Zinc acetate (ZnC₄H₆O₄) pure (CDH Laboratory), Hexamine (C₆H₁₂N₄) Extra Pure (Loba Chemie PVT. LTD.), sodium hydroxide (NaOH)AR grade (Thomas Baker), Cetyltrimethyl Ammonium Bromide (CTAB) LR grade (SDFC Ltd.) and Ammonium Molybdate ((NH₄)₆Mo₇O₂₄) tetrahydrate (Qualigens, Thermo Fisher Scientific India Pvt. Ltd.) distilled water and ethanol were used as received without further purifications.

Mo-ZnO@NF was synthesized by adding 1-5 wt% ammonium molybdate to a 0.1 M zinc acetate solution prepared in 150 mL distilled water. To this solution, 0.2 M hexamine and 1 g of Cetyltrimethyl Ammonium Bromide (CTAB) were added. The solution was stirred at room temperature, and 1 M NaOH solution was gradually added drop by drop to maintain a pH of 8–9. After stirring for 30 min, the solution was transferred into Teflon-lined sealed stainless-steel autoclaves and placed in an oven at 180 °C for 12 h. The resulting solution was washed with distilled water and ethanol several times, followed by drying in an oven at 70 °C for 24 h. The dried material was then subjected to calcination for 1 h at 300 °C in a muffle furnace. Plain ZnO was prepared using the same method without adding ammonium molybdate. The synthesized substance was subsequently utilized for further analysis and practical applications. Scheme 1 illustrates the synthesis process of Mo-ZnO@NF.

The crystalline phase identification of the synthesized Mo-ZnO@NF nanomaterials is analysed by X-ray diffractometer using a Bruker D-8 Advance XRD, employing CuKα radiation (λ = 1.54 Å) within a 2θ range of 20°–80°. Uv-Visible spectra recorded using a Jasco V-750 spectrophotometer. Photoluminescence (PL) emission spectra were obtained using a Shimadzu RF-5301pc spectroflurophotometer. Raman spectroscopic analysis was conducted using a Renishaw InVia Raman microscope to differentiate molecular vibrations. Surface morphology was examined via field emission scanning electron microscopy (FE-SEM), utilizing a Hitachi S-4800 instrument at accelerating voltages of 5 kV and 10 kV. For detailed morphological, structural analysis and elemental mapping, high-resolution transmission electron microscopy (HRTEM) was employed, using a JEOL JEM 2100 PLUS instrument. Surface chemistry was analysed using X-ray photoelectron spectroscopy (XPS) with a PHI 5000 Versa Probe III photoelectron spectrometer. The surface area of the material was determined through BET surface area analysis using Autosorbi Quantachrome Inc., United States of America.

The photocatalytic reaction was conducted in a specialized 70 mL cylindrical quartz reactor, precisely sealed to prevent any exchange of air, and equipped with a cooling jacket for efficient water circulation. The solar irradiation intensity, averaging around 145,000 Lux, was carefully controlled. In each photocatalytic experiment, photocatalyst (15 mg) was dispersed within a 25 mL aqueous solution containing 20% methanol (v/v). To ensure homogenous distribution of the photocatalyst, the remaining 45 mL of the reactor’s volume was sealed with a rubber septum and subjected to ultrasonication for 5 min. Prior to initiating the reaction, the reaction mixture underwent nitrogen gas purging to eliminate any residual gases within the reactor. Gas chromatography (GC) was employed to analyse the composition of gases within the reactor’s free space both before and after exposure to solar irradiation, with analyses performed immediately and at specific time duration.

Mo-ZnO@NF was employed to evaluate its catalysis activity through the degradation of methylene blue (MB)dye. The experiment involves, dispersion of catalyst (25 mg) in a 50 mL solution of MB (10 ppm) dye with continuous stirring under sunlight. Throughout the photocatalytic reaction, 2 mL samples were extracted from the flask at the specified time interval (10 min), followed by centrifugation to eliminate suspended catalyst particles. The absorbance of each sample was then measured using a UV-Vis spectrophotometer to determine measure the absorption intensities of MB.

The photocatalytic degradation efficiency of MB in presence of ZnO and Mo-ZnO@NF were determined by using the following Eq. 1a: %   D e g r a d a t i o n = C o − C t C o × 100 (1a) where C _o and C _t represent the original and final concentration of MB before and after photo-catalytic degradation, respectively.

