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Article

DFT-Based Investigation of Pd-Modified WO3/Porous Silicon Composites for NO2 Gas Sensors: Enhanced Synergistic Effect and High-Performance Sensing

by
Xiaoyong Qiang
,
Zhipeng Wang
,
Yongliang Guo
and
Weibin Zhou
*
College of Electronic Information and Automation, Tianjin University of Science and Technology, Tianjin 300457, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(5), 570; https://doi.org/10.3390/coatings15050570
Submission received: 23 April 2025 / Revised: 7 May 2025 / Accepted: 8 May 2025 / Published: 9 May 2025
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Abstract

:
Pd-WO3 coatings on porous silicon (PSi) substrates are engineered to enhance interfacial charge transfer and surface reactivity through atomic-scale structural tailoring. This study combines first-principles calculations and experimental characterization to elucidate how Pd nanoparticles (NPs) optimize the coating’s electronic structure and environmental stability. The hierarchical PSi framework with uniform nanopores (200–500 nm) serves as a robust substrate for WO3 nanorod growth (50–100 nm diameter), while Pd decoration (15%–20% surface coverage) strengthens Pd–O–W interfacial bonds, amplifying electron density at the Fermi level by 2.22-fold. Systematic computational analysis reveals that Pd-induced d-p orbital hybridization near the Fermi level (−2 to +1 eV) enhances charge delocalization, optimizing interfacial charge transfer. Experimentally, these modifications enhance the coating’s response to environmental degradation, showing less than 3% performance decay over 30 days under cyclic humidity (45 ± 3% RH). Although designed for gas sensing, the coating’s high surface-to-volume ratio and delocalized charge transport channels demonstrate broader applicability in catalytic and high-stress environments. This work provides a paradigm for designing multifunctional coatings through synergistic interface engineering.

1. Introduction

Surface-engineered coatings with multifunctional capabilities are pivotal in addressing emerging challenges across environmental monitoring, industrial safety, and healthcare. Among these applications, nitrogen dioxide (NO2) detection stands out due to its critical impact on air quality and human health, driving demand for advanced materials that combine high sensitivity, selectivity, and operational stability under real-world conditions. To meet these demands, hierarchical coating architectures have emerged as a promising paradigm, where tailored interfacial properties and nanostructural design synergize to enhance performance. Among these coating materials, metal oxide semiconductors (MOSs) such as TiO2 [1], ZnO [2], V2O5 [3], SnO2, CuO [4], and WO3 [5] have emerged as promising candidates owing to their tunable electrical properties, high surface reactivity, and compatibility with microfabrication processes [6,7]. However, conventional MOS-based sensors face persistent challenges, including sluggish adsorption/desorption kinetics, prolonged response/recovery times, and insufficient selectivity and long-term stability. These limitations hinder their reliability in practical settings, necessitating innovative strategies to enhance both electronic and structural performance [8,9].
Porous silicon (PSi) has gained prominence as a substrate due to its hierarchical nanopore architecture and high surface-to-volume ratio, which provide an ideal platform for anchoring functional MOS coatings [9,10,11]. The tunable surface chemistry of PSi (e.g., Si–H and Si–OH bonds) facilitates strong interfacial interactions, as demonstrated by Kumar et al. in their development of Pd/SiC nanocoatings for hydrogen detection [12]. Despite such advancements, standalone PSi-supported coatings often exhibit inadequate selectivity and stability for practical NO2 detection, particularly in complex gas environments [13].
Among MOSs, n-type WO3 stands out for its ultrahigh sensitivity to NO2, attributed to its pronounced conductivity modulation upon gas adsorption. While WO3 exhibits exceptional thermal stability and surface reactivity [14], its performance is limited by poor charge transfer kinetics, leading to poor selectivity between NO2 and interfering gases (e.g., H2S, NH3) and suffering from performance degradation over time [15,16,17,18]. To address these gaps, surface modification strategies such as heterostructure engineering and noble metal decoration have been widely explored [19,20]. Specifically, noble metal modification, such as Pd nanoparticle decoration, has emerged as a powerful approach to modulate interfacial reactivity and electronic properties, offering a promising route to enhance interfacial properties through d-p orbital hybridization and defect engineering, alongside catalytic activation and spillover effects [21,22,23,24]. For instance, Cui et al. demonstrated that Pd doping induces metallic behavior in HfSe2 monolayers upon NO2 adsorption, significantly altering electronic properties [21], while Li et al. achieved temperature-dependent dual selectivity in Pd/Au-decorated SnO2 nanosheets through tailored surface interactions [23]. Nevertheless, the atomic-scale mechanisms underlying Pd-induced electronic restructuring in WO3 coatings, particularly on porous substrates, remain underexplored, limiting rational design of high-performance sensors.
Building on advancements in porous substrate engineering and noble metal modification, this study develops Pd-decorated WO3 coatings on PSi (Pd-WO3/PSi) to bridge atomic-scale design with macroscopic functionality. The hierarchical porosity of PSi enables conformal growth of WO3 nanorods (50–100 nm diameter), while Pd nanoparticles (15%–20% surface coverage) strengthen interfacial Pd–O–W bonds, reducing the bandgap and amplifying electron density at the Fermi level by 2.22-fold. Systematic computational and experimental investigations reveal that Pd-induced d-p orbital hybridization near the Fermi level (−2 to +1 eV) enhances charge delocalization, optimizing interfacial charge transfer and adsorption energetics. Experimentally, it demonstrates a <3% performance decay over 30 days under combined thermal (25–300 °C) and humidity (45 ± 3% RH) stress, outperforming unmodified WO3/PSi. The hierarchical architecture and interfacial stability of Pd-WO3/PSi extends its applicability to catalytic and high-stress environments. By integrating DFT-guided insights with durability validation [25,26,27,28], this work advances the fundamental understanding of TMO interactions in porous coatings and establishes a blueprint for multifunctional material design through synergistic interface engineering [29,30].

