Silver-Doped TiO₂ Nanophotocatalytic Coatings for Urban Air Purification: Visible-Light Activation, Environmental Stability, and Mechanistic Insight

Silver-Doped TiO₂ Nanophotocatalytic Coatings for Urban Air Purification: Visible-Light Activation, Environmental Stability, and Mechanistic Insight | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Silver-Doped TiO₂ Nanophotocatalytic Coatings for Urban Air Purification: Visible-Light Activation, Environmental Stability, and Mechanistic Insight Asgar Hosseinnezhad, Hadi Sabri This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8735400/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Urban air pollution, dominated by nitrogen oxides (NO), volatile organic compounds (VOCs), and humidity-driven particulate precursors, remains a pressing global health challenge. Here, we report the design and evaluation of silver-modified TiO₂ nanophotocatalytic coatings optimized for realistic urban conditions, including fluctuating irradiance, variable humidity, and high pollutant loads. The coatings, prepared via a sol–gel dip-coating process, exhibited anatase-phase TiO₂ with homogeneously dispersed Ag nanoparticles, band gap narrowing from 3.18 eV to 2.72 eV, and a distinct plasmonic absorption band. Photocatalytic tests under simulated solar irradiation demonstrated superior performance, with 78% NO and 65% toluene removal within 120 min—corresponding to a twofold increase in the apparent rate constant compared to pristine TiO₂. Normalized rates reached 0.35 µmol·m⁻²·s⁻¹ with a visible apparent quantum yield of 0.6%. Durability tests confirmed > 90% retention after ten light/dark cycles and negligible Ag leaching (< 10 ppb). Mechanistic studies combining electrochemical impedance spectroscopy, photoluminescence, radical scavenger experiments, and density functional theory revealed that Ag nanoparticles act as electron sinks, suppressing recombination and enabling O₂•⁻-driven oxidation pathways. The coatings also showed improved tolerance to high relative humidity, with only 18% efficiency loss at 80% RH compared to 32% for pristine TiO₂. These findings establish Ag–TiO₂ coatings as a scalable, stable, and environmentally compatible strategy for passive urban air purification, bridging laboratory performance with real-world deployment. Physical sciences/Chemistry Earth and environmental sciences/Environmental sciences Physical sciences/Materials science Physical sciences/Nanoscience and technology Ag–TiO₂ nanophotocatalysis Urban air purification Visible-light photocatalysis Environmental stability Charge separation mechanism Humidity tolerance Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Air pollution in urban environments has become one of the most critical global health challenges, with the World Health Organization estimating that millions of premature deaths annually are linked to exposure to airborne pollutants such as nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and fine particulate matter (PM₂.₅) [ 1 ]. Conventional air purification technologies, including filtration and adsorption, are often limited by high energy consumption, secondary waste generation, and reduced efficiency under continuous operation [ 2 ]. In this context, photocatalytic nanomaterials have emerged as a promising alternative due to their ability to harness solar energy to degrade pollutants into harmless byproducts such as CO₂ and H₂O [ 3 ]. Titanium dioxide (TiO₂) has been the most widely studied photocatalyst because of its chemical stability, low cost, and strong oxidative potential [ 4 ]. However, its practical application in urban air purification is hindered by two major limitations: its wide band gap (3.0–3.2 eV), which restricts light absorption primarily to the ultraviolet region that constitutes only about 5% of the solar spectrum, and the rapid recombination of photogenerated electron–hole pairs, which reduces quantum efficiency [ 5 ]. These limitations are particularly problematic in urban environments where light intensity is often reduced by smog, tall buildings, and atmospheric scattering, and where pollutant concentrations are significantly higher than in rural areas [ 6 ]. To overcome these challenges, researchers have explored various strategies such as doping with nonmetals, coupling with narrow-band-gap semiconductors, and surface modification with noble metals [ 7 ]. Among these approaches, silver nanoparticle (AgNP) doping has attracted significant attention because it simultaneously enhances visible light absorption through localized surface plasmon resonance (LSPR) and improves charge separation by acting as an electron sink [ 8 ]. The synergistic effect of TiO₂ and AgNPs has been shown to extend photocatalytic activity into the visible spectrum, thereby enabling pollutant degradation even under low-light conditions [ 9 ]. Despite these advances, there remains a research gap in the systematic evaluation of Ag-doped TiO₂ nanophotocatalytic coatings under realistic urban conditions characterized by fluctuating light intensity and high pollutant loads. Most existing studies have been conducted under controlled laboratory conditions that do not fully replicate the complexity of urban air environments [ 10 ]. In the context of urban air pollution, the most critical pollutants include nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and fine particulate matter (PM₂.₅). Each of these contributes differently to smog formation, respiratory diseases, and environmental degradation. The development of Ag-doped TiO₂ coatings directly targets the photocatalytic degradation of NOₓ and VOCs, while also contributing indirectly to the reduction of PM₂.₅ by oxidizing gaseous precursors. Thus, the proposed nanophotocatalytic coatings address the key components of urban air pollution. Unlike many previous studies that primarily evaluated Ag–TiO₂ photocatalysts under idealized laboratory conditions, our work emphasizes performance under simulated urban environments characterized by fluctuating light intensity and high pollutant concentrations. This focus on realistic conditions highlights the novelty of our approach and provides a more direct pathway toward practical deployment in urban infrastructures [ 10 , 11 ]. Therefore, the present work aims to design, synthesize, and evaluate Ag-doped TiO₂ nanophotocatalytic coatings specifically tailored for urban air purification. By addressing the dual challenges of reduced light availability and high pollutant concentration, this study seeks to provide a scalable and sustainable solution for improving air quality in cities. To align laboratory testing with urban reality, we define “near-realistic” conditions as pollutant levels of 200–400 ppb NO and 0.5–2.0 ppm toluene, relative humidity of 40–70%, ambient temperature of 20–30°C, and simulated solar irradiance at 100–500 W.m − 2 with controlled on–off cycles to mimic diurnal fluctuations. These ranges reflect typical urban episodes and enable meaningful translation of performance to outdoor environments. 2. Methods The nanophotocatalytic coatings were synthesized using a sol–gel method combined with silver nanoparticle incorporation. Titanium isopropoxide (TTIP) was used as the Ti precursor, while silver nitrate (AgNO₃) served as the silver source. The TTIP was hydrolyzed in ethanol under controlled stirring, followed by the addition of AgNO₃ solution to achieve doping concentrations ranging from 0.5 to 3 wt%. The resulting sol was aged for 24 hours, deposited onto glass substrates by dip-coating, and subsequently calcined at 450°C for 2 hours to obtain crystalline Ag-doped TiO₂ films. During synthesis, the pH of the sol was maintained at ~ 3.5 using dilute nitric acid, and the solution was stirred at 400 rpm. The dip-coating process was repeated three times to achieve an average film thickness of ~ 250 nm, as measured by profilometry. These details ensure reproducibility of the coating preparation [ 12 ]. To demonstrate practical applicability, the coatings were deposited not only on glass substrates but also on prototype construction materials such as ceramic tiles and concrete panels. This engineering step highlights the potential of integrating Ag-doped TiO₂ into real urban infrastructures, where glass facades and building surfaces can act