Abstract
Monitoring the surroundings relies on aerospace sensors to provide immediate information on the purity of the air, temperature, humidity, and atmospheric pressure. Aerospace and space mission environmental monitoring essentials sophisticated humidity sensors for aerospace. Humidity sensor efficiency, reliability of data, and future expansion are the challenges. Selection of polymer-based nanocomposites yields innovative humidity sensor hybrid nanocomposites with sensitive, stable, robust, low cost, simple to handle, and real-time features. The research field lacks knowledge regarding the effects of these metal oxides (MOs) on essential polymer composition and humidity sensitivity, despite their ability to increase active sides and surface area for humidity detection. A theoretical investigation was conducted to examine the impact of different MOs on the sensitivity, stability, and electrical characteristics of a Polyvinyl Alcohol (PVA) matrix. The interaction among various MOs and the PVA matrix produces the formation of new active chemical groups, thereby improving the sensitivity and reactivity of PVA. The observed reactivity is associated with the elevated surface energy exhibited by PVA, with the optimal modification observed being the incorporation of Zinc oxide (ZnO) nanofiller. Fabricated sensors with PVA-ZnO based have potential for advancement by integrating recent additional ingredients particularly graphene (G), which may create innovative composite PVA-ZnO-G to immediately enhance the surface characteristics of reactivity, thereby improving humidity detection sensitivity. The PVA-ZnO-G hybrid nanocomposite was synthesized, and its surface and intermolecular characteristics were studied, along with reactivity, hardness, softness, sensitivity and selectivity, were analyzed for humidity sensing. When evaluating the PVA-ZnO-5H2O complex against the PVA-ZnO-G -5H2O complex, it is evident that the latter exhibits superior electronic characteristics. Notably, it possesses a larger dipole moment of 21.0848 Debye, a reduced band gap of 0.503 eV, and a significantly more favorable adsorption energy of -0.7592 eV. The corresponding increase in the equilibrium constant (log K = 12.8380) further indicates that PVA-ZnO-G is thermodynamically practical and exhibit enhanced stability and sensitivity. In particular, the mechanical and thermal properties were enhanced, and the electrical environment was fundamentally altered to facilitate increased H2O interaction because of the hybridization of ZnO and the incorporation of G into the matrix.
Introduction
Many sectors, including aerospace, agriculture, industry, meteorology, and national security, have invested heavily in humidity sensing systems1. There are three primary areas of focus in aerospace Smart Sensor Platforms: the creation of sensor components, their incorporation into the hardware, and the subsequent evaluation of these systems in real-world application settings2. It is critical to create and enhance materials used in humidity sensors, requiring a thorough and organized understanding of the current state of material characteristics and their application3. Novel polymer-based composites are developed through the selective integration of the strengths of various material sorts, concerning polymer/inorganic composites being among the most significant4. To emphasize the potential advantages of nano-sized materials for humidity sensing, it is important to recognize that such substances typically exhibit a highly surface area along with other distinctive chemical and physical properties, particularly MOs nanoparticles, which are classified as highly responsive to humidity5,6. Notwithstanding its electrical qualities, chemical resistance, cost-effectiveness, biocompatibility, water solubility, and extensive availability, PVA composites are rapidly becoming one of the most prominent synthetic polymer composites employed in humidity sensing7,8. Nanoparticles such as ZrO29, Fe3O410, and silver11 play a crucial role in improving the flexibility, sensitivity, and electronic properties of PVA. Furthermore, the exploration of PVA/PEO/CuO12, PVA/MgO/SiC13 nanocomposites for use as humidity sensors encompassed assessments of their electrical properties, reactivity, sensitivity, and changes in intermolecular structure. Molecular modeling serves as a theoretical approach essential for thoroughly examining the compound properties, including molecular geometries14, infrared vibrations (IR)15, molecular energies16, and the physical characteristics of chemical structures17. A range of molecular simulation methods exists for examining the physical and chemical properties of materials, commonly applied in the fields of polymers and MOs chemistry18. In the last decade, density-functional theory (DFT) calculations have emerged as a leading theory for quantum mechanical simulation, now extensively employed for simulating molecular structures16. In specific scenarios, DFT demonstrates superior alignment with experimental outcomes compared to alternative methods19,20.
This study investigates the impact of various nano metal oxides, including MgO, Al2O3, SiO2, TiO2, Fe3O4, NiO, CuO, ZnO, and ZrO2, on improving the reactivity and sensitivity of the PVA matrix. The optimal enhancement of the metal oxides was hybridized with G and synthesized for subsequent applications in humidity sensing. Humidity sensors deliver essential data to the Environmental Control and Life Support Systems, which oversees the spacecraft’s environmental conditions, thereby preventing breakdowns in equipment and preserving the health of astronauts. This study investigates the application of ZnO and G heterostructure within a PVA matrix through the fabrication of their film, which contributes to enhanced charge carrier transportation, and expanded surface area to supply water molecule attraction, thereby additionally improving its response. Afterwards, a PVA-ZnO-G nanocomposite is created by experimental interactions with G and ZnO. Molecular modeling is used to track the impact of ZnO and other MOs on PVA. Additionally, in order to employ the PVA-ZnO composite as a humidity sensor, the impact of G on its electrical characteristics was investigated theoretically. Furthermore, the performance of the sensor was evaluated in the context of a contactless sensing application. The PVA-ZnO-G heterostructure demonstrates responsiveness in contactless sensing, positioning it as a promising candidate. This study emphasizes the capabilities of PVA-ZnO-G heterostructure-based sensors in enhancing humidity sensors and provides key insights into sensors applications in diverse space industries, medical diagnostics, and non-contact sensing.