2. Materials and Methods

2.1. First-Principles Calculations

First-principles calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) code within the DFT framework. The generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional was employed to describe exchange correlation interactions, while DFT-D3 van der Waals corrections accounted for dispersion forces. A plane-wave basis set with a kinetic energy cutoff of 571.4 eV was used, and Brillouin zone integration employed a 1 × 1 × 1 k-point grid for geometry optimization. This selection was validated through convergence testing (Table S1), which demonstrated that total energy differences between 1 × 1 × 1, 2 × 2 × 2 and 3 × 3 × 3 grids remained below 6 meV, with negligible atomic force (<0.03 eV/Å) and bond length variations (<0.01 Å). Convergence thresholds were set to 1 × 10−5 eV/atom for energy and 0.03 eV/Å for maximum force. To address the morphological simplification of the computational model relative to experimental nanorod structures, the DFT framework focuses on capturing atomic-scale interfacial mechanisms rather than replicating exact experimental morphologies. Structural models were constructed as follows: (i) PSi lattice was defined with lattice constants a = 10.86 Å and c = 5.43 Å; (ii) WO3 lattice was initialized with a = 7.46 Å and c = 3.83 Å. The WO3 layer was positioned within PSi pores, and a 15 Å vacuum layer along the z-direction minimized periodic interactions.

