Electronic and Structural Elucidation of ZnO-ZnS–Aniline Nanocomposite Interactions via DFT and Microscopy Techniques

INTRODUCTION
The organic compound-based semiconductor surfaces are a cornerstone of many technologies, such as chemical sensors, photocatalysis, and the creation of electronic devices. A deep understanding of these interactions at a molecular scale is essential for the intelligent design of advanced materials [1-4]. Aniline, a basic aromatic amine, provides a useful model for exploring how π-conjugated systems interact with semiconductor surfaces. The nitrogen atom’s lone electron pair, along with delocalized π-electron system, makes aniline well-suited for investigating adsorption behaviors. Zinc oxide (ZnO) and zinc sulfide (ZnS) are II–VI semiconductors characterized by their large band gaps and are widely researched for their electronic and optical characteristics. The creation of ZnO/ZnS heterostructures has attracted considerable interest due to their improved photocatalytic performance and adjustable electronic characteristics, positioning them for use in diverse applications, including sensors and optoelectronic components [5-7]. Prior research utilizing Density Functional Theory (DFT) has investigated the adsorption of organic molecules on both ZnO and ZnS surfaces, yielding valuable data regarding adsorption energies, charge transfer phenomena, and alterations in electronic structure. Nevertheless, a investigation incorporating topological, electrostatic, and orbital viewpoints is still lacking [8-10]. In this study, we undertake a comprehensive DFT analysis of aniline adsorption on a ZnO-ZnS hybrid cluster. Through the application of Quantum Theory of Atoms in Molecules (QTAIM), Electrostatic Potential (ESP), Electron Localization Function (ELF), Non-Covalent Interactions (NCI), and Frontier Molecular Orbital (FMO) analyses, we clarify the characteristics of the interaction, the redistribution of electron density, and the resulting effects on the electronic properties of the hybrid system. This diverse methodology offers a complete comprehension of the adsorption process, thereby enriching the broader understanding of organic–inorganic interface chemistry.

MATERIALS AND METHODS
Preparation of ZnO-ZnS nanocomposites
According to the previous work [11], a hydrothermal methodology was employed to synthesize the target materials. Initially, 0.1 mol of zinc chloride (ZnCl2) was introduced into 20 mL of distilled water and subjected to continuous agitation for 1 h. The pH level of the resulting solution was carefully regulated through the gradual addition of sodium hydroxide (NaOH) solution. Following complete dissolution of the zinc chloride, the solution was transferred to a Teflon beaker, which was subsequently sealed within a 50 mL capacity autoclave. This autoclave was then maintained at a temperature of 150 °C for 3 h. In a separate procedure for the synthesis of ZnS, zinc chloride (ZnCl2) and sodium sulfide (Na2S) were combined in a 1:1 molar ratio and dissolved in 20 mL of distilled water. This mixture was also continuously stirred for 1 h. Upon complete dissolution, this solution was transferred to a Teflon beaker and sealed in a 50 mL autoclave. This autoclave was heated at 160 °C for 12 h. Following the hydrothermal treatment, both autoclaves were allowed to cool naturally to ambient temperature. The resulting precipitates from each reaction were then collected and subjected to repeated washing cycles using both water and ethanol. Finally, the washed precipitates were dried at 60 °C to obtain the final products.

Computational Details
All geometry optimizations and wavefunction analyses were performed using Gaussian 16 (Rev. C.01) with the B3LYP functional and a 6-31G(d,p) basis set for all non-metal atoms. For Zn and S, the LANL2DZ pseudopotential was applied to account for relativistic effects. The optimized structures were verified as minima via harmonic frequency analysis (no imaginary frequencies).
The wavefunction outputs were post-processed using Multiwfn 3.8 to perform:
QTAIM analysis: for investigation of bond critical points (BCPs), atomic charges, and delocalization indices;
ESP maps: for investigation of charge distribution and electrostatic complementarity;
ELF analysis: for investigation of regions of localized electron density and lone pair participation;
NCI analysis: using reduced density gradient (RDG) and sign(λ₂)·ρ plots to classify van der Waals and donor–acceptor interactions;
HOMO–LUMO visualization and energy analysis: for examination of orbital alignment and interfacial charge transfer potential.
All visualizations were rendered using VMD 1.9.2 and VESTA 3.5.8.