as passive air-purifying matrices. The coatings were characterized by X-ray diffraction (XRD) to confirm phase composition, scanning electron microscopy (SEM) for surface morphology, UV–Vis diffuse reflectance spectroscopy (DRS) for optical properties, and X-ray photoelectron spectroscopy (XPS) for chemical state analysis [ 13 ]. Surface area and porosity were measured by N₂ adsorption–desorption (BET and BJH), film roughness by AFM, and Ag plasmonic features by UV–Vis DRS (400–600 nm). XPS core levels (Ag 3d, Ti 2p, O 1s) were collected to quantify Ag⁰/Ag⁺ fractions and Ti³⁺ states. Steady-state PL and time-resolved PL (excitation at 325 nm) were recorded to assess recombination dynamics. The photocatalytic performance of the coatings was evaluated by monitoring the degradation of nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) under simulated solar irradiation. A 300 W xenon lamp equipped with AM 1.5G filters was used to simulate sunlight, and pollutant concentrations were measured using a chemiluminescence NOₓ analyzer and gas chromatography for VOCs. The incident irradiance at the sample plane was set to 250 ± 10 W.m − 2 (measured by a calibrated pyranometer), with a visible fraction of 190 ± 8 W.m − 2 . The initial NO concentration was 300 ± 20 ppb and toluene was 1.0 ± 0.1 ppm, under controlled relative humidity of 55 ± 5% at 25 ± 2 ^C. The degradation efficiency ( η ) was calculated using the following equation: $$\:\eta\:\left(\%\right)=\:\frac{{C}_{0}-{C}_{t}}{{C}_{0}}\times\:100,\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(1\right)$$ where \(\:{C}_{0}\) is the initial pollutant concentration and \(\:{C}_{t}\) is the concentration at time \(\:t\) . This equation quantifies the relative reduction in pollutant concentration as a function of irradiation time and provides a direct measure of photocatalytic efficiency [ 14 ]. The apparent reaction kinetics were analyzed using a pseudo-first-order model expressed as: $$\:ln\:\frac{{C}_{0}}{{C}_{t}}={k}_{app}.t,\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(2\right)$$ where \(\:{k}_{app}\) is the apparent rate constant and \(\:t\) is the irradiation time. The slope of the linear plot of \(\:ln\:\frac{{C}_{0}}{{C}_{t}}\) versus \(\:t\) yields the value of \(\:{k}_{app}\) , which serves as a comparative parameter for evaluating the catalytic activity of different coatings [ 12 ]. In this model, the pollutant concentration decreases exponentially with time, and the rate constant reflects the intrinsic activity of the photocatalyst under the given experimental conditions. The optical band gap of the coatings was determined using Tauc plots derived from UV–Vis DRS data. The absorption coefficient (α) was calculated from reflectance spectra, and the band gap energy ( \(\:{E}_{g}\) ) was obtained from the relation: $$\:{\left(\alpha\:h\nu\:\right)}^{n}=A\left(h\nu\:\:-{E}_{g}\right),\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(3\right)$$ where \(\:h\nu\:\) is the photon energy, \(\:A\) is a proportionality constant, and \(\:n\) depends on the nature of the electronic transition, with \(\:n\:=\:2\) for indirect allowed transitions typical of TiO₂. By extrapolating the linear portion of the plot of \(\:{\left(\alpha\:h\nu\:\right)}^{n}\) versus \(\:h\nu\:\) , the band gap energy was estimated [ 15 ]. This analysis allows the comparison of pristine TiO₂ and Ag-doped TiO₂ coatings in terms of their ability to absorb visible light. Electrochemical impedance spectroscopy (EIS) was also employed to evaluate charge transfer resistance at the electrode–electrolyte interface. Nyquist plots were recorded in the frequency range of 0.1 Hz to 100 kHz, and the data were fitted to an equivalent circuit model to extract the charge transfer resistance (Rct). A lower Rct value indicates more efficient separation and transport of photogenerated charge carriers, which directly correlates with enhanced photocatalytic activity [ 16 ]. Control experiments were conducted in the dark, with bare substrates (glass, ceramic, concrete), and with Ag-decorated substrates lacking TiO₂ to decouple plasmonic and adsorption effects. Prior to illumination, all samples underwent 30 min adsorption–desorption equilibration under flow to ensure stable baselines. No significant dark removal (< 3%) was observed for active coatings, while bare and Ag-only substrates showed (< 2%) removal, confirming photocatalytic dominance. The NO and toluene removal rates were normalized to illuminated area and photon flux. The area-specific removal rate \(\:{r}_{A}\) was calculated as: $$\:{r}_{A}=\:\frac{F\left(C0\:-\:{C}_{t}\right)}{A},\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(4\right)$$ where \(\:F\) is the volumetric flow rate, \(\:C\) is the concentration (mol fraction), and \(\:A\) is the illuminated area. Apparent quantum yield (AQY) in the visible range was obtained from the number of removed pollutant molecules divided by incident visible photons per unit time. 3. Results The synthesized Ag-doped TiO₂ nanophotocatalytic coatings exhibited distinct structural, optical, and photocatalytic properties compared to pristine TiO₂. X-ray diffraction (XRD) patterns confirmed the anatase phase of TiO₂ with no detectable secondary phases, indicating that silver was incorporated at the nanoscale without altering the crystalline structure. All photocatalytic experiments were performed in triplicate, and the reported degradation efficiencies represent mean values with standard deviations below 5%, confirming the reproducibility of the results. As confirmed by XRD patterns shown in Fig. 1 . A slight shift in the (101) diffraction peak was observed, suggesting lattice distortion due to Ag incorporation [ 17 ]. Scanning electron microscopy (SEM) images revealed uniform coatings with nanoscale grains, while energy-dispersive X-ray spectroscopy (EDS) confirmed the homogeneous distribution of silver nanoparticles across the TiO₂ matrix. EDS mapping confirmed uniform Ag distribution, and XPS showed Ag 3d₅/₂ at 368.2 ± 0.1 eV (Ag⁰) and 367.6 ± 0.1 eV (Ag⁺), with an Ag⁰ fraction of 65 ± 5%. Ti 2p peaks indicated a minor Ti³⁺ component ( \(\:\sim\) 3%), consistent with band tailing. DRS revealed a weak plasmon band centered at 460 ± 10 nm, corroborating visible-light enhancement. UV–Vis diffuse reflectance spectroscopy (DRS) demonstrated a significant red shift in the absorption edge of Ag-doped TiO₂ compared to pristine TiO₂. The calculated band gap energy decreased from 3.18 eV for pure TiO₂ to 2.72 eV for the 2 wt% Ag-doped sample, confirming enhanced visible light absorption. (See Fig. 3 ). The BET surface area increased from 18.5 ± 0.8 m 2 .g − 1 (pristine) to 24.3 ± 1.0 m 2 .g − 1 (2 wt% Ag), with mesopore diameters of 8–12 nm, supporting improved VOC mass transfer. Tauc plot analysis confirmed the band gap reduction in Ag-doped TiO₂, in agreement with previous findings [ 18 ]. Photocatalytic activity tests under simulated solar irradiation showed that Ag-doped TiO₂ coatings achieved markedly higher pollutant degradation efficiencies. For nitrogen oxides (NOₓ), the 2 wt% Ag-doped TiO₂ coating degraded 78% of NO within 120 minutes, compared to only 46% for pristine TiO₂ under identical conditions. (See Fig. 4 ). Similarly, for toluene as a representative VOC, the Ag-doped coating achieved 65% degradation, while pristine TiO₂ reached only 38%. The degradation efficiency decreased slightly at higher Ag concentrations (3 wt%), likely due to excessive silver acting as recombination centers, consistent with earlier reports [ 19 ]. Kinetic analysis using the pseudo-first-order model revealed that the apparent rate constant (kapp) for NO degradation was 0.012 min⁻¹ for Ag-doped TiO₂, compared to 0.006 min⁻¹ for pristine TiO₂. (See Fig. 5 ). Linear fits yielded R 2 = 0.987 (Ag–TiO₂) and R 2 = 0.962 (pristine) over 0–60 min; slight deviations at higher conversions were attributed to active site saturation. Triplicate tests (n = 3) showed \(\:{k}_{app}\) = 0.0120 ± 0.0004 min − 1 (Ag–TiO₂) and 0.0061 ± 0.0005 min − 1 (pristine). Error bars in Figs. 4 – 5 represent one standard deviation. This twofold increase in reaction rate highlights the role of silver nanoparticles in enhancing charge separation and extending light absorption. Electrochemical impedance spectroscopy (EIS) further confirmed this improvement, with Nyquist plots [ 20 ] showing a significantly smaller semicircle radius for Ag-doped TiO₂, corresponding to a lower charge transfer resistance (Rct) of 18 Ω compared to 42 Ω for pristine TiO₂. (See Fig. 6 ). Charge density difference mapping from density functional theory (DFT) simulations revealed electron accumulation around silver nanoparticles and electron depletion near TiO₂ sites, confirming the role of Ag as an electron sink. (See Fig. 7 ). This redistribution of charge carriers explains the reduced recombination rate and enhanced photocatalytic activity under visible light. Product analysis indicated nitrate formation on the coating surface with negligible HONO release (< 5 ppb) during illumination). For toluene, GC–MS detected benzaldehyde and benzoic acid as transient intermediates, decreasing over time, consistent with progressive mineralization. Total organic carbon (TOC) in the effluent decreased by (62 ± 4%) at 120 min. Diurnal cycling was emulated by six 30 min light on/off cycles; performance retention and recovery were recorded to mimic urban daylight variability. The coatings retained 92 ± 3% of initial NO removal after ten 2-hour irradiation cycles. Under 72 h continuous operation at 60% RH, performance decreased by < 8%. Tape abrasion (ASTM D3359, 4B) caused no measurable Ag loss; ICP-MS of condensates detected Ag at < 10 ppb, indicating minimal leaching. Increasing RH from 30% to 80% reduced NO removal by 18% (Ag–TiO₂) vs 32% (pristine), indicating improved competition management between H₂O and pollutants on Ag–TiO₂ surfaces. Overall, the results demonstrate that Ag-doped TiO₂ nanophotocatalytic coatings exhibit superior structural stability, enhanced visible light absorption, improved charge separation, and significantly higher pollutant degradation efficiency compared to pristine TiO₂. These findings validate the hypothesis that silver doping can effectively address the limitations of TiO₂ in urban air purification applications. 4. Discussion The results obtained in this study clearly demonstrate that silver-doped TiO₂ nanophotocatalytic coatings provide a significant improvement in air purification performance compared to pristine TiO₂. The observed decrease in band gap energy and the red shift in absorption edge confirm that silver incorporation successfully extends the photocatalytic response into the visible spectrum, which is crucial for operation under low-light urban conditions. This finding is consistent with previous reports that noble metal doping enhances the optical properties of TiO₂ by introducing localized surface plasmon resonance effects [ 21 ]. For example, Yu et al. [ 19 ] reported only ~ 55% NOx removal within 2 hours using Ag–TiO₂ powders, whereas our optimized coatings achieved 78% under similar irradiation conditions, underscoring the superior performance of our system. The enhanced degradation efficiency of NOₓ and VOCs observed in this work highlights the synergistic role of silver nanoparticles in both light harvesting and charge carrier dynamics. The pseudo-first-order kinetic analysis revealed a twofold increase in the apparent rate constant for pollutant degradation, which aligns with earlier studies showing that Ag nanoparticles act as electron sinks, thereby suppressing electron–hole recombination and prolonging charge carrier lifetimes [ 22 ]. The photocatalytic mechanism of Ag-doped TiO₂ is schematically illustrated in Fig. 8 . The electrochemical impedance spectroscopy results further support this mechanism, as the reduced charge transfer resistance in Ag-doped TiO₂ indicates more efficient interfacial charge transport. Radical scavenger experiments (isopropanol for \(\:{OH}^{{\bullet\:}}\) , p-benzoquinone for \(\:{O}_{2}^{{\bullet\:}-}\) , and EDTA for \(\:{h}^{+}\) ) showed the strongest inhibition with p-benzoquinone, indicating \(\:{O}_{2}^{{\bullet\:}-}\) -dominated pathways under visible light. TRPL revealed extended lifetimes ( \(\:{\tau\:}_{avg}=\:3.8\:\pm\:\:0.2\:ns)\) vs 2.1 ± 0.1 ns for pristine), corroborating suppressed recombination via Ag electron sinks. The optimal Ag loading of 2 wt% reflects a balance between plasmonic field enhancement and recombination center formation. At 3 wt%, increased Ag–Ag coupling dampens LSPR and obstructs TiO₂ active sites, elevating nonradiative pathways, as evidenced by higher PL intensity and modest \(\:{R}_{ct}\) reductions. The charge density difference analysis from DFT simulations provided direct evidence of electron redistribution at the Ag–TiO₂ interface, confirming the role of silver as an electron trap. This theoretical insight complements the experimental findings and underscores the importance of interfacial engineering in designing efficient nanophotocatalysts. DFT simulations employed PBE + U ( \(\:{U}_{Ti}=4.2\:eV\) ) on anatase (101) with Ag₁₃ clusters. Calculated charge density differences showed \(\:\sim0.12\:{e}^{-}\) transfer to Ag and band tailing consistent with the observed ~ 0.46 eV apparent band gap reduction, supporting the electron sink model. Similar computational studies have also emphasized the role of metal–semiconductor interactions in enhancing photocatalytic activity [ 23 ]. From an application perspective, the superior performance of Ag-doped TiO₂ under simulated urban conditions suggests that such coatings could be deployed on building facades, glass surfaces, and other urban infrastructures to passively reduce air pollution. Minimal Ag leaching under humid air suggests low ecological risk; nevertheless, long‑term outdoor tests should monitor Ag release and potential antimicrobial impacts on urban microbiota. The dip‑coating throughput (3 passes, 250 nm total at ~ 1 m 2 .h − 1 ) indicates feasibility for façade panels; scale-up can leverage roll‑to‑roll on glass and glaze firing on ceramics. Unlike conventional filtration systems, these coatings require no external energy input beyond ambient light and can operate continuously with minimal maintenance. This scalability and sustainability make them particularly attractive for large-scale urban air purification strategies [ 11 ]. In summary, the discussion highlights that the integration of silver nanoparticles into TiO₂ not only addresses the intrinsic limitations of TiO₂ but also provides a pathway toward practical implementation of nanophotocatalytic coatings in real-world urban environments. The combination of experimental validation and theoretical modeling strengthens the case for Ag-doped TiO₂ as a next-generation material for sustainable air purification. 5. Conclusions Ag‑doped TiO₂ coatings overcome UV‑only limitations of pristine TiO₂ by reducing the apparent band gap, enhancing visible absorption, and improving charge separation, yielding 78% NO and 65% toluene removal in 120 min under 250 W.m − 2 simulated sunlight. Area‑normalized NO removal reached 0.35 ± 0.02 \(\:\mu\:\:mol\:.\:{m}^{-2}\:.\:{s}^{-1}\) with a visible AQY of 0.6 ± 0.1.The kinetic analysis confirmed a twofold increase in the apparent rate constant for pollutant degradation in Ag-doped TiO₂, while electrochemical impedance spectroscopy revealed a substantial reduction in charge transfer resistance. These findings were further supported by density functional theory simulations, which showed electron redistribution at the Ag–TiO₂ interface, validating the role of silver as an electron sink that suppresses recombination. The coatings maintained > 90% performance over ten cycles with negligible Ag leaching (< 10 ppb), supporting environmental compatibility for passive urban deployment. The optimal silver concentration (2 wt%) balances LSPR-induced field enhancement and site blocking; higher loadings (3 wt%) increase recombination and attenuate plasmonic benefits. The conclusions drawn from this study are consistent with previous reports that highlighted the benefits of noble metal doping in enhancing the photocatalytic activity of TiO₂. In particular, the observed improvements in visible light absorption and charge carrier dynamics align with the findings of earlier works on Ag–TiO₂ composites. Moreover, the optimal dopant concentration identified here agrees with studies that emphasized the importance of balancing plasmonic enhancement with the risk of recombination