Calculation details
The model structure that estimated the interactions of PVA with various MOs, such as MgO, Al2O3, SiO2, TiO2, Fe3O4, NiO, CuO, ZnO, and ZrO2, was evaluated using density-functional theory (DFT) calculations. The choice of a basis set is fundamentally influenced by the characteristics of the structure under investigation. The 6-31G(d, p) basis set is commonly used for examining polymer interactions with nanomaterials, especially metal oxides and carbon-based nanomaterials, due to its strong alignment with experimental findings21,22,23. The accuracy of computational methods, functionals, and basis sets typically enhances with more advanced configurations; however, utilizing an overly complicated basis set does not necessarily ensure reliability, as it may lead to an increased probability of overcalculation. Consequently, comprehending the structural characteristics of the system is essential prior to choosing a suitable basis set. The 6-31G function, known as a split-valence double-zeta basis set, accurately quantifies either core as well as valence orbitals. It includes (d) polarization functions for heavier atoms and (p) polarization functions for hydrogen atoms. This improves the depiction of polar bonding connections, rendering it especially beneficial for systems characterized by high electron density, including halogen ions, electronegative molecules, or radical forms. The 6–31(d, p) basis set is appropriately aligned with the electronic and structural characteristics of the investigated system, providing an effective balance between computational efficiency and accuracy. When compared to other computational models, smaller basis sets such as 3-21G and STO-3G provide reduced computational costs; however, they frequently fall short in delivering the precision necessary for accurately modeling intricate hybrid systems, especially in the context of polymer–nanomaterial interactions. On the other hand, larger basis sets like 6-311G++, LANL2DZ, Def2-TZVP, and aug-cc-pVDZ offer a more comprehensive depiction of electronic interactions and charge distribution, which may enhance prediction accuracy. Nevertheless, this can also lead to a considerable rise in computational requirements without guaranteeing equivalent improvements in predictive performance. Furthermore, alternative functionals like M06-2X and PBE0 have demonstrated enhanced performance in accurately capturing dispersion interactions; however, their benchmarking for polymeric and hybrid nanomaterial systems is not as comprehensive24,25. Considering these factors, B3LYP/6–31(d, p) offers an ideal balance of computational efficiency, accuracy, and applicability, positioning it as a suitable option for modeling both the electronic and structural features of the system under investigation. The dependability of the estimated accuracy limitations is significantly influenced by the specific choice of the functional, basis set, and convergence parameters for the system being analyzed. Typically, employing a standard functional such as B3LYP with the 6-31G(d, p) basis set offers an appropriate equilibrium between accuracy and computational expense for typical applications; nevertheless, inaccuracies may be considerable depending on the feature in concern. The anticipated accuracy for B3LYP/6-31G(d, p) computations may fluctuate, with average mean absolute errors (MAE) of around 0.4 eV for bandgaps, 1–5 kcal/mol for adsorption or interaction energies, and roughly 5–15% for the overall dipole moment. The precision is contingent upon the system under examination. The B3LYP/6-31G(d, p) technique typically yields high to exceptional accuracy for standard organic compounds26,27.
The GAUSSIAN0928 program was used to calculate all models at the National Research Centre in Egypt’s Molecular Spectroscopy and Modeling Unit. To compute the structures, the B3LYP/6–31(d, p) model was used29,30,31. To strengthen the anticipated interactions between PVA and MOs (MgO, Al2O3, SiO2, TiO2, Fe3O4, NiO, CuO, ZnO, and ZrO2), modifications in electrical and molecular reactivity features were examined utilizing the Frontier Molecular Orbital (FMO). The FMO highlights the significance of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). This significance involved an extensive examination of critical characteristics for proposed structures, including total dipole moment (TDM), HOMO and LUMO orbitals distribution, HOMO energy (EHOMO), energy (ELUMO), and bandgap energy (ΔEg), as outlined in the following equation that the value of EHOMO and ELUMO represent the input:
The subsequent equation (Eq. 2) describes the electrical conductivity (σ) variable, which was used to study the changes in conductance caused by nanomaterials integration, depending on the energy of the band gap (ΔEg)32:
that the parameters are defined as: the temperature (T Kelvin), constant (A), and Boltzmann constant (KB) is the.
Two further density functions that describe the levels electron dispersion in a material with respect to energy are the total density of states (TDOS) and the partial density of states (PDOS). While the TDOS shows the total of all contributions, the PDOS breaks them down into their component parts to show the electrical structure in greater depth. Therefore, these features were investigated in each structure to figure out information about electron flow and sensitivity. Likewise, to strengthen the understanding of the interaction mechanism, reactivity, and both electrophilic and nucleophilic aspects of the composites under examination, the contour MESP map was assessed for each composite to identify potential attack sites. Important descriptors for investigating durability and sensitivity have been postulated by Mulliken and Koopman33,34. As a first step in developing these descriptions, we use also the input values of EHOMO and ELUMO to derive the ionization potential (IP) as well as electron affinity (EA).
Electronegativity(χ) represents a molecule’s ability to grab electrons, whereas chemical potential (µ) indicates an electron’s ability to give away. Electronegativity (χ) and chemical potential (µ) are related in a reverse manner, which is described as follows35:
Additional significant descriptors include the global hardness (η), thereby is defined as the occurrence of polarization and its susceptibility to the impact of mechanical movement. This property is dependent on the degree that the arrangement of electrons is modified due to the field of charge. On the other hand, global softness (σ) signifies the inverse of hardness36:
The calculation of η and σ, beside ΔNMax were analyzed for all composites using the previously specified equations. Furthermore, the ability of a material to take electrons was assessed using the electrophilicity factor (ω), which was calculated using the subsequent equation37:
The optimum new PVA-ZnO-G composite’s sensitivity, selectivity, and reactivity were evaluated by calculating the adsorption energy (ΔEAds) using the associated equation:
The EAds associated with adsorption are calculated by measuring the energy difference among the absorbed composite and its separate components, namely the water (adsorbate) and the PVA-ZnO-G substrate. In addition, thermodynamic principles are required for the calculation of Gibbs free energy variations in adsorbent systems, stability constant. These features have a role in determining the adsorbate-adsorbent interaction type, as well as the spontaneity and capability for adsorption. This analysis was performed by applying the following equations:
that gas constant (R), and temperature in Kelvin (T).
Materials and methods
Materials
Polyvinyl alcohol (sigma aldrich, Mwt 85000–124000, 87–89%), Zinc (II) acetate dihydrate (Fisher chemical, 99%), and Sodium hydroxide (Fisher chemical, ≥ 97%) for preparing of PVA composite and ZnO nanoparticles. The production of G utilized high-purity chemicals, comprising graphite powder, hydrochloric acid (HCl, 37%), sulfuric acid (H2SO4), hydrogen peroxide (H2O2, 30%), and phosphoric acid (H3PO4). There was no further purification process before using any of the compounds. For this experiment, we utilized Milli-Q water that was recently deionized (DI).
