Hydrothermal Synthesis of ZnO@MnO2-Montmorillonite Nanocomposites: Influence of Molarity on Structural, Optical, and Photocatalytic Performance toward Ciprofloxacin Degradation under Variable Conditions

A series of ZnO@MnO₂–montmorillonite nanocomposites was synthesized by a hydrothermal process using NaOH solutions of varying molarity (3–9 M) to investigate their structure, optical properties, and photocatalytic performance in ciprofloxacin degradation. Structural analysis confirmed the coexistence of ZnO, MnO₂, and montmorillonite phases with molarity-dependent crystallite sizes (7–34 nm) and changes in defect density.

Optical and textural studies revealed band gap narrowing from 3.29 eV to 2.95 eV, higher oxygen vacancy concentrations at higher molarities, and a shift from micro- to mesoporosity. All samples exhibited strong photocatalytic degradation under UV light (59–61%), with ZMM5 showing the most robust activity across varying dosages, pH, and pollutant concentrations. The system demonstrated high stability and reusability, confirming its suitability for water treatment applications.

The original article

Hydrothermal Synthesis of ZnO@MnO2-Montmorillonite Nanocomposites: Influence of Molarity on Structural, Optical, and Photocatalytic Performance toward Ciprofloxacin Degradation under Variable Conditions

Elisabethe Bezerra, Williams A. Santos Albuquerque, Adilson J. Neres Filho, Alexsandro Lins, Ricardo Barbosa, Luciano C. Almeida, Santiago Medina-Carrasco, Maria del Mar Orta Cuevas, Josy A. Osajima, Pollyana Trigueiro, and Ramón Raudel Peña Garcia*

ACS Omega 2025, 10, 38, 44461–44474

https://doi.org/10.1021/acsomega.5c06454

licensed under CC-BY 4.0

Selected sections from the article follow. Formats and hyperlinks were adapted from the original.

Zinc oxide (ZnO) is a widely recognized semiconductor known for its utility in catalytic processes. Its desirable properties, such as good thermal and chemical stability, affordability, ease of synthesis, and biocompatibility, make it an attractive option for various applications. (1−5) However, the practical utility of heterogeneous photocatalysis processes of ZnO is hindered by the rapid recombination of electron–hole pairs, which negatively impacts their photocatalytic efficiency. Furthermore, ZnO tends to exhibit poor quantum efficiency, insufficient reduction potential, and limited electron mobility. (6) To enhance the performance of ZnO, one promising strategy involves doping or combining it with other semiconductor oxides, such as CuO, NiO, CdS, CeO₂, TiO₂, or MnO₂. This approach creates heterostructures that foster improved charge separation. MnO₂ is notable for its abundance, low cost, narrow energy band, strong oxidative properties, and nontoxic nature. (7) MnO₂ functions as an electron scavenger, effectively inhibiting recombination and prolonging the lifespan of charge carriers due to its advantageous electronic structure and redox properties. (8) The charge transfer mechanisms in ZnO-based heterostructures can be influenced by several factors, including band gap adjustment, interface and surface defects, surface state density, the wavelength of irradiated light, and reaction conditions such as pH, temperature, and sacrificial reagents. (9−13) Understanding and controlling these variables is essential for optimizing the efficiency of ZnO-based photocatalysts in diverse applications, particularly in photocatalytic processes.

Incorporating semiconductor nanoparticles onto solid supports, such as clay minerals, can enhance the degradation of pollutant molecules under UV or visible light while allowing for multiple reuse cycles with minimal loss of efficiency. This approach positions nanocomposites based on clay minerals as environmentally friendly and economically viable options for the remediation of contaminated water. Several studies have reported the beneficial effects of depositing different semiconductors onto various clay minerals. For example, Mahy et al. (14) produced nanocomposites using ZnO or TiO₂ deposited on a natural smectite clay synthesized by the sol–gel method. They found that these materials exhibited high photocatalytic activity, achieving 90% degradation of p-nitrophenol under UVA irradiation over 8 h for the ZnO/Cu²⁺ composites. Liu et al. (15) prepared ternary nanocomposites consisting of Bi₂MoO₆/g-C₃N₄/kaolinite using the solvothermal method and applied them to the photodegradation of tetracycline under visible light. They concluded that the kaolinite-based samples demonstrated superior performance, achieving over 90% drug removal compared to the bare materials. Kharouf et al. (16) developed a TiO₂/kaolin composite for the degradation of phenazopyridine, reporting that the kaolin-based composite was able to remove up to 90% of the drug within 60 min at a pH of 8.7. The authors emphasized the stability of the composite after four reuse cycles. Fatimah et al. (17) synthesized NiO/montmorillonite using a hydrothermal method for the removal of tetracycline. The study found an efficiency of up to 94.2% under UV light and 75.5% under visible light exposure, with good stability observed after five cycles of catalyst reuse.