introduced by excessive metal loading. In summary, we conclude that Ag-doped TiO₂ nanophotocatalytic coatings represent a promising and scalable solution for urban air purification. Their ability to maintain high photocatalytic activity under low-light conditions and high pollutant concentrations makes them particularly suitable for deployment in real-world urban environments. The consistency of our results with prior literature further strengthens the case for Ag–TiO₂ as a next-generation material for sustainable environmental remediation. Future work will focus on scaling up the coating process for large-area building materials and evaluating long-term stability under real outdoor conditions, which are critical steps toward commercialization. Declarations Competing Interests The authors declare no competing interests. Funding No Funding Author Contribution All authors contributed equally to the conception, design, data analysis, theoretical modeling, and manuscript preparation. Both authors have read and approved the final version of the manuscript Acknowledgement This work is supported by the Research Council of the University of Tabriz. Data Availability The simulation datasets are available from the corresponding author upon reasonable request. References Organization, W. H. Ambient air pollution: A global assessment of exposure and burden of disease. Clean. Air J. 26 , 6–6 (2016). Imarhiagbe, O., Okafor, A. C. & Ogwu, M. C. Air Pollution Control Technologies and Strategies, Evaluating Environmental Processes and Technologies, Springer2025, pp. 231–258. Fujishima, A. & Honda, K. Electrochemical photolysis of water at a semiconductor electrode, nature, 238 37–38. (1972). Diebold, U. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8735400","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":591990958,"identity":"79feab70-d285-457e-b2da-15cfc7cb5822","order_by":0,"name":"Asgar Hosseinnezhad","email":"data:image/png;base64,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","orcid":"","institution":"University of Tabriz","correspondingAuthor":true,"prefix":"","firstName":"Asgar","middleName":"","lastName":"Hosseinnezhad","suffix":""},{"id":591990960,"identity":"7a9b5143-a053-4ed4-bf61-76aae82911d7","order_by":1,"name":"Hadi Sabri","email":"","orcid":"","institution":"University of Tabriz","correspondingAuthor":false,"prefix":"","firstName":"Hadi","middleName":"","lastName":"Sabri","suffix":""}],"badges":[],"createdAt":"2026-01-29 22:09:50","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8735400/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8735400/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102922730,"identity":"8db5fed2-e4d3-4731-9e14-67fd33b7e75f","added_by":"auto","created_at":"2026-02-18 12:56:30","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":485386,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of pristine and Ag-doped TiO₂ coatings.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/4e088c18acb61513421910a2.png"},{"id":102922727,"identity":"fa13a860-1326-40bb-a95f-a914cef3dba5","added_by":"auto","created_at":"2026-02-18 12:56:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1056357,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of Ag-doped TiO₂ nanophotocatalytic coatings at different magnifications.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/e036911c3a8760d7b76bc200.png"},{"id":102922693,"identity":"524b9e5c-774a-43bf-a04d-33d117375513","added_by":"auto","created_at":"2026-02-18 12:56:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":235297,"visible":true,"origin":"","legend":"\u003cp\u003eUV–Vis diffuse reflectance spectra and Tauc plots of pristine and Ag-doped TiO₂coatings.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/41f9eaca6d197298592a0392.png"},{"id":102922700,"identity":"c6fd3193-fe88-4bd6-9f67-4e2e43290bfa","added_by":"auto","created_at":"2026-02-18 12:56:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":401161,"visible":true,"origin":"","legend":"\u003cp\u003ePhotocatalytic degradation efficiency of NO and toluene under simulated solar irradiation.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/1f2e0f110caaab8f6e9ea5b9.png"},{"id":102922695,"identity":"fbc8cf42-74ad-4545-aa04-343f19f4be2e","added_by":"auto","created_at":"2026-02-18 12:56:14","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":175674,"visible":true,"origin":"","legend":"\u003cp\u003eKinetic analysis of NO degradation (pseudo-first-order plots).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/0a10121f9cad44efc34cf99b.png"},{"id":102922699,"identity":"16662b21-cb36-4c87-8dbb-6a1967fa035f","added_by":"auto","created_at":"2026-02-18 12:56:15","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":277535,"visible":true,"origin":"","legend":"\u003cp\u003eNyquist plots from electrochemical impedance spectroscopy (EIS).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/7f3a8ecd79dffe3738ba3fda.png"},{"id":102922704,"identity":"7fdff68c-1b7d-405a-8b2e-450c4310d097","added_by":"auto","created_at":"2026-02-18 12:56:25","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":226918,"visible":true,"origin":"","legend":"\u003cp\u003eCharge density difference mapping from DFT simulations.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/e47869c76cf5617b14615d13.png"},{"id":102922696,"identity":"ddc07942-39ce-4a3d-a2cd-314efedd9843","added_by":"auto","created_at":"2026-02-18 12:56:14","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":230573,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic illustration of photocatalytic mechanism in Ag-doped TiO₂.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/0d61aec319c998843a6fabb9.png"},{"id":102922706,"identity":"4142bd97-1e2f-4ed5-b054-d0d9e6771d03","added_by":"auto","created_at":"2026-02-18 12:56:26","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":138988,"visible":true,"origin":"","legend":"\u003cp\u003eDiurnal on–off irradiance cycling stability of NO removal.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/80999e86bddb7ee6963b65a2.png"},{"id":102922694,"identity":"9ea5b8c9-fcb0-461c-9e6d-fb29a295408c","added_by":"auto","created_at":"2026-02-18 12:56:13","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":233144,"visible":true,"origin":"","legend":"\u003cp\u003eRelative humidity dependence of NO and toluene removal.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/8ba56a9ab50ede04f61056a2.png"},{"id":104782967,"identity":"25ce4045-ef8d-4e49-a2b4-358f17c4466e","added_by":"auto","created_at":"2026-03-17 07:58:02","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3664161,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8735400/v1/728e86a7-0241-46fc-804e-4002ac78d4d2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Silver-Doped TiO₂ Nanophotocatalytic Coatings for Urban Air Purification: Visible-Light Activation, Environmental Stability, and Mechanistic Insight","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAir pollution in urban environments has become one of the most critical global health challenges, with the World Health Organization estimating that millions of premature deaths annually are linked to exposure to airborne pollutants such as nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and fine particulate matter (PM₂.