Synthesis of PVA-ZnO-G composite
Synthesis of G and ZnO nanoparticles
The ZnO nanoparticles were processed by the precipitation method in order to be prepared. In a usual manner, a solution of zinc (II) acetate dihydrate (1 M, 100 mL DI water) was added dropwise while stirring to a solution of sodium hydroxide (2 M, 100 mL DI water) till the PH reached 10. This combination was maintained until the temperature of the mixture reached 70 o C for 30 min. In order to remove the precipitate, centrifugation at a speed of 10,000 rpm was used once the temperature had been brought down to room temperature. Following this, it was washed many times with DI water, dried in an oven that was set at 80 o C for 24 h, and then calcined at 500 °C for a period of 2 h. Furthermore, the G preparation was first synthesized in the laboratory via the Hammers procedure. During this procedure, 1 gram of graphite was initially combined with 35 milliliters of H2SO4 and 3 g of KMnO4, and subsequently stirred for approximately 1 h using an ice-water bath at a temperature below 20 °C. Afterward, 30% H2O2 (approximately 105 milliliters) was meticulously incorporated into the solution over the course of an hour and then heated to approximately 100 °C. The comprehensive methodology for the G preparation process can be found in the previous research38.
Synthesis of PVA-ZnO-G membrane
A casting method was used to make the PVA-ZnO-G composite membrane. It proceeded to prepare a 10 wt% PVA solution via dispersing PVA crystals in distilled water with stirring constantly at 85 o C and 1200 rpm until a thick, transparent liquid was produced. At the same time, ultrasonication was used for 45 min to disperse pre-synthesized G NPs and ZnO particles in distilled water in a consistent manner. Next, the PVA solution gradually supplemented nanoparticle dispersion while being continuously stirred by magnetic forces. In order to preserve consistency in quality and composition, a set wt% ratio of 70:20:10 was preserved for the PVA-ZnO-G. To get consistent, flaw-free composite membrane, the resultant mixture was poured onto sterile Petri dishes and let to dry overnight in a vacuum oven set at 65 °C. The membrane was meticulously removed from Petri dish and sliced into the necessary sizes and shapes after they had dried before further analysis.
Characterization techniques
The intermolecular structural, morphological, and chemical characteristics of the produced PVA-ZnO-G composite membrane were assessed using a full range of analytical approaches. The Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectra were recorded in a spectral range of 4000–400 cm− 1 with a spectral resolution of 4 cm− 1 using an FTIR spectrometer (Vertex 70, Bruker) to study the intramolecular structure of the PVA-ZnO-G composite. Bruker D8 Advance diffractometer that operates with Cu Kα radiation (λ = 1.5406 Å) was used to analyze the composites’ crystalline composition and purity using X-ray diffraction (XRD) investigation. By recording how diffraction occurs using 2θ variated from 10° to 80° with 0.02° step, the production of the nanocomposite could be confirmed, and crystalline features could be identified. The PVA-ZnO-G membranes’ microstructural and surface morphology features were initially studied with FESEM on an FEI Nova NanoSEM 450 system. The QUANTA 200 FEG, manufactured by FEI of Japan, was used to perform FESEM in conjunction with Energy-Dispersive X-ray (EDX) spectroscopy in order to achieve elemental composition and higher-resolution image.
Results and discussion
DFT study of PVA-MOs interactions
Building model molecule
Since humidity sensors find extensive use in a variety of industrial settings, it is crucial to have a system for reliably measuring the amount of moisture in a variety of settings and substances39. Since humidity sensors find extensive use in a variety of industrial settings, it is crucial to have a system for reliably measuring the amount of moisture in a variety of settings and substances40. The three primary elements that constitute up a humidity sensing system are the flexible substrate, the hydroscopic substances, and the external interface circuits41. The basis of sensor technology unquestionably lies in the generation and study of hydroscopic substances. The fast advancement of flexible humidity sensors, on the other hand, has contributed to the rise in importance of organic polymeric substances, which provide the benefits of affordable prices and simplicity of handling42. PVA, one of these materials, has demonstrated significant responsiveness in humidity detection; however, it does have limitations, including poor porosity, a low surface area, and high inherent electrical conductivity, which collectively contribute to its reduced sensitivity when used separated43. It was recently reported that in PVA-based composites, the volume of absorbed water may be up to ten times greater than the initial volume of the PVA44. Recently, PVA-based sensors have emerged as the most prevalent type of humidity sensors, owing to their electrical conductivity enhancement that utilizes changes in impedance resulting from water adsorption45. Numerous efforts have been made to improve sensitivity, including the use of porous PVA as sensor films and the doping of PVA with various chemicals to enhance conductivity. The incorporation of MOs into the PVA polymer matrix significantly enhances both the physical adsorption capacity and the sensitivity to water molecules. The study examines the impact of different MOs, including MgO, Al2O3,SiO2,TiO2,Fe3O4, NiO, CuO, ZnO, and ZrO2 nanofillers, on the electrical properties of the PVA polymer matrix, with the objective of improving both ionic conductivity and chemical stability. PVA consists of a chain of carbon atoms linked to the primary functional group, which is hydroxyl groups (-OH). Composition is facilitated by the donating electron couple established through van der Waals bonding, which arises from these -OH active groups. In accordance with the model structure postulate, four monomers of PVA interacting with various MOs were simulated, as illustrated in Fig. 1. The interaction amongst nano MOs and PVA was studied, optimized, and simulated using the DFT: B3LYP/6–31(d, P) model. By monitoring changes in electronic features such as adjusting bandgap, electron transfer ability, sensitivity, and reactivity features, as well as EHOMO, ELUMO, HOMO-LUMO molecular orbitals, and TDM were used to investigate the impact of nano MOs on the electronic and stability characteristics of PVA matrices.