2.2. Synthesis of Pd-WO3/PSi Composite and Its Gas Sensing Measurements

The fabrication of Pd-WO3/PSi composites (Figure 1) was carried out using a meticulously optimized multi-step protocol under controlled conditions. A p-type (100) silicon wafer substrate (resistivity: 10–15 Ω·cm; thickness: 400 ± 5 μm) was sequentially cleaned via ultrasonic treatment in boiling acetone (80 °C, 10 min), anhydrous ethanol (5 min), and 10% HF solution (1 min) to remove organic residues and the native oxide layer, with ultrapure water rinsing and N2 drying between steps. PSi formation was achieved by electrochemical etching in a hydrofluoric acid (HF, 48 wt.%)/N, N-dimethylformamide (DMF, 99.5 wt.%) solution (1:3 v/v) under constant current density (100 mA cm−2) for 5 min, yielding uniform nanopores. For WO3 synthesis, a 100 ± 5 nm tungsten film was deposited via DC magnetron sputtering (base pressure: 5 × 10−5 Pa, Ar flow: 50 sccm, power: 100 W) and oxidized in a tube furnace (500 °C for 30 min, 5 °C min−1 ramp rate, 20% O2/Ar atmosphere, 100 sccm total flow) to form crystalline WO3. Pd modification was performed by sputtering Pd nanoparticles (NPs) (110 W power, 0.5 Pa working pressure, 10 s deposition time) at 25 °C.
Interdigitated Pt electrodes (finger width: 150 μm, spacing: 25 μm) were deposited via electron beam evaporation (0.2 nm s−1 deposition rate) to achieve a 20 ± 5 nm thickness, completing the sensor device. All fabrication parameters were rigorously controlled across three independent batches to ensure material reproducibility. Device-to-device variability was quantified through triplicate measurements, with <10% batch-to-batch variation observed in key metrics. Sensor performance was evaluated using a custom gas sensing system (Figure 2) with precise temperature control (25–300 °C) and gas concentration regulation (0.05–500 ppm). For long-term stability testing, sensors underwent cyclic NO2 exposure (3 cycles/day) at 25 ± 1 °C, with each cycle comprising 10 min of NO2 exposure followed by 10 min of recovery in synthetic air. Data were recorded every 24 h over a 30-day period. The sensor response (R = Ra/Rg, where Ra and Rg represent the resistance in air and target gas, respectively) was measured at 25 ± 1 °C and 45 ± 3% relative humidity. Response (τres) and recovery (τrec) times were defined as the durations to reach 90% of the maximum response and baseline recovery, respectively. Error bars in the gas sensing performance plots reflect the standard deviation of triplicate measurements.

3. Results and Discussion

3.1. Morphology Analysis

The SEM images in Figure 3a–c reveal the hierarchical nanostructural features of the Pd-WO3/PSi composite. The PSi substrate (Figure 3a) exhibits uniformly distributed pores with smooth walls and diameters of 200–500 nm. Figure 3b demonstrates WO3 nanorods grown on PSi, exhibiting highly ordered alignment along pore walls. These nanorods predominantly grow at certain angles near pore openings, with diameters of 50–100 nm and lengths of 1–2 μm. The nanorods display smooth surfaces and minimal defects, indicative of high crystallinity. This ordered architecture facilitates directional charge transport while maximizing surface reactivity through its high surface-to-volume ratio. Figure 3c details the Pd-WO3/PSi morphology, where Pd NPs are uniformly dispersed on WO3 nanorod surfaces without aggregation. The cross-sectional SEM image (inset of Figure 3c) reveals a dense WO3 nanorod layer conformally coated on PSi pore walls. This architecture minimizes interfacial stress during thermal cycling, as evidenced by 95% structural retention post-testing. Statistical analysis across multiple SEM images confirms 20–30 Pd NPs per WO3 nanorod, corresponding to 15%–20% surface coverage. This optimized distribution balances active site density with WO3 surface accessibility.
EDS analysis (Figure 3d,e) reveals distinct compositional differences between WO3/PSi and Pd-WO3/PSi. For WO3/PSi (Figure 3d), the spectrum exhibits dominant peaks at ~1.75 keV (W) and ~0.5 keV (O), confirming the tungsten oxide phase, along with minor contributions from Si (~1.75 keV) and trace C (~0.25 keV) likely due to environmental contamination. In contrast, Pd-WO3/PSi (Figure 3e) shows additional peaks, characteristic of Pd, alongside retained W and O signatures. The absence of Pd peaks in WO3/PSi and their unambiguous presence in Pd-WO3/PSi conclusively demonstrate successful Pd incorporation into the composite system.
Figure 4 presents the TEM characterization, selected area electron diffraction (SAED) patterns, and EDS elemental mapping of the Pd-WO3/PSi composite. The TEM image (Figure 4a) reveals WO3 nanostructures with uniformly distributed Pd NPs (average size: 25 ± 5 nm) anchored on their surfaces. The Pd NPs exhibit well-defined crystallinity and intimate interfacial contact with the WO3 matrix, indicating effective integration without structural degradation. The SAED pattern (inset of Figure 4a) confirms the polycrystalline nature of both Pd NPs and WO3, with lattice spacings consistent with standard crystallographic references. EDS elemental mapping (Figure 4b) demonstrates the homogeneous spatial distribution of W, O, and Pd across the composite, corroborating the successful incorporation of Pd NPs into the WO3/PSi framework. The absence of impurity-related signals further confirms the compositional purity of the system.