₅) [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Conventional air purification technologies, including filtration and adsorption, are often limited by high energy consumption, secondary waste generation, and reduced efficiency under continuous operation [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In this context, photocatalytic nanomaterials have emerged as a promising alternative due to their ability to harness solar energy to degrade pollutants into harmless byproducts such as CO₂ and H₂O [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTitanium dioxide (TiO₂) has been the most widely studied photocatalyst because of its chemical stability, low cost, and strong oxidative potential [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. However, its practical application in urban air purification is hindered by two major limitations: its wide band gap (3.0\u0026ndash;3.2 eV), which restricts light absorption primarily to the ultraviolet region that constitutes only about 5% of the solar spectrum, and the rapid recombination of photogenerated electron\u0026ndash;hole pairs, which reduces quantum efficiency [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. These limitations are particularly problematic in urban environments where light intensity is often reduced by smog, tall buildings, and atmospheric scattering, and where pollutant concentrations are significantly higher than in rural areas [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTo overcome these challenges, researchers have explored various strategies such as doping with nonmetals, coupling with narrow-band-gap semiconductors, and surface modification with noble metals [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Among these approaches, silver nanoparticle (AgNP) doping has attracted significant attention because it simultaneously enhances visible light absorption through localized surface plasmon resonance (LSPR) and improves charge separation by acting as an electron sink [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The synergistic effect of TiO₂ and AgNPs has been shown to extend photocatalytic activity into the visible spectrum, thereby enabling pollutant degradation even under low-light conditions [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eDespite these advances, there remains a research gap in the systematic evaluation of Ag-doped TiO₂ nanophotocatalytic coatings under realistic urban conditions characterized by fluctuating light intensity and high pollutant loads. Most existing studies have been conducted under controlled laboratory conditions that do not fully replicate the complexity of urban air environments [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. In the context of urban air pollution, the most critical pollutants include nitrogen oxides (NOₓ), volatile organic compounds (VOCs), and fine particulate matter (PM₂.₅). Each of these contributes differently to smog formation, respiratory diseases, and environmental degradation. The development of Ag-doped TiO₂ coatings directly targets the photocatalytic degradation of NOₓ and VOCs, while also contributing indirectly to the reduction of PM₂.₅ by oxidizing gaseous precursors. Thus, the proposed nanophotocatalytic coatings address the key components of urban air pollution. Unlike many previous studies that primarily evaluated Ag\u0026ndash;TiO₂ photocatalysts under idealized laboratory conditions, our work emphasizes performance under simulated urban environments characterized by fluctuating light intensity and high pollutant concentrations. This focus on realistic conditions highlights the novelty of our approach and provides a more direct pathway toward practical deployment in urban infrastructures [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Therefore, the present work aims to design, synthesize, and evaluate Ag-doped TiO₂ nanophotocatalytic coatings specifically tailored for urban air purification. By addressing the dual challenges of reduced light availability and high pollutant concentration, this study seeks to provide a scalable and sustainable solution for improving air quality in cities. To align laboratory testing with urban reality, we define \u0026ldquo;near-realistic\u0026rdquo; conditions as pollutant levels of 200\u0026ndash;400 ppb NO and 0.5\u0026ndash;2.0 ppm toluene, relative humidity of 40\u0026ndash;70%, ambient temperature of 20\u0026ndash;30\u0026deg;C, and simulated solar irradiance at 100\u0026ndash;500 W.m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e with controlled on\u0026ndash;off cycles to mimic diurnal fluctuations. These ranges reflect typical urban episodes and enable meaningful translation of performance to outdoor environments.\u003c/p\u003e"},{"header":"2. Methods","content":"\u003cp\u003eThe nanophotocatalytic coatings were synthesized using a sol\u0026ndash;gel method combined with silver nanoparticle incorporation. Titanium isopropoxide (TTIP) was used as the Ti precursor, while silver nitrate (AgNO₃) served as the silver source. The TTIP was hydrolyzed in ethanol under controlled stirring, followed by the addition of AgNO₃ solution to achieve doping concentrations ranging from 0.5 to 3 wt%. The resulting sol was aged for 24 hours, deposited onto glass substrates by dip-coating, and subsequently calcined at 450\u0026deg;C for 2 hours to obtain crystalline Ag-doped TiO₂ films. During synthesis, the pH of the sol was maintained at ~\u0026thinsp;3.5 using dilute nitric acid, and the solution was stirred at 400 rpm. The dip-coating process was repeated three times to achieve an average film thickness of ~\u0026thinsp;250 nm, as measured by profilometry. These details ensure reproducibility of the coating preparation [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. To demonstrate practical applicability, the coatings were deposited not only on glass substrates but also on prototype construction materials such as ceramic tiles and concrete panels. This engineering step highlights the potential of integrating Ag-doped TiO₂ into real urban infrastructures, where glass facades and building surfaces can act as passive air-purifying matrices. The coatings were characterized by X-ray diffraction (XRD) to confirm phase composition, scanning electron microscopy (SEM) for surface morphology, UV\u0026ndash;Vis diffuse reflectance spectroscopy (DRS) for optical properties, and X-ray photoelectron spectroscopy (XPS) for chemical state analysis [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Surface area and porosity were measured by N₂ adsorption\u0026ndash;desorption (BET and BJH), film roughness by AFM, and Ag plasmonic features by UV\u0026ndash;Vis DRS (400\u0026ndash;600 nm). XPS core levels (Ag 3d, Ti 2p, O 1s) were collected to quantify Ag⁰/Ag⁺ fractions and Ti\u0026sup3;⁺ states. Steady-state PL and time-resolved PL (excitation at 325 nm) were recorded to assess recombination dynamics.\u003c/p\u003e \u003cp\u003eThe photocatalytic performance of the coatings was evaluated by monitoring the degradation of nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) under simulated solar irradiation. A 300 W xenon lamp equipped with AM 1.5G filters was used to simulate sunlight, and pollutant concentrations were measured using a chemiluminescence NOₓ analyzer and gas chromatography for VOCs. The incident irradiance at the sample plane was set to 250\u0026thinsp;\u0026plusmn;\u0026thinsp;10 W.m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e (measured by a calibrated pyranometer), with a visible fraction of 190\u0026thinsp;\u0026plusmn;\u0026thinsp;8 W.m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. The initial NO concentration was 300\u0026thinsp;\u0026plusmn;\u0026thinsp;20 ppb and toluene was 1.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 ppm, under controlled relative humidity of 55\u0026thinsp;\u0026plusmn;\u0026thinsp;5% at 25\u0026thinsp;\u0026plusmn;\u0026thinsp;2 ^C.\u003c/p\u003e \u003cp\u003eThe degradation efficiency (\u003cem\u003eη\u003c/em\u003e) was calculated using the following equation:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\eta\\:\\left(\\%\\right)=\\:\\frac{{C}_{0}-{C}_{t}}{{C}_{0}}\\times\\:100,\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(1\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{0}\\)\u003c/span\u003e\u003c/span\u003e is the initial pollutant concentration and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{t}\\)\u003c/span\u003e\u003c/span\u003e is the concentration at time \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:t\\)\u003c/span\u003e\u003c/span\u003e. This equation quantifies the relative reduction in pollutant concentration as a function of irradiation time and provides a direct measure of photocatalytic efficiency [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe apparent reaction kinetics were analyzed using a pseudo-first-order model expressed as:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:ln\\:\\frac{{C}_{0}}{{C}_{t}}={k}_{app}.t,\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(2\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{k}_{app}\\)\u003c/span\u003e\u003c/span\u003e is the apparent rate constant and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:t\\)\u003c/span\u003e\u003c/span\u003e is the irradiation time. The slope of the linear plot of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:ln\\:\\frac{{C}_{0}}{{C}_{t}}\\)\u003c/span\u003e\u003c/span\u003e versus \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:t\\)\u003c/span\u003e\u003c/span\u003e yields the value of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{k}_{app}\\)\u003c/span\u003e\u003c/span\u003e, which serves as a comparative parameter for evaluating the catalytic activity of different coatings [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. In this model, the pollutant concentration decreases exponentially with time, and the rate constant reflects the intrinsic activity of the photocatalyst under the given experimental conditions.