Optemized structure of PVA and PVA interacted with different MOs such as (b-MgO, c-Al2O3, d-SiO2, e-TiO2, f-Fe3O4, g-NiO, h-ZnO, and i-ZrO2).
Frontier molecular orbital (FMO)
To investigate the influence of selected MOs (MgO, Al2O3, SiO2, TiO2, Fe3O4, NiO, CuO, ZnO, and ZrO2) on the electrical and reactivity characteristics of the PVA polymer matrix, calculations were performed to determine the HOMO and LUMO energies, orbital distribution, the bandgap energy difference (ΔEg), and the TDM for all proposed structures46. Figure 2 illustrates the results of an examination about a modification in the configuration of HOMO and LUMO orbitals. Table 1 further presents the values of the investigated model structures’ TDM, EHOMO, ELUMO, and ΔEg gap.
The computed TDM of PVA interactions with the assumed MOs displayed a notable increase for all involved MOs, ascending from 6.4337 Debye in the subsequent order: TiO2 surpasses ZrO2, followed by ZnO, then NiO, with SiO2, Al2O3, MgO, and Fe3O4 trailing behind. TDM elevates as a result of interactions with MOs, arising from the significant electronegativity difference between molecules, which induces alterations in charge distribution and molecular orientation during contact. Furthermore, the incorporation of MOs, particularly TiO2˃ZrO2˃ZnO, enhanced the electronic properties of PVA through substantial dipole–dipole associations that create a polarization effect as a result of the contact. The enhancement of TDM arises from the significant charge separation induced by the pronounced difference, which elevates the material’s sensitivity and reactivity, especially in detection, adsorption, and electrical devices. On the contrary, the advantages of ELUMO indicate that PVA seems to exhibit a greater affinity for electron attraction. The separate difference between EHOMO and ELUMO, as outlined in Eq. 1, represents the HOMO-LUMO gap, which acts as an index for assessing the chemical properties of a material’s durability and sensitivity. A diminished band gap in a compound indicates enhanced sensitivity compared to other compounds, as it necessitates less energy for electron transitions through orbitals. The calculated band gap energy ΔEg, derived from Eq. 1, of the proposed structures decreased from a substantial band gap of 6.9890 eV to a more refined band gap, arranged in the following order: ZrO2 > TiO2 > MgO > SiO2 > NiO > Fe3O4 > Al2O3 > ZnO, with ZnO showcasing the most minimal band gaps. This indicates that ZnO demonstrate the most enhancement MOs on PVA matrix. The observed reduction in the band gap, along with its relationship to conductivity (σ) (Eq. 2), indicates that the electrical conductivity (σ) of the composites, especially those made from PVA-ZnO, exhibits more enhancements. The enhancement of TDM and a reduction in gap energy ∆Eg resulted in improved stability and electrical performance of the polymer matrix, as previously noted47. The combination of semiconductor MOs with PVA enhances electron transport, and the coupling produces intra-bandgap results that reduce the bandgap owing to a decline in the conduction band. As a result, PVA-ZnO showcases excellent electrical characteristics, featuring heightened sensitivity and a resilient structure. This signifies that it is the most refined material crafted for sustainable sensing applications48.
DFT: B3LYP/6–31(d, p) calculated HOMO-LUMO orbital distribution for PVA and its interaction with various MOs, such as (MgO, Al2O3, SiO2, TiO2, Fe3O4, NiO, ZnO, and ZrO2).
Similarly, as seen in Fig. 2, the PVA HOMO orbitals were uniformly distributed along the PVA chain, whilst the LUMO orbitals were concentrated at the active -OH groups; the distribution of HOMO and LUMO orbitals was influenced by the inclusion of MOs. The impact of MgO, TiO2, and ZrO2 does not alter the distribution of HOMO orbitals; however, the distribution of LUMO orbitals has become more localized around the MO atoms. Nevertheless, the impact of various MOs influenced the distribution of HOMO and LUMO orbitals, which are localized around the metal oxide atoms. These MOs exhibit active adsorption regions that engage quickly with water molecules within the bonding adsorption of hydroxyl groups, attributed to their abundance of oxygen vacancy.
Additionally, changes in electronic properties upon interaction with moisture further demonstrate their efficacy as sensitive detectors for humidity. Thus, the integration of MOs into organic polymer materials greatly elevates their ability to attract and hold water molecules. This results in the creation of a refined water molecule within the combined material, enhancing the abundance of electron transport pathways and ensuring a steady capacitance rate of variation49.
Tolal density of States (TDOS)
The TDOS serves as a sophisticated visualization tool, crucial for understanding the electrical conductivity of materials and evaluating their properties. The TDOS elegantly measures the abundance of quantum states for each energy range across the entire system. Figuring out band gaps, conducting a detailed investigation of the electronic characteristics, and evaluating features such as conductivity and reactivity are crucial steps that clarify previous findings regarding HOMO-LUMO orbitals, energies and band gap50. Figure 3. elegantly showcases the estimated TDOS for both PVA and PVA-composites, allowing for a thorough exploration of the nuances in electronic features and physical interactions between the compounds. The alteration of molecular orbitals notably diminishes the LUMO energy, aligning its levels nearer to the Fermi level, suggesting enhanced molecular interactions. The PVA matrix exhibited a heightened tendency for electron transfer due to the enhancement of electronic transformations prompted by molecular orbitals, as evidenced by this variation. Integrating diverse MOs into PVA enhances the LUMO levels, bringing them into closer alignment with the Fermi level and elevating the overall alignment of the LUMO levels. The observations regarding the HOMO-LUMO and band gap are elegantly supported by this behavior, particularly noticeable in the case of ZnO. The following observation harmonizes with our discoveries, revealing that the changes in electron distribution stemming from the interaction between PVA and MOs elevated charge transfer, electrical properties, and conductivity51. The detailed sequence of transformations is crucial for grasping the nanocomposite’s elegance and versatility at the molecular level. This underscores previous discoveries regarding the distribution and energy of HOMO and LUMO orbitals, along with the band gap, suggesting that PVA-ZnO could offer enhanced conductivity, reactivity, and structural stability. Understanding the stability and molecular sensitivity of the nanocomposite requires familiarity with the manner of rearranging molecules. The findings corroborate earlier research regarding the distribution of HOMO-LUMO orbitals, and band gap energy, indicating that PVA-ZnO structures exhibit more reactivity, stability, and conductivity.