The structural properties of both ZnO and 1%–5% Mo-ZnO@NF samples were analyzed using X-ray diffraction patterns (XRD), as depicted in Figure 1A. The diffraction peaks observed at specific angles, such as 31.74°, 34.41°, 36.25°, 47.49°, 56.65°, 62.89°, 66.43°, 68.02°, and 69.08°, correspond to the crystallographic planes (100), (002), (101), (102), (110), (103), (200), (112), and (201), respectively. These peaks align with the wurtzite structure of ZnO (hexagonal phase) according to the Joint Committee on Powder Diffraction Standards (JCPDS) database reference number 36–1,451. Notably, no distinct Mo-related peaks are obvious, likely attributable to the substitution of Mo⁶⁺ ions into the Zn²⁺ positions within the ZnO lattice. Given the comparable ionic radii of Mo⁶⁺ (0.062 nm) and Zn²⁺ (0.074 nm), the observed shift in the (101) and (100) planes towards lower angles further supports the incorporation of Mo⁶⁺ ions into the Zn²⁺ sites [23]. The shifts in XRD peaks and reduced intensity post-molybdenum (Mo) doping in zinc oxide (ZnO) can be attributed to lattice distortion, phase formation, substitution of Mo ions for Zn, strain induction, and defect generation (Figure 1B). In X-ray diffraction (XRD), peak shifts result from changes in lattice parameters or crystal structure. Initially, small Mo additions cause significant shifts as they disrupt the lattice. However, with increasing Mo percentage, saturation occurs as the lattice accommodates more Mo atoms, leading to diminishing effects on lattice parameters and reduced peak shifts. Beyond a threshold, further Mo additions result in less significant changes in the XRD pattern, despite continued Mo incorporation. The calculated crystallite size of ZnO, 1%Mo-ZnO, 2%Mo-ZnO, 3%Mo-ZnO, 4%Mo-ZnO and 5%Mo-ZnO using Scherrer formula is 18.47 nm, 14.84 nm, 15.33 nm, 13.43 nm, 15.49 nm and 16.39 nm respectively.

Raman analysis was conducted to verify the structural phase of ZnO and Mo-ZnO@NF as shown in Figure 2, revealing consistent findings with XRD data, particularly regarding the hexagonal phase of ZnO. The wurtzite-type ZnO demonstrated optical phonon modes characteristic of the C6v space group, including A₁, 2B₁, E₁, and 2E₂. Polar A₁ and E₁ modes exhibited frequencies corresponding to transverse optical (TO) and longitudinal optical (LO) phonon modes, while the B₁ modes remained silent. The non-polar E₂ mode displayed two frequencies, E₂ (high) associated with oxygen atom motion and E₂ (low) linked to Zn sub-lattice motion [24]. A dominant peak at 436 cm⁻¹ was observed, matching to the nonpolar phonon mode E₂ (high) related with the oxygen sublattice [25], primarily reflecting oxygen atom vibrations. Peaks near 330 cm⁻¹ were assigned to E₂ (high)− E₂ (low), resulting from multi-phonon processes, while the peak at approximately 380 cm⁻¹ corresponds to A1 (TO). The emergence of the E1 (LO) mode suggested the existence of impurities and structural irregularities [26, 27]. Remarkably, the E₂ (high) phonon mode of Mo-doped ZnO samples exhibited minimal positional changes related to ZnO. This stability can be attributed to the fact that the E₂ (high) mode primarily involves oxygen atom vibrations, rendering it less sensitive to mass substitution at the cation site. The introduction of Mo doping effectively compensated for changes in mass and stress within the core, thereby mitigating considerable peak shifts in E₂ (high) [3].

UV–Visible spectroscopy was employed to investigate the optical properties of the synthesized materials, with Figure 3 illustrating the UV–vis spectra of both ZnO and 1%–5% Mo-ZnO samples. ZnO demonstrated absorption in the UV region, typical of its bandgap structure. However, upon Mo doping, a bathochromic shift was observed, indicative of the formation of transitional energy levels within the ZnO lattice. This shift towards longer wavelengths extended the absorption spectrum into the visible region [28], a phenomenon commonly referred to as a redshift or band narrowing effect. These alterations in optical characteristics due to variations in the electronic structure of ZnO induced by metal doping, reflecting the complex interaction between dopant incorporation and the material’s optical response [29].