RESULTS AND DISCUSSION
Structural and morphological characterization
The crystalline structure of the ZnO-ZnS nanocomposite was characterized using X-ray diffraction (XRD), and the pattern is shown in Fig. 1a. The diffraction peaks are observed at 2θ values of 18.81°, 22.02°, 24.41°, 28.94°, 32.06°, 35.06°, 38.06°, and 55.51°, corresponding to the reflection planes (111), (100), (002), (101), (102/220), (110/311), (103), and (112), respectively. These reflections indicate the presence of both hexagonal wurtzite ZnO (JCPDS No. 36-1451) and cubic ZnS (JCPDS No. 05-0566) phases. The distinct and sharp peaks reflect the good crystallinity of the synthesized nanocomposite. Notably, the coexistence of ZnO and ZnS phases in the XRD pattern confirms the successful formation of a heterostructured composite, which is essential for achieving enhanced charge separation and optical performance [12].The broadening of the peaks is also indicative of the nanocrystalline nature, which is further confirmed by electron microscopy results. The SEM image (Fig. 1b) provides further insight into the surface morphology and microstructure of the ZnO-ZnS composite. The surface appears to be uniformly covered by densely packed, quasi-spherical nanoparticles, forming a continuous porous network. This microstructure favors surface adsorption and electron mobility by offering high surface area and effective interfacial contact between ZnO and ZnS grains. Moreover, the absence of large particle agglomerates indicates good dispersion and control over particle growth during synthesis. These features are beneficial for heterojunction-based applications, where charge separation and transport depend on the intimate contact between different semiconductor phases [13]. The combined XRD, TEM, and SEM analyses confirm the successful synthesis of a well-crystallized ZnO/ZnS nanocomposite, exhibiting: coexisting crystalline phases of ZnO and ZnS, nanoparticle morphology with uniform size distribution (~100 nm or smaller), surface roughness and porosity advantageous for molecular adsorption. TEM micrograph (Fig. 1c) reveals the presence of aggregated nanoparticle clusters with an average particle size below 100 nm. The particles appear spherical, with some degree of overlap and surface contact, which is favorable for producing interfacial heterojunctions between ZnO and ZnS domains. The visible contrast between darker and lighter regions may indicate different compositions (i.e., ZnO-rich vs ZnS-rich regions), highlighting the phase coexistence observed in the XRD results [10]. The nanometric dimensions observed in TEM support the XRD conclusion regarding the small crystallite size and confirm the suitability of the material for applications requiring high surface-to-volume ratios, such as adsorption, catalysis, or surface functionalization. These structural attributes make the material an excellent candidate for surface-sensitive applications, including chemical sensing and catalysis, which is consistent with its interaction characteristics observed in your quantum chemical analyses (ESP, QTAIM, NCI, etc.).

 
ESP and ELF analysis
The ESP distribution was calculated for the ZnO-ZnS–aniline hybrid system and visualized using dual isosurfaces at (±0.002 a.u.). The red isosurface corresponds to regions of negative ESP (electron-rich), while the yellow isosurface highlights positive ESP (electron-poor) zones as shown in Fig. 2a. ESP maps are powerful tools for identifying reactive sites, understanding molecular interactions, and predicting non-covalent bonding behavior[14]. The nitrogen atom in the aniline’s –NH2 group is within a negative ESP zone, as shown in Fig. 2a, confirming its lone pair’s role as an electron donor. Zn atoms are surrounded by positive ESP, indicating Lewis acidic character and supporting the possibility of N→Zn donor–acceptor interaction [15]. The π-system of the aromatic ring contributes to a delocalized, slightly negative ESP region, allowing for π–metal interactions [15]. The complementary nature of ESP regions between the adsorbate and surface supports a stable, electrostatically driven adsorption mechanism.
The ELF was computed to assess electron pairing and bond localization within the composite. Isosurfaces at an ELF value of 0.80 a.u. were visualized, as illustrated in Fig. 2b. ELF is a quantum-topological descriptor widely used to reveal bonding character and electron pairing in molecules and solids [16]. Donut-shaped ELF lobes over the aromatic ring are consistent with π-electron delocalization. The –NH2 group exhibits pronounced ELF near nitrogen, confirming the presence of a localized lone pair [17]. Surface oxygen and sulfur atoms show localized ELF regions, while Zn atoms lack ELF features, reflecting their role as electron acceptors. The ELF results validate the ESP-derived conclusion: no covalent bonding is observed, but directional electrostatic stabilization via localized donor–acceptor interactions dominates the adsorption process.