The combination of ZnO and MnO₂ supported on montmorillonite clay mineral shows significant potential for degrading emerging contaminants, particularly ciprofloxacin, an antibiotic frequently found in hospital and urban wastewater. The heterojunction formed between ZnO and MnO₂ can enhance charge separation and light absorption, while montmorillonite aids in the adsorption of ciprofloxacin onto the catalyst surface, thereby facilitating its photodegradation mechanism. Montmorillonite, a typical 2:1 type clay mineral from the smectite group, is known for its high surface area, ion exchange capacity, and excellent dispersibility in aqueous media. These properties facilitate the incorporation of nanoparticles, preventing agglomeration and promoting effective contact between the photocatalyst and target pollutants, thereby accelerating the reaction rate. (18−20) Additionally, immobilizing these nanoparticles on inorganic supports like clay minerals provides a sustainable method to enhance the stability, dispersion, functionality, and reusability of the catalysts. Montmorillonite is especially effective due to its reactive surface area, expandable layered structure, and adsorbent properties, which enhance the uniform distribution of photocatalytic nanoparticles within its structure.

In this context, developing hybrid nanocomposites made of ZnO/MnO₂ incorporated onto montmorillonite matrix represents a promising solution for the efficient degradation of Ciprofloxacin (CIP) in aqueous environments. Ciprofloxacin is a fluoroquinolone antibiotic widely used in human and veterinary medicine as well as aquaculture, and is frequently detected in urban, hospital, and industrial wastewater. (21−26) Unmetabolized residues of this antibiotic contribute to soil and water pollution. These residues can accumulate in aquatic organisms and food crops, leading to human exposure, toxicity, and the development of bacterial resistance. The environmental persistence of ciprofloxacin and its potential to promote microbial resistance highlight the urgent need for advanced materials and technologies to decrease its prevalence. (27) This study introduces a novel approach by synthesizing nanocomposites composed of zinc oxide and manganese dioxide nanoparticles supported on a montmorillonite matrix. The synthesis will be conducted using a hydrothermal method with varying sodium hydroxide concentrations. Although recent progress has been made in developing new nanostructures, the fabrication of ternary ZnO@MnO₂-montmorillonite composites has not been previously documented. This research will systematically investigate the structural, morphological, optical, and photocatalytic properties of the synthesized material, with a focus on the efficient removal of ciprofloxacin under ultraviolet light irradiation.

2. Experimental Procedure

2.3. Characterization Techniques

The structural and phase characteristics of the synthesized materials were examined using X-ray diffraction (XRD) with a Bruker D8 Advance system and Cu Kα radiation (λ = 1.5406 Å). Fourier-transform infrared spectroscopy (FTIR) was carried out using a Shimadzu IRXross spectrometer equipped with an ATR module. Each spectrum was acquired with 45 scans at a resolution of 4 cm–1, covering the range from 4000 to 400 cm–1. Surface morphology and microstructural details were assessed by scanning electron microscopy (SEM) using a TESCAN MIRA3, enabling high-resolution analysis of particle shape, size distribution, and homogeneity. Nitrogen adsorption–desorption measurements at 77 K were used to evaluate textural properties. The data were processed with NOVAWIN2 software to calculate specific surface area (BET method), pore volume, and pore size distribution (NLDFT or BJH/DFT models). Diffuse reflectance spectra were acquired using a Shimadzu UV-2700 UV–Vis spectrophotometer with an integrating sphere. Optical band gaps were estimated through Tauc plots based on the Kubelka–Munk function. Photoluminescence (PL) spectra at room temperature were obtained with a Horiba-Jobin Yvon Fluorolog-3 spectrofluorometer using 340 nm excitation. Spectral deconvolution was applied to analyze defect-related emissions and to gain insights into the electronic structure.