TDOS of of PVA and PVA interacted with different MOs such as (b-MgO, c-Al2O3, d-SiO2, e-TiO2, f-Fe3O4, g-NiO, h-ZnO, and i-ZrO2).
Partial density of States (PDOS)
The intricate relationships of bonding, composition, and various electrical interactions can be more profoundly appreciated through a detailed representation of the PDOS calculation, showcasing the contributions of distinct atomic orbitals or components. By highlighting the intricate connections among various atomic orbitals or fragments, including s, p, and d orbitals, PDOS offers an exquisite view of the TDOS. The atomic orbitals engaged in bonding and antibonding interactions can be more elegantly comprehended through the utilization of PDOS. This approach enhances our comprehension of how various components or orbitals influence the compound’s electrical architecture and characteristics52. Figure 4. illustrates that the expected PDOS for the new PVA composites varies according to the magnitudes of the H-1s, C-2p, and O-2p orbital patterns. The composition is elegantly diverse, reflecting the exquisite range of MOs employed within the PVA matrix. The PVA PDOS analysis unveiled that the contributions of H-1s, C-2p, and O-2p are on par with the HOMO values. An exquisite arrangement of carbon, hydrogen, and oxygen atoms extending across the HOMO and LUMO levels is generated when each the O-2p of molecular orbitals of MOs intertwine with the O-2p of PVA orbital interaction. The combination has been validated, that the preceding instance illustrates that the PVA molecule must relinquish a considerable number of electrons to forge elegant connections with molecular orbitals53. The influence of MOs is reflected in the notable increase in the intensity of PDOS peaks elegantly spread across the HOMO and LUMO levels, spanning from − 13 to 0. Surrounding the exquisite crystalline structure, one can behold these magnificent peaks. The interaction between the O-2P atomic orbitals of molecular orbitals and PVA, along with the energy dissipation resulting from molecular orbital binding through a van der Waals physical connection, plays a significant role in this phenomenon. Both events complement each other beautifully, showcasing the influence and engagement of MOs on the PVA matrix.
PDOS of of PVA and PVA interacted with different MOs such as (b-MgO, c-Al2O3, d-SiO2, e-TiO2, f-Fe3O4, g-NiO, h-ZnO, and i-ZrO2).
Molecular electrostatic potential (MESP)
Likewise, MESP is a crucial investigation for quantifying electrostatic potential due to interactions between chemical systems. MESP was employed to examine the sensitivity of a chemical system, elucidating its reactivity and stability. MESP is valuable since it correlates the effects of the overall charge distribution with electronegativity, dipole moment, partial charges, and the sites of chemical reactivity within the proposed structure54. Various MESP contour maps manifest on the molecule’s surface as a color map, with hues arranged from red to yellow. The color differentiation indicated as red on the MESP surface denotes the area of greatest charge density, while the yellow coloration signifies the region of minimal charge density, and the green hue reflects a state of zero electrostatic potential. The most potential is often located in red areas, while the lowest potential is situated in yellow areas. Figure 5 displays the MESP contour map of PVA and PVA reacting with various MOs. Consequently, red spots are employed to measure activity. The PVA map indicated that the PVA active region was predominantly localized near the hydroxyl group of the alcohol. Upon interaction with different MOs, PVA exhibited a red coloration that dispersed throughout the upper and lower terminals of the polymer chain. This result revealed that the reactivity of PVA was enhanced due to its interaction with MOs and the active sites of PVA. Upon interaction with MOs, PVA exhibited localized red areas mostly surrounding the oxygen atom of the MOs. Consequently, the electrical properties and reactivity of PVA enhanced significantly, rendering interaction with MOs more sensitive, stable and reactive for utilization in various applications, including electronics and sensors.
DFT: B3LYP/6–31(d, p) MESP mapping calculations of PVA and PVA interacted with different MOs such as (b-MgO, c-Al2O3, d-SiO2, e-TiO2, f-Fe3O4, g-NiO, h-ZnO, and i-ZrO2).
Reactivity and stability describtors
To thoroughly assess the impact of these MOs on PVA structure, it is essential to quantify several physical qualities associated with durability and responsiveness, such as hardness, softness, and electrophilicity. The influence of MOs on physical and chemical stability, as well as the sensitivity of PVAs, may be elucidated by the examination of these characteristics. These characteristics are primarily ascertained from the measurements of HOMO and LUMO energies, grounded on Mulliken and Koopmans’ assumptions. All examined descriptors are shown in Table 2. At the outset, the calculations for IP and EA were performed using Eq. 3 and Eq. 4, drawing upon the EHOMO and ELUMO, which are profoundly shaped by these variables. An alternative viewpoint suggests that the ionization potential signifies the essential energy needed to extract electrons from a molecule, thereby generating free radicals. On the other hand, electron affinity (EA) is defined as the energy necessary to draw electrons towards a molecule, leading to the formation of negative ions that enhance the organism’s nucleophilicity55. Moreover, electronegativity (χ) represents the exquisite arrangement of electrons within substances. Measurements indicating heightened electronegativity reveal an increased electronegativity, thus amplifying the substance’s sensitivity and capacity for detection. PVA-ZnO exhibited the most enhanced electronegativity structure among the composites studied, boasting an impressive total value of 6.0563 eV. This showcases their responsiveness, highlighted by the band gap data and the MESP result. The chemical hardness (η) of a material elegantly measures its resilience against structural deformation or charge separation56. Moreover, it is commonly understood that hardness (η) stands in direct contrast to softness (σ), a vital attribute for composites possessing delicate qualities. The composite boasts an exquisite blend of softness and more stability, elevating its durability to unparalleled heights. This renders the composite and selection for use in electronics and sensing applications57. This reactivity of PVA-ZnO, gathered outstanding softness ratings due to its enhanced sensitivity and durability features. Ultimately, the molecular reactivity of a substance is characterized by its electrophilicity (ω) and nucleophilicity (ε). The likelihood of a material engaging in the absorption or donation of electrons from its environment is influenced by these two characteristics. The electrophilicity (ω) number is presented as an indication of a substance’s capacity to attract electrons. In contrast, the significance of nucleophilicity (ε), regarded as the opposite of electrophilicity (ω), highlights a substance’s capacity to elegantly donate or transfer electrons in relation to a specific chemical composition58. Substances can be classified into three distinct categories based on their electrophilicity: strong electrophiles, characterized by a ω value of 1.5 eV or greater; mid- electrophiles, which possess an ω value between 0.8 and 1.5 eV; and low electrophiles, identified by an ω value below 0.8 eV59. A low electrophile epitomizes the essence of PVA. Integrating molecular orbitals into composites enhances their electrophilicity, leading to the formation of highly resilient electrophiles, particularly in the interaction between PVA and ZnO. Therefore, it can be concluded that the integration of ZnO nanoparticles into the PVA matrix significantly improved its surface area responsiveness and absorption capabilities. The findings concerning MESP and band gap revealed that PVA-ZnO exhibited heightened reactivity and sensitivity. This not only shows the effects of composition but also highlights that PVA-ZnO is an extraordinarily precious chemical for optical and electrical applications.