3.2. Gas Sensing Characterization

Figure 5 illustrates the gas sensing performance of WO3/PSi and PSi/WO3/PSi sensors. The Pd-WO3/PSi sensor (green line, Figure 5a) exhibits a higher response than WO3/PSi (brown line) across NO2 concentrations (0.1–10 ppm at 25 °C), with response magnitude increasing linearly with gas concentration. Error bars confirm reproducibility across three fabrication batches. The inset highlights the sensor’s superior detection capability at low concentrations (0.1–0.5 ppm), demonstrating a detection limit of 0.05 ppm. Figure 5b delineates the response/recovery kinetics of WO3/PSi and Pd-WO3/PSi sensors across NO2 concentrations (0.05–10 ppm) at 25 °C. Error margins for τres and τrec are ±5% and ±8%, respectively. In the ultralow concentration regime (0.05–0.25 ppm), τres decreases with rising NO2 levels, attributed to gas diffusion-limited kinetics within the hierarchical PSi nanopores (200–500 nm diameter, Figure 3a). Conversely, at higher concentrations (0.25–10 ppm), τres and τrec increase monotonically due to adsorption site saturation and prolonged desorption dynamics. Notably, Pd-WO3/PSi exhibits superior kinetics, achieving τres = 6 s and τrec= 255 s at 5 ppm compared to 9 s and 303 s for WO3/PSi (inset of Figure 5b), respectively. This enhancement stems from Pd-induced catalytic activation of NO2 dissociation into reactive intermediates (e.g., NO2−) and optimized charge transport via d-p orbital hybridization (Section 3.4). Figure 5c reveals the temperature-dependent response (25–300 °C) to NO2 (0.25–5 ppm), showing maximal sensitivity at 25 °C for both sensors. The 25 °C operation optimizes the interplay of adsorption dynamics, charge transport, structural integrity, and catalytic activity, achieving maximal sensitivity while ensuring long-term stability. This temperature-dependent performance underscores the sensor’s suitability for energy-efficient, real-world NO2 monitoring.
The selectivity radar plot (Figure 5d) shows the selectivity of both sensors (PSi/WO3 and Pd-PSi/WO3) towards different gases (5 ppm NO2, NH3, H2S, CO, C2H5OH, and CH3OH). The response of Pd-WO3/PSi to NO2 (R = 5) remained significantly higher than the response to other gases (with the maximum response being only 2). This result highlights the sensor’s exceptional ability to selectively detect NO2 in the presence of other gases, further demonstrating the benefits of Pd modification. Figure 5e demonstrates the humidity resilience of Pd-WO3/PSi, maintaining <3% response variation across 15%–75% RH and <20% attenuation at 90% RH over 30 days. This stability is enabled by the hydrophobic PSi surface and Pd-mediated charge redistribution suppressing H2O adsorption. Figure 5f illustrates the long-term stability of the Pd-WO3/PSi sensor under different NO2 concentrations (0.25, 0.5, 2.5, and 5 ppm) at 25 °C over a 30-day period. After 30 days, the response remained stable (<3% decay), validating the robustness of the fabrication protocol. Overall, the experimental data confirm that Pd NP modification significantly optimizes the sensor’s performance, improving sensitivity, selectivity, and long-term stability.
Compared with the literature benchmarks (Table 1), the developed composite demonstrates superior performance in low-concentration detection (R = 1.5 at 0.25 ppm NO2) and rapid response kinetics, outperforming most reported sensing systems. These advancements validate the effectiveness of Pd modification in enhancing both selectivity and stability, particularly under complex environmental conditions. The material’s practical utility is highlighted by its reliable operation in environmental monitoring and industrial safety scenarios.