 
NCI and RDG analyses
To further investigate the nature of the interaction between aniline and the ZnO-ZnS surface, a NCI analysis was performed using the RDG approach implemented via Multiwfn and visualized with VMD. This method, developed by Johnson et al.[18], enables visualization of weak interactions in real space and provides insight into their nature—whether dispersive, attractive, or repulsive. The RDG scatter plot (Fig. 3a) provides a quantitative distribution of interaction types: The sharp peak near sign(λ₂)·ρ ≈ 0 corresponds to dispersion interactions, affirming the dominance of van der Waals contacts in the system. The blue shoulder extending into negative sign(λ₂)·ρ values (~ –0.02 to –0.03 a.u.) confirms the presence of moderate attractive interactions, such as N···Zn electrostatic coordination [18, 19]. A small population at positive sign(λ₂)·ρ indicates repulsive contacts, but their relative weakness suggests that they do not hinder overall complex stability. Fig. 3b shows the NCI isosurface of the ZnO-ZnS–aniline, mapped at an RDG value of 0.5 a.u. and color-coded by sign(λ₂)·ρ. This descriptor combines the electron density (ρ) and the second eigenvalue (λ₂) of the Hessian matrix of the electron density to classify interaction types: Green isosurfaces indicate van der Waals (vdW) interactions, which are weak and non-directional. These interactions are broadly distributed along the contact region between the aniline π-system and the ZnO-ZnS surface—highlighting the importance of dispersion forces in stabilizing the adsorbed geometry [18, 20]. A blue region is observed between the nitrogen atom of aniline and a nearby Zn atom, suggesting a localized, attractive donor–acceptor interaction, consistent with lone pair donation (N → Zn). This is in agreement with previous findings from ESP and ELF analyses, which showed a region of electron localization at the nitrogen site and electrostatic complementarity with positively polarized Zn centers [21, 22]. Red isosurfaces, indicating repulsive interactions, are sparse and confined to limited H···O/S contacts. Their minimal extent confirms a sterically favorable adsorption geometry, with negligible Pauli repulsion. The combination of NCI isosurface and scatter plot confirms a multi-modal interaction mechanism for aniline adsorption on the ZnO-ZnS surface: Localized attractive interactions (blue zone) signify site-specific electron donation (N → Zn), likely a Lewis acid–base interaction. Dispersive forces (green region) provide a broad, stabilizing π-surface interaction, playing a critical role in non-covalent surface anchoring [23]. Minimal steric hindrance (red zones) confirms well-optimized adsorption geometry. These findings reinforce conclusions from prior ELF and ESP analyses and highlight the importance of weak, non-covalent forces in determining the stability of hybrid organic–inorganic systems[22].

QTAIM Analysis
Fig. 4a shows QTAIM atomic graph of aniline: The QTAIM molecular graph of aniline in isolation is depicted, illustrating atomic charges (q(A)) and bond paths (indicated by solid lines) connecting atomic nuclei. The nitrogen atom (N1) carries a significant negative charge (–0.969 a.u.), which aligns with its capacity to act as a lone pair donor. The C–C bond paths constitute a continuous hexagonal structure, with bond ellipticities in the range of ~0.14–0.18, which are indicative of π-electron delocalization. All C–H and N–H bonds are characterized by elevated electron density and strong covalent character (ρ > 0.26 a.u.), as anticipated for a stable aromatic amine. Fig. 4b illustrates QTAIM atomic graph of ZnO-ZnS samples. This figure presents the surface topology of the ZnO-ZnS cluster, detailing the connectivity and charge distribution among Zn, O, and S atoms. Zinc atoms display positive charges, ranging from +0.93 to +1.11 a.u., while oxygen atoms exhibit substantial negative charges (–1.13 a.u.), and sulfur atoms show moderate negative charges (–0.75 a.u.). The presence of Zn–O and Zn–S bonds is clearly demonstrated through bond paths; these bonds are characterized by low electron density (ρ) and positive Laplacians, confirming their polar ionic nature. Fig. 4c depicts QTAIM molecular graph of aniline adsorbed on ZnO-ZnS. This detailed illustration combines the aniline molecule with the ZnO-ZnS surface, highlighting key interactions: a newly formed bond path (BCP) is observed between N9 (of aniline) and Zn1, providing evidence for a donor-acceptor interaction (ρ = 0.075 a.u., ∇²ρ = +0.278 a.u.). The π-system of aniline maintains its ring integrity, showing no distortion or shift in the ring critical point (RCP). The persistence of all Zn–O/S bonds suggests that the surface structure remains stable following aniline adsorption.
Table 1 presents a summary of the bond characteristics observed in this study. Analysis of the topology of aniline adsorbed on the ZnO-ZnS surface reveals significant electronic rearrangement following adsorption, particularly between the nitrogen atom of aniline and the Zn1 atom located on the surface. A new bond critical point (BCP) is formed between Zn1 and N9, characterized by an electron density (ρ) of 0.075 a.u., a Laplacian of the electron density (∇²ρ) of +0.278 a.u., and an ellipticity of 0.0487. These values suggest a non-covalent donor-acceptor interaction, typically categorized as a closed-shell electrostatic or coordinate bond.[19]. This finding supports previous ESP and NCI results that revealed: a negative ESP region around nitrogen (electron-rich), a positive ESP region around Zn (electron-deficient), a blue NCI region at the same N···Zn interface. The relatively low electron density (ρ < 0.1 a.u.) and positive Laplacian at the Zn–N BCP confirm the non-covalent nature of the interaction, yet indicate significant charge transfer stabilization.

