extensive applications.[17] In comparison with MnO2, the CuO metal atom exists in the +2 oxidation state, and the MnO2 metal atom is in the +4 oxidation state. Many applications using MnO2 exist due to its physiochemical characteristics such as photocatalytic activity, ion exchange, catalysis, molecular adsorption, biosensing, and energy storage.[18] Due to its cost benefit, excellent stability, abundance in nature, environmentally benign nature, intriguing electrochemical performance, and small band gap energy (1–2 eV), it is an attractive source for photocatalysis.[19,20] Around two decades ago, the photocatalytic activity of MnO2 was verified by a process involving oxidation of 2-propanol.[19] Furthermore, to evaluate the use of MnO2 in conjunction with polymers, cellulose nanocrystals (CNCs) are used as dopants.[21] CNCs gained much attention during the past decade as the most abundant renewable polymer resource and inspired researchers to develop cellulose-based materials with novel functions.[21,22] CNCs have a large surface area, good biocompatibility, thermal stability, and good mechanical and chemical properties. Adsorbent-based CNCs are highly effective at removing dyes, heavy metal ions, and numerous contaminants in wastewater. It constitutes recapping of β-D46 glucopyranose units, which categorize it as a carbohydrate polymer. Cellulose is used as an additive and can be employed as an adsorbent for water purification. CNCs are insoluble in conventional solvents due to their high concentration of hydroxyl groups and strong hydrogen bonds.[23] The CNC itself is not suitable for photocatalysis due to its small size and wide band gap. However, CNC-based semiconducting materials show outstanding performance in dye degradation processes.[24] A smart cotton fabric (CF) covered with rGZn was developed, and its photocatalytic self-cleaning ability was proven by the destruction of MB, rhodamine B dyes, and tea stains on it even when exposed to sunlight.[25] Due to their significant visual absorption capabilities, the composite as a highly stable 2-D MAX material can be employed as a potential photocatalysts, like pure CuS and CuS/Ag2S(9).[26,27]

Recently, the most competent way to treat a dye is by combining adsorption and degradation effects. In this respect, metal oxides such as zinc oxide (ZnO), titanium oxide (TiO2), and manganese dioxide (MnO2) have been doped on the CNC surface to efficiently degrade dyes without the generation of secondary pollutants.[25] Doping of transition metal zirconium is used to change the optical properties of MnO2. At room temperature, Zr has a monoclinic structure, and with increasing temperature, it switches to tetragonal and cubic structures.[26] Zr is a ceramic material with a large surface area, adsorbent properties, and thermal and mechanical stability. A wide band gap (5.25 eV) makes it unsuitable for photocatalytic activity, but its use as a dopant in oxides results in a positive effect on their photocatalytic properties.[27,28] Various techniques including sol–gel, hydrothermal, co-precipitation, and thermal decomposition have been used to form different sample morphologies.[27] In the hydrothermal method, the development of nanomaterials can occur over an extended temperature range, that is, from room temperature to very high temperatures. To control the morphology of prepared materials, either high-pressure or low-pressure conditions can be used, which depends on the vapor pressure of main composition during the reaction. Using this approach, several types of nanomaterials have been synthesized. Hydrothermal synthesis has noteworthy advantages over others and can produce nanomaterials which are not stable at elevated temperatures. Numerous papers on hydrothermal synthesis of nanoparticles, nanotubes, nanorods (NRs), graphene nanosheets, and hollow nanospheres have been published.[29]

In this work, CNCs, MnO2, CNC-MnO2, and Zr/CNC-MnO2 were synthesized using the hydrothermal technique. To our best knowledge, this is first study utilizing Zr as a doping agent in CNC-MnO2 containing a particular morphology of nanorods validated through numerous characterizations. The antimicrobial evaluation of MnO2, CNCs, and Zr/CNC-MnO2 nanorods evaluated with Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) directly isolated from ovine mastitis milk, whereas the photocatalytic activity with effective degradation of MB dye using codoped MnO2 nanorods is also of unique importance. To model the endurance and bonding behavior of Zr/CNC-MnO2 systems, first-principles calculations using density functional theory were performed. This fabricated material depicted promising potential of bactericidal and photocatalytic response to treat pathogenic etiologies and wastewater.

Experimental Section

Materials

Cellulose ((C6H10O5), 99.5%), manganese sulfate monohydrate (MnSO4·H2O (99%), potassium permanganate (KMnO4 (99.5%)), zirconium nitrate (Zr(NO3),H2O), and MB were obtained from Sigma-Aldrich, Germany, and used out of their original packaging.