Photoluminescence spectroscopy is useful method for investing the electronic structure and optical characteristics of semiconductor nanomaterials. By investigating factors like surface oxygen vacancy, defects, surface states and behaviour of photogenerated carriers, it shows key structures insight, which provides progress and enhancement in the performance of nanomaterials. Figure 4 illustrates the photoluminescence (PL) spectra of both pure ZnO and 1%–5% Mo-doped ZnO nanoflowers (Mo-ZnO@NF) excited at a wavelength of 355 nm. Across all samples, two consistent peaks are observed. The UV luminescence peak, cantered at 392 nm, aligns closely with the band edge, indicative of a band gap energy of 3.13 eV, attributed to the free excitonic recombination process within ZnO. Moreover, a secondary peak emerges in the visible range, spanning from 430 to 580 nm, attributed to electron transitions from excited states to the valence band, originating from oxygen defect levels [30]. Despite a gradual decrease in peak intensity and a redshift towards the visible region with Mo doping compared to pure ZnO, the observed redshift in the emission peak is attributed to the presence of Mo/Zn interstitials in Mo-ZnO doping. Furthermore, the reduction in photoluminescence intensity in Mo-ZnO@NF directly correlates with a decrease in the recombination rate of photo-generated electron-hole pairs. This reduction in recombination speed signifies a favorable condition for enhancing the photocatalytic degradation efficiency of Mo-ZnO@NF [31]. The relationship between PL and photocatalytic activity is crucial in understanding the efficiency of photocatalytic processes. High PL intensity often signifies rapid recombination of photoexcited electron-hole pairs, limiting the availability of long-lived charge carriers and subsequently diminishing photocatalytic performance. Conversely, low PL intensity suggests efficient charge carrier separation and reduced recombination rates, indicative of enhanced photocatalytic activity. Changes in PL spectra can offer insights into surface defects, dopant effects, or alterations in the electronic band structure, all of which influence photocatalytic behaviour.

The FESEM images depicted in Figure 5 reveal the unique flower-like morphology of both ZnO and Mo-ZnO@NF, offering detailed insights into their structural characteristics. In the images, small circular ZnO particles with dimensions ranging from 17 to 25 nm are observed, arranged in a flake-like manner, owing to highly porosity. These flakes further aggregate to form the distinctive flower-like structures. Notably, the flowers exhibit a porous nature, with visible spaces and voids between the arranged petal-like structures. The width of the flakes varies significantly, spanning from nanometers to micrometers. The observed morphological features, characterized by the intricate arrangement of ZnO particles into highly porous flakes to flower-like structures leading to high surface area which underscore the importance of nanostructure engineering in tailoring the properties of semiconductor materials for various applications. Additionally, the significant enlargement of flakes in Mo-ZnO@NF highlights the potential for optimizing photocatalytic performance through precise control of morphology and surface area.

HR-TEM is a powerful tool employed to explore the morphology of nanomaterials, giving insights into their structural characteristics at the atomic scale. In Figure 6, Mo-ZnO nanoparticles, with dimensions approximately ranging from 2 to 3 nm, are observed to assemble into distinctive flakes. These flakes further aggregate to construct flower-like structures, showcasing the intricate organization of the nanomaterial. Moreover, elemental mapping conducted within the TEM framework confirms the presence of zinc (Zn), oxygen (O), and molybdenum (Mo) within the nanoparticles. This elemental mapping provides spatial information about the distribution of each constituent element within the nanomaterial, facilitating a comprehensive understanding of its chemical composition and structural arrangement.

X-ray photoelectron spectroscopy (XPS) was conducted to analyse the chemical valance states of various elements and the surface elemental composition in 1% Mo-ZnO@NF. The XPS survey spectrum in Figure 7 revealed the presence of Mo, Zn, O, C in the Mo-ZnO@NF sample. In Figure 7B, Zn 2p spectrum displayed a peak at a binding energy of 1021.45 eV assigned to the Zn 2p _3/2 peak of a Zn²⁺, and another 1044.25 eV assigned to the Zn 2p _1/2 of a Zn²⁺. Notably, metallic Zn with a binding energy 1021.45 eV was not detected, confirming the exclusive presence of oxidised Zn. The Mo 3d XPS spectrum (Figure 7C) exhibits two peaks at 225.1 eV and 234.9 eV corresponding to Mo3d_5/2 and Mo3d_3/2, respectively. Figure 7D presented the O 1s XPS spectrum, where the binding energy at 530.25 eV was assigned to O1s [32–34].

The photocatalytic efficacy of a material is linked to its specific surface area, with a greater surface area typically correlating with enhanced photocatalytic activity. Determination of surface area involves the generation and analysis of N₂ absorption-desorption isotherms, employing the Brunauer-Emmett-Teller (BET) method. In this study, the BET surface area of ZnO, as well as Mo-doped ZnO samples at varying doping levels (1%–5% Mo-ZnO), were calculated as 11.32 m²/g, 22.21 m²/g, 19.35 m²/g, 17.15 m²/g, 15.46 m²/g, and 13.14 m²/g, respectively (Figure 8). The elevation in surface area observed in Mo-ZnO samples underscores their potential utility in photocatalytic applications. This increase in surface area, particularly notable in 1% Mo-ZnO, facilitates greater availability of active sites, thereby promoting interaction between the photocatalyst and target molecules, thus generating a favourable environment for catalytic reactions. Consequently, 1% Mo-ZnO exhibits enhanced photocatalytic performance relative to both pristine ZnO and higher doping levels (2%–5% Mo-ZnO). This observation underscores the pivotal role of surface characteristics in governing photocatalytic activity and highlights the potential of Mo-ZnO catalysts in diverse applications.