Frontier molecular orbital and binding energy
Fig. 5 illustrates the optimized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) configurations for aniline, ZnO-ZnS, and the ZnO-ZnS–aniline hybrid system. To further probe the chemical behavior of the ZnO-ZnS–aniline system, including its reactivity, stability, and charge transfer properties, we calculated global conceptual DFT descriptors derived from frontier molecular orbital energies. These descriptors encompass the ionization potential (IP), electron affinity (EA), chemical potential (μ), global hardness (η), electrophilicity index (ω), and global softness (S). These results are presented in Table 2. These descriptors, rooted in conceptual DFT principles [24], offer valuable insights into the reactivity of a molecule or complex towards nucleophilic and electrophilic interactions, with their values undergoing notable changes upon adsorption. 
HOMO–LUMO gap and global hardness (η): Aniline exhibits the largest HOMO–LUMO gap (5.43 eV), suggesting substantial electronic stability and limited reactivity. The ZnO-ZnS surface displays a gap of 4.23 eV, which is characteristic of wide-bandgap semiconductors [25]. The hybrid system (ZnO-ZnS–aniline) shows a significant reduction in the energy gap to 2.73 eV, indicating enhanced reactivity attributed to orbital polarization and interaction effects. Consistently, the global hardness (η = ½ ΔE) decreases across the sequence (aniline: 2.72 eV → hybrid: 1.36 eV), indicating a reduced resistance to charge transfer [26], as detailed in Table 3.
Chemical potential (μ), IP, and EA: The chemical potential (μ) becomes increasingly negative upon adsorption (shifting from –2.65 eV in aniline to –6.11 eV in the hybrid), suggesting a greater electron-attracting capability. The ionization potential (IP = –EHOMO) increases substantially, rising from 5.37 eV in aniline to 7.47 eV in the hybrid, demonstrating significant HOMO stabilization. Conversely, the electron affinity (EA = –ELUMO) increases sharply from –0.06 eV (aniline) to 4.75 eV in the hybrid, indicating enhanced LUMO stabilization and an increased capacity to accept charge, a typical indicator of surface-induced polarization [27]. Electrophilicity index (ω) and global softness (S): The electrophilicity index (ω = μ²/2η), which quantifies a system’s ability for accepting electron density, increases from 1.29 eV (aniline) to 13.67 eV in the hybrid, suggesting a shift from a weakly to a strongly electrophilic system [28]. This is further supported by the increase in global softness (S = 1/η) [29], which nearly triples upon adsorption, consistent with enhanced electronic flexibility, as presented in Table 3.. These trends collectively indicate that aniline adsorption on ZnO-ZnS alters the overall reactivity profile, leading to increased electronic softness, enhanced charge acceptance, and greater electrophilicity. These characteristics are crucial for interfacial charge transfer processes, which are vital in surface sensing applications, catalysis, and optoelectronic devices [30].

CONCLUSION
In this study, the adsorption behavior of aniline on a ZnO-ZnS hybrid surface was investigated using a combination of ESP, ELF, and NCI analyses based on DFT calculations. These complementary methods provide a comprehensive understanding of the electronic and interaction landscape governing the stability and orientation of the adsorbed complex. QTAIM analysis reveals the formation of a bond critical point between aniline’s nitrogen and a Zn atom on the surface, indicating a non-covalent donor–acceptor interaction. Frontier molecular orbital analysis shows significant spatial and energetic separation of HOMO (localized on aniline) and LUMO (on ZnO-ZnS), with a reduced band gap for the hybrid, supporting a charge-transfer adsorption mechanism. The integrated findings provide a complete electronic and topological profile of the interaction, reinforcing the importance of weak forces in adsorption and suggesting potential utility in surface-based electronic and sensing devices. Overall, these results indicate that the adsorption of aniline onto ZnO-ZnS is governed by a synergistic balance of non-covalent interactions, rather than strong covalent bonding. 

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.