Preparation of Cellulose

The hydrothermal technique was used to synthesize cellulose nanocrystals employing sulfuric acid (64% w/w) and water as the solvent, and 10.01 g of Avicel was added to the solvent mixture at 100 °C (Figure a). The resultant solution was allowed to cool to room temperature before being centrifuged for 10 min at 7100 rpm with distilled water. Finally, the material was dried for 2 days to conduct further characterization.

Figure 1

Schematic diagram of synthesis of (a) CNC and (b) Zr/CNC-MnO2 NRs.

Preparation of MnO2, CNC-MnO2, and Zr/CNC-MnO2

For the control MnO2 sample, KMnO4 and MnSO4·H2O (2:3) as precursors were dissolved in 100 mL of distilled water under constant stirring at 40 °C for 20 min (Figure b). To prepare the CNC-MnO2 sample, a fixed amount of CNC (2.5 mL) was added in the above-stated stirred MnO2 solution. For doping of Zr in CNC-MnO2, Zr(NO3), H2O as a source of Zr (2%) was added in the CNC-MnO2 solution. Each solution was transferred to a Teflon-lined stainless steel autoclave and maintained at 150 °C for 17 h before being cooled to ambient temperature. Following that, the solution was rinsed with distilled water and dried at 50 °C to yield samples of black powder (MnO2, CNC-MnO2, Zr/CNC-MnO2).

Segregation and Characterization of S. aureus and E. coli

Milk swabs from sheep (ovine) were obtained from multiple farms in Punjab, Pakistan. The specimens were plated upon 5% sheep blood agar (SBA) and incubated at 37 °C overnight. The acquired populations were plated on MacConkey agar (McA) and mannitol salt agar (MSA) to isolate E. coli as Gram negative (G −ve) and S. aureus as Gram positive (G +ve). Gram staining, catalase, and coagulase assays were used to characterize the biochemical and topographical properties of pure cultures.

Antimicrobial Activity

Antimicrobial evaluation of produced materials was performed using an agar well diffusion method, which included swabbing 1.5 × 108 CFU/mL S. aureus and E. coli on MA and MSA, individually. Bacterial isolates were swabbed upon Petri plates, and 6 mm diameter boreholes were created using a sterilized cork borer. CNCs, MnO2, and Zr were added in distinct ratios (0.5 and 1.0 mg/50 μL) and sorted using ciprofloxacin (0.005 mg/50 μL) and DI water (50 μL) as positive (+ve) and negative (−ve) standards, accordingly. Following 24 h of dosing on Petri dishes at 37 °C, inhibitory zones were measured in millimeters (mm) employing a Vernier caliper. Antibacterial viability of samples was determined with a one-way analysis of variance (ANOVA).[30]

Photocatalysis

The degradation proportion was obtained along with the percentage of MB to determine the photocatalytic efficiency of materials using solar light. A stock solution of MB (5 g/500 mL) was made, and 10 mg of prepared samples was mixed with 30 mL of stock solution and kept in the dark for 30 min prior to illumination to achieve an adsorption–desorption equilibrium between the MB and the photocatalyst. Later, 30 mL of the produced solution was moved to a photoreactor using a 400 W mercury bulb (400–700 nm). After 30 min, 3 mL of solution was taken from the reactor to evaluate the MB dye deterioration using a UV–vis spectrometer. The following equation was used to calculate the degradation rate:where C0 and C are the first and last concentrations of dye.

Material Characterization

Structural assets and crystallite size were determined with X-ray diffraction (XRD) (PAN analytical X’pert pro XRD) with Cu Kα radiation (λ= 0.154 nm) and the diffraction angle varying from 20 to 80°. Fourier transform infrared (FTIR) spectroscopy was employed to expose the existence of functional groups using a PerkinElmer spectrometer. High-resolution transmission electron microscopy (HRTEM) was employed to study the surface morphologies with a JEM 2100F instrument, correspondingly joined with an energy-dispersive spectrometer. Absorption spectra were attained using a UV–visible Genesys 10S spectrophotometer in which a stock solution of 1 mg/mL strength was prepared for all samples in DI water. Then a 500 μL stock solution was further diluted to 5 mL by adding water and keeping water as the blank. To study migration and recombination of electron–hole pairs, photoluminescence (PL) spectroscopy was carried out using a spectrofluorometer (JASCO, FP-8300). Compositional analysis proceeded using X-ray photoelectron spectroscopy (XPS).