DFT study of G interaction with PVA-ZnO
Owing to the features of G, a 2D sp2-hybridized substance, it has physicochemical potential for enhanced electrical, thermal, sensitivity and durability performance in materials. The objective of G’s inquiry was to develop or enhance an environmentally sustainable nanocomposite with superior electrical, thermal, sensitivity and durability characteristics60. To enhance the material’s surface area and porousness, hence facilitating electron transport across the porous structure and augmenting the material’s appeal to electrons and conductive properties, G nanosheets remain an effective reinforcing strategy. A distinctive characteristic of G that enhances its utility in sensing applications is its capacity to modify the bandgap of structures61. Based on prior results, PVA-ZnO exhibited the highest electrical enhancement, stability, and activity, leading to its selection for interaction with the four G. The distributions of HOMO-LUMO orbitals and MESP mapping were analyzed for PVA-ZnO-G to assess the impact of G on PVA-ZnO as shown in Fig. 6. In the comparison of PVA-ZnO-G composite with PVA-ZnO, the distribution of HOMO and LUMO orbitals remained concentrated around the ZnO atoms. Upon examining the contour MESP map, it is evident that the red line areas extend beyond the ZnO to the peripheries of the G surface sheet, hence augmenting the active surface area. This indicates that the composite’s responsiveness, porosity, and electrical conductivity were all improved due to the influence of G. Furthermore, the calculated TDM, HOMO-LUMO band gap, and reactivity descriptors were investigated and listed in Table 3. The TDM of PVA-ZnO was 15.4349 Debye, and the band gap energy (∆Eg) was 0.7935 eV. Upon reaction of PVA-ZnO with four G forms, the TDM was modified to 12.3859 Debye, while the band gap energy (∆Eg) decreased to 0.3276 eV. The electrical properties of the recently developed composite exhibited a comparable reduction in band gap energy (∆Eg), corroborated by the findings of TDOS, which indicated a significant shift in HOMO levels near the Fermi level, varying from − 6.4530 to −5.0380 eV, alongside a redistribution towards the more intense levels. This variation was followed by a significant alteration in the disparity between HOMO and LUMO values. The integration of graphene sheets expands the surface area in relation to the porous structure, consequently increasing both the conductivity and sensitivity of PEO. Additionally, the formation of a hybrid with ZnO and graphene elevates electrophilicity while concurrently improving and transforming the composite into highly stable conductive materials. The findings demonstrated that G enhanced the surface activity of PVA-ZnO. Consequently, the PVA-ZnO-G composite exhibits novel composite characteristics of responsiveness and selection rendering it suitable for sensor applications.
Optimized structure, HOMO-LUMO orbital distribution, and MESP contour of the PVA-ZnO-G calculated utilizing the DFT: B3LYP/6–31(d, p) model.
Experimental result of PVA-ZnO-G composite
FTIR result
The ATR-FTIR spectrometer was implemented to examine the intermolecular composition of the interaction between ZnO nanoparticles and G with the PVA matrix. Figure 7. illustrates the ATR-FTIR transmittance spectra of the analyzed PVA, ZnO, G, and PVA-ZnO-G nanocomposite. Table 4 also presents the band assignments for PVA, ZnO, G, and the PVA-ZnO-G nanocomposite. Initially, the principal unique bands of PVA for the sample manifested as follows. 3350 cm⁻¹ for OH stretching, 2900 and 2880 cm⁻¹ for CH2 asymmetric and symmetric stretching, 1735 cm⁻¹ for the carbonyl group, 1385 cm⁻¹ for CH wagging, 1270 cm⁻¹ for C-C, and two bands at 1090 and 620 cm⁻¹ for C-O stretching and OH bending, respectively. The principal objective is not to classify each structure, but to investigate the influence of ZnO and G on the intermolecular PVA matrix by analyzing the synthesized PVA-ZnO-G nanocomposite. The spectra of the PVA-ZnO-G nanocomposite demonstrates the formation of a composite among PVA, ZnO, and G. The ATR-FTIR spectra of the PVA-ZnO-G nanocomposite demonstrates the typical peak of PVA, where the interaction of OH groups in PVA resulted in a shift to a higher wavenumber at 3264 cm⁻¹ due to their interactions with ZnO and the edges of G sheets62. The notable change observed showed robust interactions and the establishment of hydrogen bonds among the metal oxide, G sheet, and PVA matrix. A subsequent alteration in the CH2 asymmetric and symmetric stretching of PVA affected 2666 and 2885 cm− 1, respectively. The vibrational C-C bands of the G sheet subsequently appeared at 1580 cm− 1. Ultimately, the final PVA bands were seen at 1231 cm− 1 for C-C, 1300 cm− 1 showing CH wagging, 1102 cm− 1 representing C-H in-plane vibrations, and two further bands at 1083 and 639 cm− 1 suggesting C-O stretching and OH bending, respectively. Moreover, the employed metal oxide is seen at 493 cm− 1, confirming the existence of ZnO in the composite, which is influenced by a shift to a lower wavenumber. The band at 493 cm− 1 in the composite revealed that ZnO interacted via acetate, as previously mentioned63.