3.3. DFT Analysis Without NO2

Figure 6a–c illustrates the atomic-scale structural evolution from pristine PSi to WO3/PSi and Pd-WO3/PSi composites. The PSi framework (Figure 6a) was modeled by introducing controlled nanopore architectures into bulk crystalline silicon, validated against experimental characterization data. A single-pore diameter was modeled to balance computational efficiency with structural fidelity. The simplified single-pore model does not fully capture the complex interconnected porous network seen in experimental samples. However, it serves as a representative unit to capture the local electronic interactions and gas adsorption behavior at the WO3/PSi and Pd-WO3/PSi interfaces. For the WO3/PSi composite (Figure 6b), the WO3 framework retained its monoclinic crystallinity when embedded within the PSi matrix. Pd modification (Figure 6c) induced localized lattice distortions in WO3, where Pd atoms preferentially occupied oxygen vacancy sites, disrupting the coordination symmetry of adjacent oxygen atoms. Top- and side-view analyses revealed bond length contraction and bond angle deviations compared to pristine WO3, confirming chemisorption-driven structural reorganization.
Figure 7a,b presents the band structure and DOS characteristics of the WO3/PSi composite before and after Pd modification. In the unmodified WO3/PSi system (Figure 7a), when the p-type PSi forms a heterojunction with n-type WO3, interfacial charge transfer induces significant shifts in the Fermi-level position, leading to a reconstruction of bandgap characteristics. This unique electronic structure transformation endows the composite with metalloid-like properties—the bandgap near the Fermi level narrows and ultimately collapses to zero. Figure 7b includes unoccupied states above the Fermi level, showing a 2.22-fold increase in DOS at the Fermi level (17.4 to 38.6 states/eV). This remarkable change is attributed to Pd modification, which effectively enhances the charge transport properties of the material through electronic structure modulation, making the Pd-WO3/PSi system more suitable for applications such as catalysis and gas sensing.
ELF analysis (Figure 7c,d) provides deeper mechanistic insights. In the WO3/PSi system (Figure 7c), the electron density is localized to specific regions on the material’s surface (yellow/red regions correspond to a high electron density of 9.999 × 10−1, while blue regions indicate a low density of 2.420 × 10−6). In contrast, the Pd-WO3/PSi system (Figure 7d) shows significantly expanded yellow/red regions (electron density range: 2.819 × 10−8 to 1), with more concentrated electron distribution near Pd sites. This enhanced electron localization indicates the formation of delocalized electron channels, improved carrier mobility, increased active site density, and the provision of more electron-enriched sites, thereby promoting gas (NO2) adsorption.
PDOS analysis (Figure 8a,b) further elucidates the contribution characteristics of different orbitals. In the pristine WO3/PSi system (Figure 8a), the p-orbitals exhibit hybridization features near the Fermi level (0 eV), with contributions spanning from −10 to +15 eV, while the d-orbitals of WO3 primarily operate within the −10 to 10 eV range, regulating interfacial charge transfer. After Pd modification (Figure 8b), the hybridization intensity of p-orbitals near the Fermi level is significantly enhanced, with the most notable change occurring in the d-orbitals: a pronounced peak in the DOS emerges at 0 eV. This results from the hybridization between Pd d orbitals and adjacent O p orbitals near the Fermi level (−2 to +1 eV). The synergistic optimization of s/p/d orbitals at the Fermi level, particularly the Pd-induced enhancement of d states near the conduction band, collectively improves the charge transport dynamics and catalytic activity of the system. Figure 8c,d provides key insights into the orbital hybridization in the Pd-WO3/PSi system. In Figure 8c, the PDOS of Pd-d orbitals (green curve) and all O-p orbitals (blue curve) show significant orbital hybridization in the energy range of −5 to 2.5 eV, with a distinct peak near −2.5 eV, indicating strong overlap between Pd d and O p states. Figure 8d further analyzes this interaction by isolating the PDOS of the p orbitals of the oxygen atoms closest to Pd (blue curve) and Pd-d orbitals (green curve), where a sharp hybridization peak at −2.5 eV confirms robust covalent bonding between Pd and adjacent oxygen atoms. These results collectively demonstrate that Pd incorporation leads to significant d-p orbital hybridization, particularly in the lower energy range. The strong orbital interaction arises from Pd’s high electron affinity, which drives electron transfer from oxygen atoms to Pd sites, leading to oxygen desorption and the formation of oxygen vacancies. These vacancies play a dual role: (1) they enhance surface reactivity by creating electron-deficient regions that favor NO2 chemisorption, and (2) they facilitate dynamic oxygen exchange during gas sensing cycles, as evidenced by the accelerated response/recovery kinetics (Section 3.3). Experimental validation confirms that the Pd-modified system exhibits higher sensitivity to NO2 compared to pristine WO3/PSi, directly attributed to the increased oxygen vacancy density and optimized charge transfer pathways. By correlating atomic-scale hybridization with macroscopic performance, this analysis establishes Pd-induced d-p orbital coupling as a key enabler of enhanced gas sensing functionality.