Results and Discussion

The structural attributes and phase composition of fabricated materials were ascertained using XRD, as displayed in Figure a. The XRD array of CNCs shows diffraction peaks at 13, 19.5, 22.4, and 34° indexed to (101), (101), (002), and (112) planes, respectively, revealing its monoclinic structure (JCPDS card no. 46-0905).[31] A characteristic peak at 34° confirms the Iβ crystal form of CNC-COOH.[32] Diffraction peaks of MnO2 occurring at 12.7, 18.1, 28.8, 37.5, 42.1, 49.9, 56.2, and 60.3° were compatible with (110), (200), (310), (211), (301), (411), (600), and (521) crystal planes, respectively, exhibiting tetragonal structure that was well matched with the standard spectrum (JCPDS card no. 44-0141). The peaks detected were strong and sharp, indicating the existence of MnO2 nanorods, and no extra peak originating from an impurity was detected, which confirmed phase purity.[33] Upon CNC and Zr doping into MnO2, peaks shifted toward a lower angle, as shown in Figure a′. Peaks of CNC in MnO2 coexist at 12 and 18°, 19 and 22°,[24] and in Zr/CNC-MnO2, peaks of Zr overlap at 18, 28, 50, 56, and 60° and are well matched with standard spectrum (JCPDS card no. 00-005-0665), which shows the presence of zirconium. Crystallite size of CNCs, MnO2, CNC-MnO2, and Zr/CNC-MnO2 was 10.11, 36.6, 36.2, and 36.0 nm, respectively, as determined by Debye–Scherrer equation.

Figure 2

(a) XRD analysis, (a′) zoomed-in XRD pattern, (b) FTIR pattern, and (c–f) selected area electron diffraction images of (c) CNCs, (d) MnO2, (e) CNC-MnO2, and (f) Zr/CNC-MnO2 nanostructures.

FTIR was performed to identify the presence of vibrational and chemical bands in synthesized materials (Figure b). In CNC spectra, spotted peaks at 565 and 1059 cm–1 are ascribed to the stretching vibration of the C–OH bond of secondary and primary alcohols, and the peak at 1059 cm–1 belongs to symmetrical C–O and C–O–C bond stretching in cellulose.[31,34] The absorption peaks at 570 and 691 cm–1 are attributed to Mn–O and Mn–O–Mn, representing the formation of MnO2 nanorods.[35] An absorption peak at 875–1099 cm–1 for CNC-MnO2 represents the stretching vibration of the −OH and C–H bond.[34] In the case of Zr/CNC-MnO2, bands were similar to those in MnO2. An absorption peak at 1580 cm–1 shows the presence of a hydroxyl group of Mn–OH and O–H–O due to absorption of water during sample preparation.[36] Selected area electron diffraction profiles of CNC, MnO2, CNC-MnO2, and Zr/CNC-MnO2 are shown in Figure c–f. The obtained rings corroborated the polycrystalline phase of NRs, confirming that the samples were crystalline in nature, while the indexed rings closely matched with XRD data (Figure a).

Absorption spectra of synthesized materials were recorded with a UV–vis spectrophotometer, as illustrated in Figure a. All of the samples showed maximum absorption in the region of 300–400 nm. The absorption peak of CNC was found between 360 and 375 nm, and the MnO2 peak was observed at 348 and 390 nm, which is mostly due to the d–d transition of Mn ions.[37,38] UV–vis spectra exhibited a lower wavelength peak at ∼348 nm, which is attributed to localized π–π* electronic transition. Upon doping, CNC-MnO2 and Zr/CNC-MnO2 showed a blue shift at 365 and 358 nm, respectively, compared to MnO2 (390 nm). The measured band gap energies from the Tauc plot are 3.15 eV for CNC,[31] 3.19 eV for MnO2,[38] 3.40 eV for CNC-MnO2, and 3.46 eV for Zr/CNC-MnO2, which is attributed to a blue shift in absorption spectra corresponding to a decrease in crystallite size.[20,23] Apart from UV–vis, photoluminescence spectroscopy was utilized to determine the optical properties and to assess the electron–hole pair recombination capacity after being created by photons in synthesized samples (Figure b). A lower electron–hole recombination rate is represented by lower PL intensity and therefore coincides with promising longer lifetime of excited charge carriers. To investigate PL spectra of the prepared samples at room temperature, a wavelength of 280 nm was employed for excitation.[39] In PL spectra, an observed peak of CNCs was located ∼427 nm and a broad peak in the range of 440–500 nm. However, MnO2, CNC-MnO2, and Zr/CNC-MnO2 have the same spectra and exhibit a strong blue shift, which may be assigned to an oxygen-vacancy-related defect generated by electron and hole recombination.[37] In this spectra, the highest peak resulted in a higher rate of exciton recombination, and lower PL intensity shows a lower recombination rate which is responsible for lower photodegradation efficiency.[39] Hence, the PL spectra revealed that MnO2 nanorods are promising materials for application in visible and ultraviolet light emission devices.[37]