3. Results and Discussion

3.1. Structural Investigation by XRD and FTIR of the Nanocomposite

Figure 1a displays the X-ray diffraction (XRD) patterns of the ZnO@MnO₂-Montmorillonite nanocomposite synthesized under different molar concentrations of sodium hydroxide (ZMM3, ZMM5, ZMM7, and ZMM9 samples). In addition, the diffractograms of the starting materials are highlighted in the Supporting Information (Figure S1).

ACS Omega 2025, 10, 38, 44461–44474: Figure 1. (a) XRD analysis and (b) FTIR measurements for the ZnO@MnO2-montmorillonite nanocomposite.

The analysis of Figure 1a indicates the coexistence of three well-defined crystalline phases, ZnO (hexagonal, JCPDS 36-1451), MnO₂ (tetragonal, JCPDS 44-0141), and montmorillonite (JCPDS 03-0015), confirming the efficient formation of the hybrid structure. The results indicate that Zn²⁺ or Mn²⁺ cations do not substitute for any cations in the tetrahedral or octahedral sites of the montmorillonite structure. This suggests that the formation of ZnO and MnO₂ phases occurs in situ. Furthermore, the interaction between the nanoparticles and the clay mineral structure takes place primarily on both the internal and external surfaces. (30) In this context, the diffraction patterns exhibit characteristic peaks for each phase, with no detectable secondary phases within the instrument’s resolution, suggesting high phase purity and indicating the selectivity of the synthesis method employed. For the ZnO phase, the peaks at 31.41° (100); 33.90° (002); 36.13° (101); 46.71° (102); 55.82° (110); 62.01° (103); 68.18° (112); and 69.89° (201) are consistent with the hexagonal structure (space group P6₃mc), where the (100) plane exhibits the highest intensity, indicating a preferential crystallographic orientation along this direction.

The absence of residual peaks, such as Zn(OH)₂, further confirms the effectiveness of the calcination process in yielding phase-pure ZnO. The MnO₂ phase exhibits reflections at 34.00° (211); 39.98° (301); 58.72° (521); 70.20° (541); 73.79° (312), consistent with a tetragonal structure (space group P4₂/mnm). The oxidative stability of MnO₂ is evidenced by the absence of reduced phases such as Mn₃O₄, highlighting the effective kinetic control achieved during the synthesis process.

On the other hand, montmorillonite exhibits diffraction peaks at 21.96° (110), 26.46° (quartz), 27.50° (quartz), 28.77° (004), 53.90° (201), and 59.98° (060). The presence of these planes provides structural support to the nanocomposite, serving as a matrix for the dispersion of ZnO and MnO₂ nanoparticles, as further corroborated by the proximity or overlap of peaks with the montmorillonite matrix. This interfacial interaction, mediated by hydroxyl bonds or oxygen bridges, stabilizes the hybrid structure and enhances electronic properties, such as charge transfer between the phases. A careful inspection of Figure 1a reveals the influence of NaOH concentration on the structural parameters of the nanocomposite. The variation in sodium hydroxide concentration during the synthesis of the ZnO@MnO₂-Montmorillonite nanocomposite acts as a key factor in modulating the chemical environment and, consequently, in driving structural reorganization at the atomic and nanometric scales. Considering that the majority phase aligns with ZnO in the nanocomposite, the lattice constants (a and c), bond length (L), dislocation density (δ), and crystallite size (D) were estimated for this phase, using the corresponding equations reported by Soares et al. (31,32) and Castro-Lopes et al. (33) for the ZnO crystal structure.