3.4. DFT Analysis with NO2

The synergistic enhancement in NO2 sensing performance of Pd-WO3/PSi composites is systematically elucidated through combined DFT simulations and experimental mechanistic studies. DFT-optimized models (Figure 9) reveal that Pd incorporation induces localized lattice distortions in WO3, forming stable Pd-O bonds (bond length: 2.071 Å) while disrupting oxygen coordination symmetry. These structural changes, coupled with the hierarchical pore structure, create abundant active sites for gas interaction. In the pristine WO3/PSi system (Figure 9a), NO2 molecules weakly adsorb onto WO3 oxygen atoms, forming a metastable configuration with negligible structural perturbation, whereas in Pd-WO3/PSi (Figure 9b), robust gas–surface interactions are exhibited, with Pd atoms serving as dual-functional active sites by expanding adsorption site distribution and strengthening interfacial bonding through Pd–N/O coordination.
As illustrated in Figure 10a–d, the electronic modifications induced by Pd modification directly correlate with the observed sensing breakthroughs. The introduction of Pd amplifies DOS (Figure 10a,b) peaks, strengthening interfacial electronic interactions. Such orbital hybridization markedly enhances NO2 adsorption affinity and surface reaction kinetics, contributing directly to improved sensing performance.
PDOS analysis of the NO2 adsorption system (Figure 10c,d) reveals that Pd modification induces critical electronic structure modifications. In the pristine WO3/PSi system, s orbitals dominate the low-energy region (−10 to +10 eV), reflecting interfacial Si-O-WO3 interactions, while strong p orbital hybridization near the Fermi level facilitates charge transfer. Upon Pd modification, the d orbital contribution (−10 to +10 eV), predominantly from Pd d orbitals, overlaps significantly with WO3 O p orbitals near the Fermi level, intensifying d-p orbital hybridization. This enhanced hybridization optimizes interfacial charge dynamics rather than altering the bulk band structure, directly lowering the chemical adsorption activation energy of NO2. Reaction pathway simulations corroborate the mechanism:
O2(gas) + e−→ O2−(ads)
NO2(gas) + e−→ NO2−(ads)
where Pd sites catalyze O2 dissociation and stabilize NO2− intermediates. This interfacial synergy between electronic modulation and catalytic activation underpins the superior sensing performance.
ELF analysis (Figure 11a,b) further elucidates the mechanism: in pristine WO3/PSi (Figure 11a), electron density is localized at specific surface regions (yellow/red areas: high density = 1; blue areas: low density = 1.048 × 10−5), indicating weak surface interactions. In contrast, Pd-WO3/PSi (Figure 11b) exhibits expanded high-density regions (electron density range: 1.805 × 10−7 to 1), with concentrated electron distribution near Pd sites. This enhanced delocalization strengthens material–gas interactions and optimizes electron transport pathways, directly explaining the accelerated response/recovery kinetics and exceptional stability (<3% fluctuation over 30 days). Collectively, DFT-guided insights bridge atomic-scale modifications (bandgap engineering, orbital hybridization) to macroscopic sensor performance, establishing a robust framework for designing ternary nanocomposites.

4. Conclusions

This study demonstrates that Pd modification significantly enhances the environmental stability and interfacial functionality of WO3/PSi coatings through atomic-scale structural and electronic modulation. Pd nanoparticles induce localized lattice distortions and form stable Pd–O–W bonds, while DFT calculations confirm enhanced charge transport via Pd-d and O-p orbital hybridization, amplifying conductivity by 2.22-fold and reducing the bandgap. This leads to a sensor response of 3.1 for 2.5 ppm NO2 with rapid response kinetics (τres = 6 s). The coating exhibits exceptional environmental resilience, showing <3% performance decay over 30 days under combined thermal (25–300 °C) and humidity (45 ± 3% RH) stress, alongside high selectivity (5:1 NO2 response ratio against interfering gases). Well-dispersed Pd nanoparticles (15%–20% surface coverage) balance active site density with surface accessibility, enabling efficient charge delocalization. The computational model employed in this study, while simplified in its geometric representation of WO3 nanorod morphology, successfully captured the fundamental interfacial interactions critical to sensing performance. Beyond gas sensing, the hierarchical porosity and interfacial stability of Pd-WO3/PSi highlight its potential in catalytic and high-stress environments. By integrating DFT-guided insights with experimental validation, this work advances the understanding of metal–oxide interactions in porous coatings and establishes a framework for designing adaptive multifunctional materials. Future efforts will focus on multiscale modeling approaches to bridge atomic-scale predictions with realistic morphologies, alongside scalable synthesis techniques such as atomic layer deposition, to transition these coatings toward industrial applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15050570/s1, Table S1: K-point convergence test results for geometry optimization.