Figure 3

(a) UV–vis analysis, (b) PL spectra, and (c–f) band gap of CNCs, MnO2, CNC-MnO2, and Zr/CNC-MnO2 nanostructures using a Tauc plot.

The morphological features of synthesized nanoparticles were clarified through HRTEM, as shown in Figure a–d. The nanoclusters of CNCs were observed (Figure a), and the presence of nanorods of MnO2 was confirmed, which is attributed to the hydrothermal reaction time for product formation.[40] The combination of nanoclusters and increase in nanorod diameter was observed in CNC-MnO2 (Figure c). The presence of nanoclusters along with nanorods with high agglomeration confirmed the presence of dopants (CNCs and Zr) with increasing diameter of nanorods, as shown in Figure d. HRTEM results are very inconsistent with XRD. The interlayer d-spacing was determined using HRTEM images, as revealed in Figure a′–d′. The d-spacing calculated through Gatan software for CNCs, MnO2, CNC-MnO2, and codoped MnO2 was measured as 0.35, 0.30, 0.303, and 0.304 nm and were assigned to (002), (211), (310), and (200) planes of CNC and MnO2 (JCPDS card no. 44–0141), respectively.

Figure 4

(a–d) HRTEM images and (a′–d′) d-spacing measurements for CNC, MnO2, CNC-MnO2, and Zr/CNC-MnO2 nanostructures.

The energy-dispersive spectroscopy analysis of synthesized materials provides surface elemental composition, as depicted in Figure S2a–d. The presence of carbon and oxygen peaks confirmed the CNC constituents, and Figure S2b–d confirms the dominant presence of elements Mn, O, C, and Zr. Furthermore, the presence of additional peaks of Na due to NaOH control pH, whereas the rest of the peaks could originate from the sputtered gold coating and the Cu tape and holder used to carry the sample during examination.

XPS investigation confirmed the composition of Zr/CNC-MnO2 nanostructures as O, Zr, and Mn, as shown in Figure S4a–c. All O, Zr, and Mn peaks detected are consistent with previously published data. The binding energies at peak positions of 529.4, 531.3, and 532.9 eV in Figure S4a correspond to lattice oxygen (Mn–O–Mn), Mn–O–H, and H–O–H, accordingly.[51,52] At 182.5 and 184.7 eV, the Zr 3d3/2 and Zr 3d5/2 were obtained, respectively (Figure S4b). Zr binding energy in catalysts was larger than that of pure metal at 180.0 eV, lower than that of ZrO2 at 182.9 eV, and comparable to that of ZrOx at 181.4 eV,[53] indicating successful integration of Zr cations.[54] The XPS spectra of Mn 3p in Figure S4c exhibits two characteristic peaks at 654.2 and 642.4 eV corresponding to Mn 2p1/2 and 2p3/2, respectively.[55]