\u003c/p\u003e \u003cp\u003eThe optical band gap of the coatings was determined using Tauc plots derived from UV\u0026ndash;Vis DRS data. The absorption coefficient (α) was calculated from reflectance spectra, and the band gap energy (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{E}_{g}\\)\u003c/span\u003e\u003c/span\u003e) was obtained from the relation:\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:{\\left(\\alpha\\:h\\nu\\:\\right)}^{n}=A\\left(h\\nu\\:\\:-{E}_{g}\\right),\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(3\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:h\\nu\\:\\)\u003c/span\u003e\u003c/span\u003e is the photon energy, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:A\\)\u003c/span\u003e\u003c/span\u003e is a proportionality constant, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:n\\)\u003c/span\u003e\u003c/span\u003e depends on the nature of the electronic transition, with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:n\\:=\\:2\\)\u003c/span\u003e\u003c/span\u003e for indirect allowed transitions typical of TiO₂. By extrapolating the linear portion of the plot of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\left(\\alpha\\:h\\nu\\:\\right)}^{n}\\)\u003c/span\u003e\u003c/span\u003e versus \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:h\\nu\\:\\)\u003c/span\u003e\u003c/span\u003e, the band gap energy was estimated [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. This analysis allows the comparison of pristine TiO₂ and Ag-doped TiO₂ coatings in terms of their ability to absorb visible light.\u003c/p\u003e \u003cp\u003eElectrochemical impedance spectroscopy (EIS) was also employed to evaluate charge transfer resistance at the electrode\u0026ndash;electrolyte interface. Nyquist plots were recorded in the frequency range of 0.1 Hz to 100 kHz, and the data were fitted to an equivalent circuit model to extract the charge transfer resistance (Rct). A lower Rct value indicates more efficient separation and transport of photogenerated charge carriers, which directly correlates with enhanced photocatalytic activity [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eControl experiments were conducted in the dark, with bare substrates (glass, ceramic, concrete), and with Ag-decorated substrates lacking TiO₂ to decouple plasmonic and adsorption effects. Prior to illumination, all samples underwent 30 min adsorption\u0026ndash;desorption equilibration under flow to ensure stable baselines. No significant dark removal (\u0026lt;\u0026thinsp;3%) was observed for active coatings, while bare and Ag-only substrates showed (\u0026lt;\u0026thinsp;2%) removal, confirming photocatalytic dominance.\u003c/p\u003e \u003cp\u003eThe NO and toluene removal rates were normalized to illuminated area and photon flux. The area-specific removal rate \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{r}_{A}\\)\u003c/span\u003e\u003c/span\u003e was calculated as:\u003cdiv id=\"Equd\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equd\" name=\"EquationSource\"\u003e\n$$\\:{r}_{A}=\\:\\frac{F\\left(C0\\:-\\:{C}_{t}\\right)}{A},\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(4\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:F\\)\u003c/span\u003e\u003c/span\u003e is the volumetric flow rate, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:C\\)\u003c/span\u003e\u003c/span\u003e is the concentration (mol fraction), and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:A\\)\u003c/span\u003e\u003c/span\u003e is the illuminated area. Apparent quantum yield (AQY) in the visible range was obtained from the number of removed pollutant molecules divided by incident visible photons per unit time.\u003c/p\u003e"},{"header":"3. Results","content":"\u003cp\u003eThe synthesized Ag-doped TiO₂ nanophotocatalytic coatings exhibited distinct structural, optical, and photocatalytic properties compared to pristine TiO₂. X-ray diffraction (XRD) patterns confirmed the anatase phase of TiO₂ with no detectable secondary phases, indicating that silver was incorporated at the nanoscale without altering the crystalline structure. All photocatalytic experiments were performed in triplicate, and the reported degradation efficiencies represent mean values with standard deviations below 5%, confirming the reproducibility of the results. As confirmed by XRD patterns shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA slight shift in the (101) diffraction peak was observed, suggesting lattice distortion due to Ag incorporation [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Scanning electron microscopy (SEM) images revealed uniform coatings with nanoscale grains, while energy-dispersive X-ray spectroscopy (EDS) confirmed the homogeneous distribution of silver nanoparticles across the TiO₂ matrix. EDS mapping confirmed uniform Ag distribution, and XPS showed Ag 3d₅/₂ at 368.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 eV (Ag⁰) and 367.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 eV (Ag⁺), with an Ag⁰ fraction of 65\u0026thinsp;\u0026plusmn;\u0026thinsp;5%. Ti 2p peaks indicated a minor Ti\u0026sup3;⁺ component (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sim\\)\u003c/span\u003e\u003c/span\u003e3%), consistent with band tailing. DRS revealed a weak plasmon band centered at 460\u0026thinsp;\u0026plusmn;\u0026thinsp;10 nm, corroborating visible-light enhancement.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUV\u0026ndash;Vis diffuse reflectance spectroscopy (DRS) demonstrated a significant red shift in the absorption edge of Ag-doped TiO₂ compared to pristine TiO₂. The calculated band gap energy decreased from 3.18 eV for pure TiO₂ to 2.72 eV for the 2 wt% Ag-doped sample, confirming enhanced visible light absorption. (See Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The BET surface area increased from 18.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 m\u003csup\u003e2\u003c/sup\u003e.g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (pristine) to 24.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.0 m\u003csup\u003e2\u003c/sup\u003e.g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (2 wt% Ag), with mesopore diameters of 8\u0026ndash;12 nm, supporting improved VOC mass transfer.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTauc plot analysis confirmed the band gap reduction in Ag-doped TiO₂, in agreement with previous findings [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e \u003cp\u003ePhotocatalytic activity tests under simulated solar irradiation showed that Ag-doped TiO₂ coatings achieved markedly higher pollutant degradation efficiencies. For nitrogen oxides (NOₓ), the 2 wt% Ag-doped TiO₂ coating degraded 78% of NO within 120 minutes, compared to only 46% for pristine TiO₂ under identical conditions. (See Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSimilarly, for toluene as a representative VOC, the Ag-doped coating achieved 65% degradation, while pristine TiO₂ reached only 38%. The degradation efficiency decreased slightly at higher Ag concentrations (3 wt%), likely due to excessive silver acting as recombination centers, consistent with earlier reports [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eKinetic analysis using the pseudo-first-order model revealed that the apparent rate constant (kapp) for NO degradation was 0.012 min⁻\u0026sup1; for Ag-doped TiO₂, compared to 0.006 min⁻\u0026sup1; for pristine TiO₂. (See Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Linear fits yielded \u003cem\u003eR\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.987 (Ag\u0026ndash;TiO₂) and \u003cem\u003eR\u003c/em\u003e\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.962 (pristine) over 0\u0026ndash;60 min; slight deviations at higher conversions were attributed to active site saturation. Triplicate tests (n\u0026thinsp;=\u0026thinsp;3) showed \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{k}_{app}\\)\u003c/span\u003e\u003c/span\u003e= 0.0120\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0004 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Ag\u0026ndash;TiO₂) and 0.0061\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0005 min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (pristine). Error bars in Figs.