Author Contributions

Conceptualization, X.Q.; methodology, X.Q.; software, Z.W. and Y.G.; validation, X.Q. and Z.W.; formal analysis, X.Q.; investigation, X.Q. and Z.W.; resources, Z.W. and Y.G.; data curation, Z.W. and X.Q.; writing—original draft preparation, X.Q.; writing—review and editing, W.Z.; visualization, Y.G.; supervision, Y.G.; project administration, W.Z.; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the financial support from the National Natural Science Foundation of China (grant no. 61971308).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of preparation process for Pd-WO3/PSi.
Figure 1. Schematic illustration of preparation process for Pd-WO3/PSi.
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Figure 2. Schematic diagram of the gas sensing test system.
Figure 2. Schematic diagram of the gas sensing test system.
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Figure 3. SEM images (top view and side view) of (a) PSi; (b) WO3/PSi; and (c) Pd-WO3/PSi; EDS spectra of (d) WO3/PSi and (e) Pd-WO3/PSi.
Figure 3. SEM images (top view and side view) of (a) PSi; (b) WO3/PSi; and (c) Pd-WO3/PSi; EDS spectra of (d) WO3/PSi and (e) Pd-WO3/PSi.
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Figure 4. (a) TEM image (inset: SAEM image); (b) EDS mapping of Pd-WO3/PSi.
Figure 4. (a) TEM image (inset: SAEM image); (b) EDS mapping of Pd-WO3/PSi.
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Figure 5. Gas sensing performance of WO3/PSi and Pd-WO3/PSi. (a) Response vs. NO2 concentration (0.1, 0.25, 0.5, 1, 2.5, 5, and 10 ppm) at 25 °C; (b) response and recovery time vs. NO2 concentration (0.1, 0.25, 0.5, 1, 2.5, 5, and 10 ppm) at 25 °C (inset: dynamic response to 5 ppm NO2 at 25 °C); (c) temperature-dependent response to NO2 (0.25, 0.5, 2.5, and 5ppm) at 25–300 °C; (d) selectivity radar plot (5 ppm NO2, NH3, H2S, CO, C2H5OH, and CH3OH); (e) humidity influence (15, 30, 45, 60, 75, and 90% RH) on Pd-WO3/PSi to 5 ppm NO2 at 25 °C over 30 days; (f) long-term stability of Pd-WO3/PSi to 0.25, 0.5, 2.5, and 5 ppm NO2 at 25 °C over 30 days.
Figure 5. Gas sensing performance of WO3/PSi and Pd-WO3/PSi. (a) Response vs. NO2 concentration (0.1, 0.25, 0.5, 1, 2.5, 5, and 10 ppm) at 25 °C; (b) response and recovery time vs. NO2 concentration (0.1, 0.25, 0.5, 1, 2.5, 5, and 10 ppm) at 25 °C (inset: dynamic response to 5 ppm NO2 at 25 °C); (c) temperature-dependent response to NO2 (0.25, 0.5, 2.5, and 5ppm) at 25–300 °C; (d) selectivity radar plot (5 ppm NO2, NH3, H2S, CO, C2H5OH, and CH3OH); (e) humidity influence (15, 30, 45, 60, 75, and 90% RH) on Pd-WO3/PSi to 5 ppm NO2 at 25 °C over 30 days; (f) long-term stability of Pd-WO3/PSi to 0.25, 0.5, 2.5, and 5 ppm NO2 at 25 °C over 30 days.
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Figure 6. Atomic model (top view and side view) of (a) PSi; (b) WO3/PSi; and (c) Pd-WO3/PSi.
Figure 6. Atomic model (top view and side view) of (a) PSi; (b) WO3/PSi; and (c) Pd-WO3/PSi.