Figure a illustrates the rate constant for fabricated materials based on pseudo-first-order kinetics. The degradation rate constant for CNC, MnO2, CNC-MnO2, and Zr/CNC-MnO2 was 0.00642, 0.03364, 0.04962, and 0.04302 min–1, respectively. MnO2 degrades the dye up to 96% within the first 120 min. The maximum degradation of 99% was shown by CNC-MnO2, where the CNC due to its adsorbent properties acts as an additive when combined with MnO2, which serves to increase the degradation rate from 96 to 99%.[41] The additive mechanism of degradation of MB by CNC/MnO2 is obtained by two decolorization processes. The first is an electrostatic interaction where positively charged MB molecules get adsorbed on the surface and fibers in interior mesoporous channels function as supplementary ion diffusion routes for the charge/discharge operations, as CNC/MnO2 contains a high number of hydroxyl groups with negative charges provided by CNC.[57] However, using MnO2, adsorbed MB molecules were oxidized and degraded to CO2 or different molecules.[42] As a consequence, by way of CNC/MnO2 decolorization of MB, both electrostatic adsorption and oxidative degradation occurred. Moreover, the high vicinity, excess porosity, and plenty of contact sites with dye facilitated the decolorization impact.[41] Zr/CNC-MnO2 showed 95% degradation, but the reason for less MB degradation in CNC-MnO2 is that Zr is preferred for mixed oxide and has the ability to hold surface hydroxyl groups, which enhances photocatalytic activity, as shown in Figure S1.[43] Therefore, the addition of Zr would possibly have accelerated hydroxyl groups to combine and deal with CNC while rejecting Mn ions due to small atomic radii on the surface of Zr/CNC-MnO2. Hence, the presence of more hydroxyl groups on the surface due to addition of zirconium nanoparticles may enhance the photocatalytic activity due to the fact that the adsorbed surface hydroxyl group on the Zr/CNC-MnO2 surface traps the hole to create a hydroxyl radical, which is a strong oxidant and is proficient at oxidizing many organics.[43,44] This trend can be correlated with PL spectra of samples, as shown in Figure f, where CNCs and Zr/CNC-MnO2 showed the highest PL peak compared to the rest of the samples, resulting in a higher rate of exciton recombination. Moreover, MnO2 and CNC-MnO2 showed lower PL intensity with a lower recombination rate, which is responsible for lower photodegradation efficiency.[39] However, material with rod-like one-dimensional morphology showed the best catalytic oxidation of dyes due to a large surface area with the highest concentration of adsorbed oxygen. The catalytic performance of different morphology of manganese oxide was in descending order such that rod-like > tube-like > flower-like > wire-like > bulk.[27]

Figure 5

Plot of −ln(C/C0) versus time spectra (a) and error bar for photocatalytic activity of CNC, MnO2, CNC-MnO2, and Zr/CNC-MnO2 nanostructures (b).

The rate constant of all samples was determined using the pseudo-first-order equation, , where C is concentration at time t, C0 is initial concentration, and k is the rate constant. These results indicate that CNC-MnO2 shows maximum potential for wastewater treatment.

For photocatalyst application, Table summarizes various MnO2 photocatalysts. The increase in the amount of semiconductor oxide or nanocarbon was due to their tendency to congregate on the surface of MnO2 and obstruct the surface area available for organic dye adsorption.[48] Due to the excess linked materials, the photocatalysts were obscured from the light source, resulting in decreased photocatalytic activity.[49] As a result, our research established a viable method for using hydrothermally produced Zr and CNC-doped MnO2 nanorods. Hence, due to the photocatalytic reactor under solar light, our synthesized materials showed excellent decolorization performance and are highly promising for wastewater dye removal.

Table 1

Literature COmparison of Different Dye Degradation with Various MnO2 Photocatalysts and Organic Dyes

photocatalysts	pollutant	light source	time (min)	degradation (%)	ref
Cu/MnO2	brilliant green	visible	180	73.1	(45)
MnO2/Co3O4/ZrO2	methylene blue	solar	100	68.24	(46)
MnO2/Nb2O4/carbon clusters	methylene blue	visible	180	75.00	(47)
Zr/CNC-MnO2	methylene blue	visible	120	95.94	this work

Photolysis under Dark Conditions

Photolysis (without catalyst) was evaluated by the reduction of MB. An experiment was executed, and a 10 mg suspension of prepared samples was mixed with 30 mL of MB under constant stirring. To confirm the establishment of adsorption equilibrium, the magnetically stirred suspension was kept in the dark for at least 60 min. Afterward, to determine the concentration of MB after contact with visible light at certain time intervals, a 5 mL suspension was obtained for UV–vis absorption. The % degradation graph of prepared samples is shown in Figure . After 30 min of light irradiation, a maximum degradation of 98% was exhibited by CNC-MnO2. MnO2 degraded 95% MB in 30–35 min, Zr/CNC-MnO2 showed 93.42%, while CNC showed the lowest degradation of 31.51% for MB after 35 min of light irradiation.

Figure 6

Plot of % degradation performance.

