ZnO/CNT nanocomposites were synthesized by incorporating zinc oxide (ZnO) nanoparticles with multi-walled carbon nanotubes (MWCNT), which have 3 - 15 walls and lengths ranging from 1 - 10 µm, procured from M/s. Alfa Aser. Green synthesis was used to synthesize ZnO nanoparticles [12] . Initially, 0.032 g of MWCNTs was dispersed in 50 mL of ethanol using ultrasonic vibrations in a sonicator for approximately 1 hour to ensure uniform dispersion. ZnO nanoparticles weighing 0.968 g were introduced to the mixture following sonication. The solution was heated to 80˚C and then stirred with a magnetic stirrer until the solvent evaporated completely, leaving behind a dry powder. The collected powder was subsequently transferred to a Planetary Ball Milling Machine (Retsch PM 100) and subjected to ball milling for 1 hour to synthesize the ZnO/CNT nanocomposites. A ball-to-powder ratio (BPR) of 20:1 was maintained, and the milling was carried out at 300 rpm. To improve milling efficiency, 1 - 2 drops of acetone were added during the loading process. Before material loading, the ball mill was thoroughly cleaned to avoid contamination. Following an hour of milling, the ZnO/CNT nanocomposite material was extracted for additional analysis and future use.

Various characterization techniques were employed to confirm the structural, chemical, and optical properties of the synthesized ZnO/CNT nanocomposite. The crystalline structure of the material was ascertained by XRD analysis using a CuKα source, and the XRD spectra were used to calculate parameters like strain and crystallite size.

XPS was conducted using a Thermo Scientific Nexsa G2 system to analyze the elemental composition and chemical states present in the material. UV-Vis Spectroscopy was carried out using an Agilent Cary 60 Spectrophotometer to investigate the optical absorption characteristics of the nanocomposite. FTIR measurements were obtained using the FTIR Spectrum 2 (Perkin Elmer) machine, covering the spectral range of 4100-400 cm⁻¹, to determine the functional groups present in the sample. Additionally, TGA was performed using an STA 7300 HITACHI instrument to examine the thermal stability and decomposition behavior of the material. These characterization techniques collectively provide comprehensive insights into the structural and functional properties of the ZnO/CNT nanocomposite.

The photocatalytic dye removal of Methylene Blue (MB) was investigated using ZnO/CNT nanocomposites as the photocatalyst. In this experiment, 2 mg of MB dye was mixed with 50 mg of ZnO/CNT photocatalyst and diluted to a total volume of 100 mL using deionized (DI) water. The prepared solution was kept in the dark for 60 minutes to establish adsorption-desorption equilibrium. The initial absorbance was then recorded, with the maximum absorption peak of MB observed at 662 nm. The dye removal experiment was conducted under UV light irradiation for a total duration of 80.5 hours and under sunlight for 1.5 hours. Changes in the absorption spectra were monitored at regular intervals to assess the photocatalytic efficiency of the ZnO/CNT nanocomposite in the removal of MB dye from the solution.

To determine the structural properties of the ZnO/CNT nanocomposite, Rietveld refinement was performed. The structural evaluation was carried out using the VESTA software, providing detailed insights into the crystal structure. The refinement results for the ZnO/CNT nanocomposite sample include: Chi-Square (χ²): 1.49, Bragg R-Factor: 25.79, Rf-Factor: 13.93. Figure 1 displays the X-ray diffraction (XRD) spectrum of ZnO/CNT nanocomposites. The presence of distinct low intensity peaks in the XRD spectrum suggests that the nanocomposite is nano-crystalline.