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e represent one standard deviation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis twofold increase in reaction rate highlights the role of silver nanoparticles in enhancing charge separation and extending light absorption. Electrochemical impedance spectroscopy (EIS) further confirmed this improvement, with Nyquist plots [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] showing a significantly smaller semicircle radius for Ag-doped TiO₂, corresponding to a lower charge transfer resistance (Rct) of 18 Ω compared to 42 Ω for pristine TiO₂. (See Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCharge density difference mapping from density functional theory (DFT) simulations revealed electron accumulation around silver nanoparticles and electron depletion near TiO₂ sites, confirming the role of Ag as an electron sink. (See Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThis redistribution of charge carriers explains the reduced recombination rate and enhanced photocatalytic activity under visible light.\u003c/p\u003e \u003cp\u003eProduct analysis indicated nitrate formation on the coating surface with negligible HONO release (\u0026lt;\u0026thinsp;5 ppb) during illumination). For toluene, GC\u0026ndash;MS detected benzaldehyde and benzoic acid as transient intermediates, decreasing over time, consistent with progressive mineralization. Total organic carbon (TOC) in the effluent decreased by (62\u0026thinsp;\u0026plusmn;\u0026thinsp;4%) at 120 min.\u003c/p\u003e \u003cp\u003eDiurnal cycling was emulated by six 30 min light on/off cycles; performance retention and recovery were recorded to mimic urban daylight variability.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe coatings retained 92\u0026thinsp;\u0026plusmn;\u0026thinsp;3% of initial NO removal after ten 2-hour irradiation cycles. Under 72 h continuous operation at 60% RH, performance decreased by \u0026lt;\u0026thinsp;8%. Tape abrasion (ASTM D3359, 4B) caused no measurable Ag loss; ICP-MS of condensates detected Ag at \u0026lt;\u0026thinsp;10 ppb, indicating minimal leaching. Increasing RH from 30% to 80% reduced NO removal by 18% (Ag\u0026ndash;TiO₂) vs 32% (pristine), indicating improved competition management between H₂O and pollutants on Ag\u0026ndash;TiO₂ surfaces.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOverall, the results demonstrate that Ag-doped TiO₂ nanophotocatalytic coatings exhibit superior structural stability, enhanced visible light absorption, improved charge separation, and significantly higher pollutant degradation efficiency compared to pristine TiO₂. These findings validate the hypothesis that silver doping can effectively address the limitations of TiO₂ in urban air purification applications.\u003c/p\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe results obtained in this study clearly demonstrate that silver-doped TiO₂ nanophotocatalytic coatings provide a significant improvement in air purification performance compared to pristine TiO₂. The observed decrease in band gap energy and the red shift in absorption edge confirm that silver incorporation successfully extends the photocatalytic response into the visible spectrum, which is crucial for operation under low-light urban conditions. This finding is consistent with previous reports that noble metal doping enhances the optical properties of TiO₂ by introducing localized surface plasmon resonance effects [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. For example, Yu et al. [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] reported only\u0026thinsp;~\u0026thinsp;55% NOx removal within 2 hours using Ag\u0026ndash;TiO₂ powders, whereas our optimized coatings achieved 78% under similar irradiation conditions, underscoring the superior performance of our system.\u003c/p\u003e \u003cp\u003eThe enhanced degradation efficiency of NOₓ and VOCs observed in this work highlights the synergistic role of silver nanoparticles in both light harvesting and charge carrier dynamics. The pseudo-first-order kinetic analysis revealed a twofold increase in the apparent rate constant for pollutant degradation, which aligns with earlier studies showing that Ag nanoparticles act as electron sinks, thereby suppressing electron\u0026ndash;hole recombination and prolonging charge carrier lifetimes [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. The photocatalytic mechanism of Ag-doped TiO₂ is schematically illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e8\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe electrochemical impedance spectroscopy results further support this mechanism, as the reduced charge transfer resistance in Ag-doped TiO₂ indicates more efficient interfacial charge transport. Radical scavenger experiments (isopropanol for \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{OH}^{{\\bullet\\:}}\\)\u003c/span\u003e\u003c/span\u003e, p-benzoquinone for \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{O}_{2}^{{\\bullet\\:}-}\\)\u003c/span\u003e\u003c/span\u003e, and EDTA for \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{h}^{+}\\)\u003c/span\u003e\u003c/span\u003e) showed the strongest inhibition with p-benzoquinone, indicating \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{O}_{2}^{{\\bullet\\:}-}\\)\u003c/span\u003e\u003c/span\u003e-dominated pathways under visible light. TRPL revealed extended lifetimes (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\tau\\:}_{avg}=\\:3.8\\:\\pm\\:\\:0.2\\:ns)\\)\u003c/span\u003e\u003c/span\u003e vs 2.1 \u0026plusmn; 0.1 ns for pristine), corroborating suppressed recombination via Ag electron sinks.\u003c/p\u003e \u003cp\u003eThe optimal Ag loading of 2 wt% reflects a balance between plasmonic field enhancement and recombination center formation. At 3 wt%, increased Ag\u0026ndash;Ag coupling dampens LSPR and obstructs TiO₂ active sites, elevating nonradiative pathways, as evidenced by higher PL intensity and modest \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{ct}\\)\u003c/span\u003e\u003c/span\u003e reductions.\u003c/p\u003e \u003cp\u003eThe charge density difference analysis from DFT simulations provided direct evidence of electron redistribution at the Ag\u0026ndash;TiO₂ interface, confirming the role of silver as an electron trap. This theoretical insight complements the experimental findings and underscores the importance of interfacial engineering in designing efficient nanophotocatalysts. DFT simulations employed PBE\u0026thinsp;+\u0026thinsp;U (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{U}_{Ti}=4.2\\:eV\\)\u003c/span\u003e\u003c/span\u003e) on anatase (101) with Ag₁₃ clusters. Calculated charge density differences showed \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\sim0.12\\:{e}^{-}\\)\u003c/span\u003e\u003c/span\u003e transfer to Ag and band tailing consistent with the observed\u0026thinsp;~\u0026thinsp;0.46 eV apparent band gap reduction, supporting the electron sink model. Similar computational studies have also emphasized the role of metal\u0026ndash;semiconductor interactions in enhancing photocatalytic activity [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFrom an application perspective, the superior performance of Ag-doped TiO₂ under simulated urban conditions suggests that such coatings could be deployed on building facades, glass surfaces, and other urban infrastructures to passively reduce air pollution. Minimal Ag leaching under humid air suggests low ecological risk; nevertheless, long‑term outdoor tests should monitor Ag release and potential antimicrobial impacts on urban microbiota. The dip‑coating throughput (3 passes, 250 nm total at ~\u0026thinsp;1 m\u003csup\u003e2\u003c/sup\u003e.h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) indicates feasibility for fa\u0026ccedil;ade panels; scale-up can leverage roll‑to‑roll on glass and glaze firing on ceramics. Unlike conventional filtration systems, these coatings require no external energy input beyond ambient light and can operate continuously with minimal maintenance. This scalability and sustainability make them particularly attractive for large-scale urban air purification strategies [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn summary, the discussion highlights that the integration of silver nanoparticles into TiO₂ not only addresses the intrinsic limitations of TiO₂ but also provides a pathway toward practical implementation of nanophotocatalytic coatings in real-world urban environments. The combination of experimental validation and theoretical modeling strengthens the case for Ag-doped TiO₂ as a next-generation material for sustainable air purification.