Reaction Mechanism

For degradation of organic molecules, the measure of reactions through photocatalytic experiments can be termed as follows, and the electronic band structure of inorganic semiconducting materials (CNC, MnO2, and Zr) is given in Figure S1.[50−52] In a photocatalytic reaction, photons of energy E = hν compared to energy larger than the material’s band gap energy is delivered on CNC, MnO2, and Zr. During this process, photoelectrons (e–) are shifted from the lower valence band (VB) to the higher conduction band (CB). The excitation process generates a hole in the VB, thus resulting in one electron–hole pair (e––h+). Figure S1 shows that electrons in the CB of CNC can transfer electrons directly to MnO2 (CB). After reaction with water, photogenerated h+ produces free radical •OH. The •OH radical is a strong oxidant agent formed on the surface of the semiconductor, which selectively targets adsorbent molecules (CNC) or those which are close to the catalyst surface. In this way, biological compounds are attacked and microorganisms are reduced to initiate decontamination. To produce superoxide radicals (O2–), photogenerated electrons are picked up by oxygen molecules and electrons interact with surface-bound water molecules (OH–). Superoxide ions (O2–) provide a hydroperoxylate radical (•H2O), and finally, H2O2 is separated into strongly reactive •OH.[53,54]

During the photodegradation process, the total organic carbon (TOC) contents in the MB solution were dynamically measured. Figure a shows that the TOC of the MB solution treated with Zr/CNC-MnO2 with visible light irradiation decreased regularly with reaction time, and the TOC removal rate at 120 min was 89.7 and 82.2% by CNC-MnO2 or Zr-doped CNC-MnO2, a little lower than that of the MB removal rate (99%, 95%) measured by a decoloring degree. These findings demonstrated that MB was favorably mineralized in the Zr/CNC-MnO2/visible light system, although there was a little residue of TOC. Catalyst reusability is crucial as it allows treatment of industrial wastewater and effluent for the most number of cycles possible. By extracting the utilized catalyst material, reusability of CNC-MnO2 and Zr/CNC-MnO2 nanostructures was investigated. It was washed, dried, and again used for degradation up to three times, as in Figure b. The efficiency of reused catalyst only changes slightly with each cycle. It is worth noticing that the deteriorating efficiency only decreases by a little proportion (2%) after three cycles, indicating that the fabricated nanostructures are quite stable.

Figure 7

(a) Variation of total organic carbon of MB solution during the photodegradation process. (b) Reusability.

In vitro bacterial potential of synthesized materials for S. aureus and E. coli bacteria was evaluated using agar well diffusion via inhibition zone measurement, as shown in Table S1. The result revealed the enhanced antibacterial activity against bacterial strains and synergism of synthesized materials and inhibition areas for statistically substantial (p < 0.05) for low and high concentration. For synthesized materials, the inhibition zone against E. coli ranged from 3.05–7.45 and 3.15–8.10, whereas for S. aureus, the range was 0–2.10 and 1.10–4.00 at minimum and maximum doses, correspondingly. All results were compared with those of ciprofloxacin that was used as a positive control (7.15 and 5.25 mm) against S. aureus and E. coli growth in contrast with DI water (0 mm). Broadly, Zr/CNC-MnO2 revealed high efficacy for G +ve and for G −ve.

The oxidative damage induced by manufactured nanoparticles is dependent on their amount and size. Reactive oxygen species were generated by nanoparticles that confine the bacterial membrane, resulting from cytoplasmic organelles, which is accountable for bacterial death.[10,55] The mechanism by which nanomaterials react with bacterial strains involves a significant contact between cations and negatively charged components of the bacterial membrane, leading toward disintegration of the micropathogen (Figure S3).[56]

To elucidate the observed mechanism in the Zr/CNC-MnO2 complex systems, the Quantum Atomistix ToolKit (quantumATK)[57] package was employed to measure the geometric optimizations and charge transfer calculations. Additionally, the method of local assembly of atomic orbitals was incorporated. Perdew, Burke, and Ernzerhof (PBE) developed the exchange correlation function in conjunction with the generalized gradient approximation (GGA).[58] The PseudoDojo[59] pseudopotential was used to analyze relationships of electrons and ions, as well as the valence electrons. In calculating the self-consistent field (SCF), a threshold limit of 10–6 Ha for energy convergence was used. The geometrical framework and ion relaxation were performed using the Broyden–Fletcher–Goldfarb–Shanno (LBFGS) method with a force on each atom of less than 0.05 eV/Å.