However, some minor peaks corresponding to specific crystal planes are observed at different diffraction angles, which are indexed as (101), (102), (110), (103), (004), (112), (200), (201), (104), (202), (203), (114), (210), (104), and (211). These peaks appear at 31.73˚, 40.15˚, 50.29˚, 51.71˚, 57.53˚, 58.34˚, 58.61˚, 60.53˚, 65.53˚, 66.29˚, 74.91˚, 79.78˚, 80.71˚, 80.99˚, and 82.64˚, respectively. The VESTA-generated structural model is shown inset of Figure 1 , which confirms the hexagonal structure of ZnO, with a unit cell volume of 73.399 ų. The Full Width at Half Maximum (FWHM) method was used to calculate additional structural parameters, and the results were as follows: Crystallite size: 21 nm, Microstrain: 0.00534, Dislocation density: 0.0027460 nm⁻². These structural findings provide critical insights into the composition and crystalline characteristics of the ZnO/CNT nanocomposite.

XPS was used to find surface contaminants, elemental composition and chemical states of the ZnO/CNT nanocomposite. The results are illustrated in Figure 2 (a)-(d). Figure 2 (a) shows the wide-range survey spectrum, verifying that the main components are carbon (C), oxygen (O), and zinc (Zn). This overall scan validates the synthesis of the ZnO/CNT nanocomposite with no significant contamination. The quantitative analysis reveals the element’s atomic percentages in the sample, as shown in Table 1 .

The high-resolution C1s spectrum is presented in Figure 2 (b). Deconvolution of this spectrum reveals peaks centered at 284.6 eV (C–C), 285.8 eV (C–OH), 287 eV (C=O) and 288.8 eV (O-C=O), indicating the presence of graphitic carbon and various carbon-oxygen functional groups on the CNT surface, confirming the active fictionalization of CNT with ZnO. Figure 2 (c) displays the O 1s core-level spectrum, where a strong peak is observed at ~530.1 eV, corresponding to lattice oxygen (O^2-) in ZnO. Additional minor features suggest the presence of oxygen vacancies, such as surface-adsorbed species etc., which may enhance photocatalytic activity.

Figure 2 (d) illustrates the Zn 2p spectrum, with well-defined peaks for Zn 2p_3/2 at 1021 eV and Zn 2p_1/2 at 1044 eV, confirming the Zn²⁺ oxidation state. These values are consistent with those reported for ZnO in the wurtzite phase. A minor binding energy shift (~0.95 eV) compared to pure ZnO suggests interaction between ZnO and CNT components. These results confirm the effective integration of ZnO and CNTs in the composite material and support the presence of chemical environments beneficial for photocatalytic dye removal. Similar observations were reported by Li et al. [13] and Hanif et al. [14] , reinforcing the findings.

The absorption behavior and band gap of the ZnO/CNT nanocomposites were investigated using UV–Visible spectroscopy. As illustrated in Figure 3 , the absorption spectrum exhibits a notable peak at 370 nm, indicating the material’s enhanced ability to absorb light in the near-UV region. This red shift compared to pure ZnO suggests strong electronic interactions between ZnO and CNTs, confirming the successful formation of the nanocomposite structure [15] . The Tauc plot, shown in the inset of Figure 3 , estimates the band gap of the ZnO/CNT nanocomposite to be 2.25 eV, which is significantly lower than that of pristine ZnO. This band gap narrowing can be attributed to quantum confinement effects and interfacial charge transfer within the composite. Other researchers have reported similar red shifts and reductions in band gap values. For example,

Zhi et al. [15] observed that pure ZnO had an absorption maximum at 362 nm and a band gap of 3.18 eV, while N-doped ZnO/CNT composites showed a red-shifted peak at 368 nm with a reduced band gap of 2.65 eV [16] . These findings emphasize the impact of composite formation on the absorption characteristics and electronic structure of ZnO-based nanomaterials, which are critical parameters for photocatalytic applications.

FTIR spectroscopy was employed to analyze the functional groups present in the ZnO/CNT nanocomposite, with spectra recorded in the range of 4000 - 400 cm⁻¹, and are illustrated in Figure 4 . A broad absorption peak at approximately 3247 cm^-1 corresponds to the symmetric stretching vibrations of hydroxyl (O-H) groups, indicative of adsorbed water molecules on the ZnO/CNT surface. The peak observed at 1405 cm⁻¹ is attributed to the stretching vibrations of the C-O-H functional group, suggesting the presence of hydroxyl functionalities.