ATR-FTIR transmittance spectra for the studied PVA, ZnO, G and PVA-ZnO-G nanocomposite.
XRD result
The nanocomposites were further analyzed using XRD diffraction to examine the crystallinity of the samples. Figure 8. illustrates the X-ray diffraction pattern of the PVA-ZnO-G nanocomposite in contrast to pure PVA, ZnO, and G. An extensive, pronounced peak emerged at the scattering angle of 2θ = 19.7°, corresponding to a ‘d’ spacing of 4.48 Å, which may be ascribed to the (−101) reflection plane of crystalline PVA64. The crystalline characteristics of PVA result from the robust interactions among PVA chains via intermolecular hydrogen bonding. The XRD pattern of ZnO nanoparticles reveals the hexagonal wurtzite structure, with crystalline peaks observed at 2θ = 31.72°, 34.37°, 36.21°, 47.49°, 56.55°, 62.80°, 66.34°, 67.92°, and 69.06°, corresponding to the reflection planes (100), (002), (101), (102), (110), (103), (200), (112), and (201), respectively, indicative of the hexagonal phase of ZnO as previously noted65,66. The computed crystallite size and lattice strain for the examined samples, according to the Scherrer equation, are presented in Table 5. The mean crystal dimensions of ZnO and G nanoparticles were ascertained to be 23.4 nm and 11.2 nm, respectively. The crystallite size of the PVA-ZnO-G nanocomposite was around 22.7 nm. Variations in crystal size and strain may result from interactions among PVA, ZnO, and G, attributable to processes such as nanoparticle aggregation and structural deformation.
XRD diffraction pattern for PVA, ZnO, G and PVA-ZnO-G nanocomposite.
SEM morphology
The FESEM analysis of PVA, ZnO, G, and PVA-ZnO-G nanocomposites, illustrated in Fig. 9, examines the alterations in surface morphology and microstructural characteristics of the PVA surface resulting from the interactions and effects of ZnO and G. The homogeneous distribution of spherical particles resulting from the dispersion of ZnO nanoparticles across the PVA surface led to an enhanced surface area and optimal contact, consequently providing an ideal environment for charge transfer67. The effect of G further enhances the surface of PVA due to the presence of layered agglomerates, attributed to the significant van der Waals connections that occur between G sheets, PVA, and ZnO. The aggregation may influence the electrical properties and sensing capabilities of the composite, requiring an ideal balance of graphene content for optimal efficiency68. Highly structural homogeneity, together with high crystalline and microporosity nature, substantiates the enhancement of electrical conductivity and reinforces the composite’s efficacy in environmental applications, including adsorption and sensing. The EDX analysis findings, illustrated in Fig. 9; Table 6, confirmed peaks for C, O, and Zn elements, hence verifying the successful production of the nanocomposite with high elemental purity69. These findings corroborate the theoretical results for band gap and DOS analyses, which suggest that the interconnected macropores are formed by the nanoparticle network generated by ZnO and G within the PVA matrix. Efficient sensing depends on these macropores, which facilitate the passage of adsorbents to interact with the composite surface, particularly in humidity detection. The concordance between both sets of measurements indicates that the PVA-ZnO-G composite holds greater potential as a humidity sensor.
FESEM and EDX of PVA-ZnO-G nanocomposite.
Sensitivity of PVA-ZnO-G as humidity sensor
The fundamental concept for humidity sensing is based on the variation of the electrical signal in response to differing humidity levels. The PVA-ZnO-G composite is noteworthy for humidity sensing because of its substantial surface area, customizable structural design, and enhanced electrical properties. The PVA-ZnO-G composite was selected as a sensing material because of its distinctive electrical properties, as monitoring and regulating humidity is a critical concern for several applications70. Five water molecules were allowed to contact the surface of the PVA-ZnO-G composite in order to mimic the adsorption process, as seen in Fig. 10, in order to examine its responsiveness for humidity sensing and compare it with PVA-ZnO. As is well-known, the humidity sensing approach relies on changes in physical characteristics, with alterations in TDM and HOMO-LUMO band gap energies serving as the key indicators of responsiveness. To evaluate the new composite’s sensitivity and/or improvement in humidity sensing, Table 7 displays the results of the computations of the stability constant (log K), adsorption energy (EAds), and Gibbs free energy (ΔGF) using Eq. 11, Eq. 12, and Eq. 13. The TDM of PVA-ZnO and PVA-ZnO-G was greatly enhanced by the physical interaction with water molecules, indicating that there are strong dipole-dipole connections between the composites and H2O. Additionally, the bandgap ΔEg was reduced, revealing a significant change in the electrical characteristics caused by H2O adsorption in both composites.
The calculated EAds for PVA-ZnO and PVA-ZnO-G were negative, indicating that the adsorption process is advantageous and that the composites attain more stability during this phase of the process. PVA-ZnO-G exhibited a more significant enhancement in stability compared to PVA-ZnO. The novel composite PVA-ZnO-G is effective for humidity monitoring, as the negative values of EAds and ΔGf signify an exothermic and spontaneous physisorption interaction62. According to the findings of the stability constant log K, PVA-ZnO-G shows the most stable structure. The findings from the stability constant calculations and the computed ΔGf show that the PVA-ZnO-G hybrid composite improves chemical stability and responsiveness by creating conductive pathways that allow electron flow along the polymeric framework71. The HOMO-LUMO orbital distribution in PVA-ZnO and PVA-ZnO-G remained unchanged following water molecule contact; the orbitals remained centered on the ZnO atoms. In contrast, the MESP colored maps reveal a notable shift in reactivity, with the reactivity surrounding ZnO and G increasing. In addition to improving the electrical characteristics of PVA-ZnO-G as a humidity sensor, the explored effect of ZnO-G into PVA polymer matrix enhanced significantly improves the structure’s responsiveness and durability composite.
Optemized structure, HOMO-LUMO orbital distrbution and MESP mapping calculations of PVA-ZnO-G with 5 water molecules using the DFT: B3LYP/6–31(d, p) model.