\u003c/p\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eAg‑doped TiO₂ coatings overcome UV‑only limitations of pristine TiO₂ by reducing the apparent band gap, enhancing visible absorption, and improving charge separation, yielding 78% NO and 65% toluene removal in 120 min under 250 W.m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e simulated sunlight. Area‑normalized NO removal reached 0.35\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\mu\\:\\:mol\\:.\\:{m}^{-2}\\:.\\:{s}^{-1}\\)\u003c/span\u003e\u003c/span\u003e with a visible AQY of 0.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1.The kinetic analysis confirmed a twofold increase in the apparent rate constant for pollutant degradation in Ag-doped TiO₂, while electrochemical impedance spectroscopy revealed a substantial reduction in charge transfer resistance. These findings were further supported by density functional theory simulations, which showed electron redistribution at the Ag\u0026ndash;TiO₂ interface, validating the role of silver as an electron sink that suppresses recombination.\u003c/p\u003e \u003cp\u003eThe coatings maintained\u0026thinsp;\u0026gt;\u0026thinsp;90% performance over ten cycles with negligible Ag leaching (\u0026lt;\u0026thinsp;10 ppb), supporting environmental compatibility for passive urban deployment. The optimal silver concentration (2 wt%) balances LSPR-induced field enhancement and site blocking; higher loadings (3 wt%) increase recombination and attenuate plasmonic benefits.\u003c/p\u003e \u003cp\u003eThe conclusions drawn from this study are consistent with previous reports that highlighted the benefits of noble metal doping in enhancing the photocatalytic activity of TiO₂. In particular, the observed improvements in visible light absorption and charge carrier dynamics align with the findings of earlier works on Ag\u0026ndash;TiO₂ composites. Moreover, the optimal dopant concentration identified here agrees with studies that emphasized the importance of balancing plasmonic enhancement with the risk of recombination introduced by excessive metal loading.\u003c/p\u003e \u003cp\u003eIn summary, we conclude that Ag-doped TiO₂ nanophotocatalytic coatings represent a promising and scalable solution for urban air purification. Their ability to maintain high photocatalytic activity under low-light conditions and high pollutant concentrations makes them particularly suitable for deployment in real-world urban environments. The consistency of our results with prior literature further strengthens the case for Ag\u0026ndash;TiO₂ as a next-generation material for sustainable environmental remediation. Future work will focus on scaling up the coating process for large-area building materials and evaluating long-term stability under real outdoor conditions, which are critical steps toward commercialization.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting Interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eNo Funding\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAll authors contributed equally to the conception, design, data analysis, theoretical modeling, and manuscript preparation. Both authors have read and approved the final version of the manuscript\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis work is supported by the Research Council of the University of Tabriz.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe simulation datasets are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eOrganization, W. H. Ambient air pollution: A global assessment of exposure and burden of disease. \u003cem\u003eClean. Air J.\u003c/em\u003e \u003cb\u003e26\u003c/b\u003e, 6\u0026ndash;6 (2016).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eImarhiagbe, O., Okafor, A. C. \u0026amp; Ogwu, M. C. 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B\u003c/em\u003e. \u003cb\u003e107\u003c/b\u003e, 77\u0026ndash;87 (2011).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Ag–TiO₂ nanophotocatalysis, Urban air purification, Visible-light photocatalysis, Environmental stability, Charge separation mechanism, Humidity tolerance","lastPublishedDoi":"10.21203/rs.3.rs-8735400/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8735400/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eUrban air pollution, dominated by nitrogen oxides (NO), volatile organic compounds (VOCs), and humidity-driven particulate precursors, remains a pressing global health challenge. Here, we report the design and evaluation of silver-modified TiO₂ nanophotocatalytic coatings optimized for realistic urban conditions, including fluctuating irradiance, variable humidity, and high pollutant loads. The coatings, prepared via a sol\u0026ndash;gel dip-coating process, exhibited anatase-phase TiO₂ with homogeneously dispersed Ag nanoparticles, band gap narrowing from 3.18 eV to 2.72 eV, and a distinct plasmonic absorption band. Photocatalytic tests under simulated solar irradiation demonstrated superior performance, with 78% NO and 65% toluene removal within 120 min\u0026mdash;corresponding to a twofold increase in the apparent rate constant compared to pristine TiO₂. Normalized rates reached 0.35 \u0026micro;mol\u0026middot;m⁻\u0026sup2;\u0026middot;s⁻\u0026sup1; with a visible apparent quantum yield of 0.6%. Durability tests confirmed\u0026thinsp;\u0026gt;\u0026thinsp;90% retention after ten light/dark cycles and negligible Ag leaching (\u0026lt;\u0026thinsp;10 ppb). Mechanistic studies combining electrochemical impedance spectroscopy, photoluminescence, radical scavenger experiments, and density functional theory revealed that Ag nanoparticles act as electron sinks, suppressing recombination and enabling O₂\u0026bull;⁻-driven oxidation pathways. The coatings also showed improved tolerance to high relative humidity, with only 18% efficiency loss at 80% RH compared to 32% for pristine TiO₂. These findings establish Ag\u0026ndash;TiO₂ coatings as a scalable, stable, and environmentally compatible strategy for passive urban air purification, bridging laboratory performance with real-world deployment.\u003c/p\u003e","manuscriptTitle":"Silver-Doped TiO₂ Nanophotocatalytic Coatings for Urban Air Purification: Visible-Light Activation, Environmental Stability, and Mechanistic Insight","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-18 12:55:14","doi":"10.21203/rs.3.rs-8735400/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"edd05369-bf4f-4d43-a297-64e9d9a5abe1","owner":[],"postedDate":"February 18th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":62990623,"name":"Physical sciences/Chemistry"},{"id":62990624,"name":"Earth and environmental sciences/Environmental sciences"},{"id":62990625,"name":"Physical sciences/Materials science"},{"id":62990626,"name":"Physical sciences/Nanoscience and technology"}],"tags":[],"updatedAt":"2026-03-15T17:39:22+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-18 12:55:14","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8735400","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8735400","identity":"rs-8735400","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}