Cellulose is a polysaccharide consisting of many hundreds or thousands of carbon, hydrogen, and oxygen atoms in a straight chain. A dimeric glucose was utilized to simulate the linearly bulky cellulose in this analysis. It is reported in previous theoretical calculations that two units of dimeric glucose are sufficiently reliable and accurate to simulate bulk cellulose.[60] The most stable α-MnO2 nanostructure is employed to form the complex system with dimeric glucose (CNC-MnO2). In this work, we considered one configuration which represents that two O atoms are bonding to two Mn atoms because this configuration is found to be the most stable from previous theoretical works of CNC-ZnO nanoclusters.[60,61] Geometry optimization was carried out, and binding energies were calculated for CNC-MnO2 and Zr/CNC-MnO2 configurations, as illustrated in Figure . The obtained results of binding energy clearly demonstrated that both configurations can be stably formed (−0.50 eV for CNC-MnO2 and −0.20 eV for Zr/CNC-MnO2). Moreover, the Mn–O/Zr–O distances are found in the range from 1.940 to 1.972 Å, noting that the calculated C–O distances in cellulose varies between 1.510 and 1.512 Å. The CNC is stabilized on the undoped and Zr-doped MnO2 nanocluster via O–Mn and O–Mn/O–Zr bonds through the terminal O atoms. The interaction of the CNC with the MnO2 and Zr doping leads to a redistribution of electron density in Zr/CNC-MnO2 complex systems, which is analyzed by calculating the charge density difference isosurface maps, as shown in Figure . For the CNC-MnO2 system, there is a charge depletion around the Mn atoms, whereas charges are accumulated around the oxygen atoms. As the O atoms are more electronegative than Mn atoms, O donates its electrons to Mn to form the Mn–O bonds. For Zr/CNC-MnO2, it is observed that there is less charge depletion around the Zr atom compared to the Mn atom.

Figure 8

Structure of (a) dimeric glucose, (b) MnO2 nanostructure, (c) CNC-MnO2, and (d) Zr/CNC-MnO2 complex systems.

Figure 9

Difference charge density of (a) CNC-MnO2 and (b) Zr/CNC-MnO2 complex systems, where the isosurface value is set to be 0.15 e/Å and the electron density accumulation and depletion are shown in yellow and green, respectively.

Scavenging (DPPH) assay was carried out on undoped CNC, MnO2, CNC-MnO2, and Zr-doped CNC-MnO2 nanorods to evaluate active radical species present in the photocatalyst and their antioxidant potential. The ability of the DPPH free radical to diminish may be evaluated spectrophotometrically by detecting the reduction in absorbance at 517 nm. The DPPH activity of the nanoparticles was shown to increase dose-dependently in this study (Figure ). When compared to CNC at a concentration of 300 g/mL, pristine MnO2 showed a significant scavenging activity (65.33%), which was increased to 75.21% when combined with CNC. MnO2 may produce reactive oxygen species such as OH, O2•–, and 1O2, all of which can interact with the DPPH free radical.[62] Doping Zr into CNC-MnO2 material, on the other hand, reduces scavenging activity. This could include adding Zr to the test sample with CNC, whereas rejecting Mn ions owing to small atomic radii results in increased turbidity and an antagonistic relationship that reduced scavenging activity.

Figure 10

DPPH radical scavenging activity of fabricated nanorods.

Conclusion

The current work investigated the influence of CNC and Zr doping on the elemental and phase constitution, optical characteristics, and morphological aspects of MnO2 nanorods. XRD revealed the monoclinic structure; moreover, the predicted interlayer spacing matched HRTEM data. For the crystallite size of CNC (10.11 nm), MnO2 (36.6 nm), CNC-MnO2 (36.2 nm), and Zr/CNC-MnO2 (36.0 nm), the Debye–Scherrer equation was used. In UV absorption spectra, the CNC absorption peak at 360–375 nm and the MnO2 peak at 348–390 nm indicated a blue shift. The CNC peak was found at 427 nm in the PL spectra, and MnO2, CNC-MnO2, and Zr/CNC-MnO2 exhibit identical peaks. EDS spectra indicated successful CNC and Zr doping in MnO2. Density functional theory explained charge transport and depletion in many configurations. The synthesized Zr/CNC-MnO2 exhibited remarkable bactericidal and photocatalytic efficacy in treating industrially hazardous wastewater and biomedical implementations.