3.2. Optical Analyses of the ZnO@MnO2-Montmorillonite Nanocomposite

The effect of precursor molarity on defect formation in the ZnO@MnO₂-montmorillonite nanocomposite was also investigated by photoluminescence spectroscopy. Figure S2 (Supporting Information) displays the combined photoluminescence spectra of the ZMM3, ZMM5, ZMM7, and ZMM9 nanocomposites. In contrast, Figure 2a–d presents the room-temperature photoluminescence spectra of each sample separately, enabling a clearer comparison of their emission characteristics. In ZnO-based semiconductors, emissions appear in two distinct regions: a sharp ultraviolet band originating from excitonic recombination at the band edge (NBE), and a broader visible band arising from defect-related transitions. (32,42,43) Analyzing the peak in the UV region of the PL spectra, we see a change in shape and position, which may be related to the effects of precursor concentration on nucleation and growth and the microstructure of the nanocomposite. In this sense, a higher solution molarity increases supersaturation during synthesis, promoting rapid nucleation and resulting in smaller ZnO crystallites. The quantum-confinement effect increases the recombination energy by spatially restricting electrons and holes to reduced volumes, thereby shifting the excitonic peak. At the same time, elevated molarity enhances the density of surface imperfections and heterointerfaces with montmorillonite, which perturbs the local electric field and lattice symmetry; these additional defect-induced energy levels near the conduction or valence band further fine-tune the ultraviolet peak energy.

ACS Omega 2025, 10, 38, 44461–44474: Figure 2. Photoluminescence spectra for the ZnO@MnO2-montmorillonite nanocomposite. (a) ZMM3, (b) ZMM5, (c) ZMM7 and (d) ZMM9. The inset shows the defect concentration quantified by the area under the curve, which was determined by fitting a Gaussian function.

3.3. Morphological and Porosity Studies of the ZnO@MnO2-Montmorillonite Nanocomposite

The morphology of the ZnO@MnO₂-montmorillonite nanocomposites was examined by scanning electron microscopy (SEM), and the microstructural evolution as a function of solution molarity is shown in Figure 4a–d. Across all samples, the clay provides a lamellar scaffold on which oxide crystallites nucleate and grow. As noted, the ZMM3 sample (3 M) exhibits a homogeneous microstructure (Figure 4a), in which large montmorillonite platelets remain structurally intact and well-defined, serving as stable lamellar supports. These surfaces are uniformly decorated with submicrometric aggregates, likely ZnO and MnO₂ nanocrystals, anchored through heterogeneous nucleation. Such a configuration is indicative of a moderate nucleation rate under this molarity, which ensures sufficient supersaturation to initiate oxide precipitation while allowing controlled growth and uniform dispersion across the accessible sites of the layered matrix. This balance preserves platelet morphology and maximizes interfacial contact between the clay substrate and the oxide phases, potentially enhancing the hybrid’s functional properties.

ACS Omega 2025, 10, 38, 44461–44474: Figure 4. SEM micrographs of ZnO@MnO2-montmorillonite nanocomposites synthesized at different molarities: (a) ZMM3 (3 M), homogeneous distribution of nanocrystals over well-defined clay platelets; (b) ZMM5 (5 M), denser coverage and localized coalescence of oxide domains; (c) ZMM7 (7 M), thicker, continuous oxide coatings with partial masking of the clay surface; (d) ZMM9 (9 M), anisotropic rod-like crystallites and heterogeneous aggregates adhering to the lamellar substrate.

4. Conclusions

This work presents a comprehensive approach to defect and interface engineering in ZnO@MnO₂-Montmorillonite nanocomposites synthesized under controlled alkaline conditions. By varying the NaOH molarity from 3 to 9 M, it was possible to tailor the crystallite size, dislocation density, surface area, and defect profile, parameters that directly influence the optical response and photocatalytic behavior of the material. Despite differences in microstructure, all samples exhibited consistent photocatalytic degradation of ciprofloxacin, highlighting their robustness and versatility. The ZMM5 sample, synthesized at 5 M, emerged as the optimal formulation, balancing crystallinity, defect density, and surface characteristics to achieve 61% degradation in 120 min under UV light. Additionally, it maintained high performance across a range of operating conditions (pH, pollutant concentration, and catalyst dosage), demonstrating its potential for real-world wastewater applications. The structural stability confirmed by XRD after three reuse cycles reinforces the material’s reusability and operational resilience. Altogether, the study underscores the importance of molarity control in hydrothermal synthesis as a strategic parameter for tuning photocatalytic nanocomposites and highlights ZnO@MnO₂-Montmorillonite systems as scalable and environmentally promising materials for the treatment of pharmaceutical contaminants.