The addition of ZnO to PVA enhances the availability of adsorption sites for water molecules and promotes the development of supplementary hydrophilic functional groups and oxygen-vacancies imperfections on the PVA surface72. Composite materials, created by combining ZnO and G with PVA, enhance their sensing efficacy. The substantial specific surface area and prevalence of hydrophilic functional groups that G provides significantly enhance features, particularly their inadequate linearity and prolonged recovery time73. Due to its elevated concentration of hydrophilic functional groups, PVA-ZnO-G enhances the volume of water detectable by humidity sensors. The sensitivity of a humidity sensor can be augmented by increasing the number of oxygen vacancy defects on the material’s surface, therefore accelerating the conversion of water molecules into conducting ions74. The analysis of the previously described humidity sensing characteristics reveals that the high humidity sensitivity of PVA-ZnO is mostly attributable to the material’s surface oxygen vacancy defects and abundant hydrophilic functional groups. The presence of hydrophilic functional groups (OH−) facilitates the adsorption of additional water molecules onto the surface75. The fast breakdown of H2O molecules is expedited by many surface oxygen vacancy defects, so achieving the objective of improving response speed. Hydrophilic materials exhibit a consistent decline in resistance and a rise in capacitance upon the absorption of water molecules72. The material’s surface is adsorbing a minute quantity of water. Hydrogen ions (H+) can transmit electricity by traversing hydroxyl groups following their dissociation into OH− and H+ due to oxygen vacancy defects. A minute number of electrons was released from the material’s surface as a result of H2O adsorption; these electrons also contribute to conduction during the process73,74. The DFT theoretical study of the PVA-ZnO-G humidity sensor’s detection process provides a clear understanding of the responsiveness of the novel composite, which may be validated by future experimental testing. A significant quantity of surface-generated hydroxyl and oxygen-rich vacancy defects was identified in the experimental data. The rapid destruction of water molecules generates a substantial quantity of conducting ions, facilitated by the synergistic effects of oxygen-rich vacancy defects and an abundance of hydroxyl groups, hence enhancing the adaptability of the humidity sensor76.
The DFT simulations summarized in Table 8 provide a comprehensive view of the intrinsic electronic properties governing water molecule interaction with various sensor materials. The PVA-ZnO-G hybrid exhibits the lowest band gap (ΔEg = 0.503 eV) and the most favorable adsorption energy (EAds = − 0.7592 eV) among the listed materials. This indicates a higher electronic polarizability and stronger interaction with H2O molecules, which theoretically translates to improved sensitivity and responsiveness in humidity sensing. In comparison, PVA-ZnO without graphene shows a higher band gap (1.1777 eV) and weaker adsorption (− 0.6558 eV), confirming that graphene incorporation enhances surface reactivity and electronic conductivity. Other theoretical systems, such as ZnO/graphene composites or pristine ZnO, exhibit weaker H2O binding (− 0.32 to − 0.28 eV), while materials with oxygen-deficient sites, such as Fe2O3, demonstrate comparable adsorption energies (∼−0.75 eV), highlighting the role of surface defects and functional groups in water molecule interaction. These results indicate that the PVA-ZnO-G hybrid is thermodynamically favorable for water adsorption, providing a strong theoretical basis for its high-performance humidity sensing potential.
Table 9 contextualizes the PVA-ZnO-G hybrid relative to experimentally reported ZnO-based composites. Materials such as Ag-doped ZnGa2O4/ZnO and WO3/ZnO composites exhibit rapid response times (1–10 s), low hysteresis (2–7%), and high sensitivity, representing state-of-the-art practical humidity sensors. PVDF/ZnO and graphite/ZnO systems show moderate response times and sensitivity, while graphene flower-based ZnO composites demonstrate ultra-fast response (~ 0.4 s), emphasizing the beneficial effect of graphene in enhancing surface area and water molecule transport. Although PVA-ZnO-G has not yet been experimentally tested, the DFT-predicted properties (ΔEg and EAds) suggest that its intrinsic sensitivity and water adsorption capacity are superior to most materials listed. The combination of ZnO with graphene within a PVA matrix enhances hydrophilic functional group density, oxygen vacancies, and electronic conductivity, which are expected to translate into improved practical performance once experimental characterization is completed. Therefore, the dual analysis of Tables 8 and 9 demonstrates that the PVA-ZnO-G hybrid is both theoretically advantageous and promising for real-world humidity sensing applications, bridging the gap between electronic structure design and experimental sensor performance.
Conclusion
Regarding the purpose of improving the performance of PVA as a low-cost and easily maintained humidity sensor, which is necessary for space applications, a novel composite material called PVA-ZnO-G was developed. Both the results of experiments and the simulations performed DFT indicated that the presence of ZnO and G significantly improved the surface characteristics of PVA and had a significant impact on its electrical, sensitivity, and reactivity properties. With the addition of ZnO and G, the HOMO-LUMO band gap of PVA decreased to 0.3276 eV, demonstrating the highest stability and minimal adsorption energy of −0.7592 Kcal/mol. Additionally, the behavior of the material shifted from that of a broadband gap to that of a narrow band gap. Through the integration of metal oxide with G, the management and/or adjustment of band gaps was made easier, hence paving the way for the utilization of new composites in sensing applications particularly humidity sensing.
Data availability
All data generated or analysed during this study will be available on request (**Corresponding Author Hend A. Ezzat** [**hend.ezzat@nriag.sci.eg**](mailto: hend.ezzat@nriag.sci.eg))
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Maroof A. Hegazy wrote the result and discussion. Hanan Elhaes and Medhat A. Ibrahim: Supervision, Formal analysis. Nadra Nada: Supervision- review & editing. Nadra Nada: Supervision- review & editing. Hend A. Ezzat: Modeling Structure Assignments, Writing – original draft, Writing – review & editing.
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Hegazy, M.A., Nada, N., Elhaes, H. et al. Advanced polymer-based humidity sensor for aerospace applications implementing ZnO-G: theoretical and experimental study. Sci Rep 16, 6339 (2026). https://doi.org/10.1038/s41598-026-35026-2
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DOI: https://doi.org/10.1038/s41598-026-35026-2