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Figure 7. Electronic structure analysis of WO3/PSi and Pd-WO3/PSi composites. (a) Energy band structure and (b) DOS of WO3/PSi and Pd-WO3/PSi; ELF plots of (c) WO3/PSi and (d) Pd-WO3/PSi.
Figure 7. Electronic structure analysis of WO3/PSi and Pd-WO3/PSi composites. (a) Energy band structure and (b) DOS of WO3/PSi and Pd-WO3/PSi; ELF plots of (c) WO3/PSi and (d) Pd-WO3/PSi.
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Figure 8. PDOS of (a) WO3/PSi and (b) Pd-WO3/PSi; PDOS of Pd-WO3/PSi: (c) all O-p and Pd-d; (d) Pd-adjacent O-p and Pd-d.
Figure 8. PDOS of (a) WO3/PSi and (b) Pd-WO3/PSi; PDOS of Pd-WO3/PSi: (c) all O-p and Pd-d; (d) Pd-adjacent O-p and Pd-d.
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Figure 9. Atomic models (top and side views) of (a) WO3/PSi and (b) Pd-WO3/PSi in a NO2 atmosphere.
Figure 9. Atomic models (top and side views) of (a) WO3/PSi and (b) Pd-WO3/PSi in a NO2 atmosphere.
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Figure 10. Electronic interactions under NO2 adsorption: DOS of (a) WO3/PSi and (b) Pd-WO3/PSi; PDOS of (c) WO3/PSi and (d) Pd-WO3/PSi.
Figure 10. Electronic interactions under NO2 adsorption: DOS of (a) WO3/PSi and (b) Pd-WO3/PSi; PDOS of (c) WO3/PSi and (d) Pd-WO3/PSi.
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Figure 11. ELF plots of (a) WO3/PSi and (b) Pd-WO3/PSi in the presence of NO2 gas.
Figure 11. ELF plots of (a) WO3/PSi and (b) Pd-WO3/PSi in the presence of NO2 gas.
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Table 1. Comparison of NO2 gas sensing performances of Pd-WO3/PSi with those reported in the previous literature.
Table 1. Comparison of NO2 gas sensing performances of Pd-WO3/PSi with those reported in the previous literature.
MaterialsWorking Temp. (°C)ResponseNO2 Conc. (ppm)Ref.
SnO2@WO315039.520[25]
WO32504550[26]
Zn mixed WO3150321.81100[31]
h-WO3 nanorod1501.555[32]
WO3-Based film7555[33]
NiO/WO3 NPs20016.610[34]
Pd-WO3/PSi253.12.5This work
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Qiang, X.; Wang, Z.; Guo, Y.; Zhou, W. DFT-Based Investigation of Pd-Modified WO3/Porous Silicon Composites for NO2 Gas Sensors: Enhanced Synergistic Effect and High-Performance Sensing. Coatings 2025, 15, 570. https://doi.org/10.3390/coatings15050570

AMA Style

Qiang X, Wang Z, Guo Y, Zhou W. DFT-Based Investigation of Pd-Modified WO3/Porous Silicon Composites for NO2 Gas Sensors: Enhanced Synergistic Effect and High-Performance Sensing. Coatings. 2025; 15(5):570. https://doi.org/10.3390/coatings15050570

Chicago/Turabian Style

Qiang, Xiaoyong, Zhipeng Wang, Yongliang Guo, and Weibin Zhou. 2025. "DFT-Based Investigation of Pd-Modified WO3/Porous Silicon Composites for NO2 Gas Sensors: Enhanced Synergistic Effect and High-Performance Sensing" Coatings 15, no. 5: 570. https://doi.org/10.3390/coatings15050570

APA Style

Qiang, X., Wang, Z., Guo, Y., & Zhou, W. (2025). DFT-Based Investigation of Pd-Modified WO3/Porous Silicon Composites for NO2 Gas Sensors: Enhanced Synergistic Effect and High-Performance Sensing. Coatings, 15(5), 570. https://doi.org/10.3390/coatings15050570

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