Additionally, a vibration peak at 1032 cm⁻¹ corresponds to the stretching of C-O bonds in carbon nanotubes, for ZnO/CNT nanocomposites. In the lower wave number region, peaks at 475 cm⁻¹, 556 cm⁻¹, and 559 cm⁻¹ are indicative of Zn-O bond stretching, confirming the formation of the ZnO phase within the composite [17] - [19] . Overall, the FTIR analysis provides information for the successful integration of ZnO with CNTs, as evidenced by the characteristic absorption bands corresponding to hydroxyl groups, C-O-H functionalities, and Zn-O bonds.

Thermogravimetric measurements were conducted on the ZnO/CNT nanocomposites to measure its thermal stability and composition. The TGA curve, depicted in Figure 5 , illustrates weight loss (wt%) over the temperature range of 43˚C to 500˚C, revealing distinct stages: (i) temperature range: 76.83˚C - 113˚C: A minor weight loss of approximately 0.25%, likely due to the evaporation of adsorbed water molecules; (ii) temperature range: 113˚C to 145˚C: about 2.04% weight reduction, potentially attributable to the desorption of residual solvents or the decomposition of organic impurities; (iii) temperature range: 145˚C to 290˚C: A further weight loss of 2.799%, which may correspond to the decomposition of functional groups associated with the CNTs; (iv) temperature range: 290˚C to 490˚C: A weight decrease of 1.66%, possibly resulting from the degradation of more stable organic components or the oxidation of CNTs.

The cumulative weight loss observed up to 500˚C was 6.749%. ZnO-CNT nanocomposites exhibited thermal stability up to approximately 230˚C, with weight loss stages corresponding to the decomposition of functional groups and oxidation of CNTs. The study noted endothermic peaks around 233.7˚C and 326.3˚C, attributed to the breaking of physical bonds between functionalized CNTs and ZnO, and the decomposition of CNT functional groups into CO₂, respectively [20] .

The photocatalytic degradation of methylene blue (MB) dye using ZnO/CNT nanocomposites was investigated under ultraviolet (UV) and natural sunlight irradiation. A 6-watt UV lamp served as the irradiation source throughout the experiment. Figure 6 (a) illustrates the degradation profile of MB dye under UV exposure, indicating a gradual decrease in absorbance over time, with complete degradation observed after 80.5 hours. The corresponding signifying reduction in dye concentration under natural sunlight is depicted in Figure 6 (c), which reveals a steady decline in the characteristic MB absorption peak at 662 nm. Figure 6 (b) Figure 6 (d) further provide this trend, demonstrating a consistent decrease in dye concentration from its initial value under UV and sunlight. Tian et al. also show that ZnO-reduced graphene oxide-CNT composites achieved a degradation efficiency of 96% under UV light irradiation for 260 minutes, underscoring the role of CNTs in the photocatalytic activity [21] .

Similarly, Chaudhary et al. [22] also reported that ZnO nanoparticles loaded on multi-walled CNTs exhibited enhanced photocatalytic activity and achieved over 90% MB degradation within a few hours through photoelectrochemical water splitting. The prolonged degradation time observed in the current study, compared to these reports, may be attributed to differences in experimental conditions such as photocatalyst concentration, light intensity, and initial dye concentration (2.56948 mg) [23] . However, the results confirm the potential of ZnO/CNT nanocomposites as an effective photocatalyst for the degradation of organic pollutants like methylene blue under UV light irradiation [24] . This highlights how effective ZnO/CNT nanocomposites can be used for the removal of dye, even when used in small concentrations. However, their performance is remarkable under sunlight. This could be because densely packed or unevenly distributed CNTs may block or absorb some of the UV light, reducing how much light actually reaches the catalyst. Also, when there’s too much catalyst, the active sites might get saturated, limiting further dye breakdown [25] . Beyond a certain amount, the particles may start to cluster, causing light scattering and shielding effects, which makes the photocatalyst less efficient overall [26] .