Next Article in Journal
The Effect of Pre-Deformation on the Microstructure and Hardness of Al-Zn-Mg-Cu Alloy
Next Article in Special Issue
Recent Progress in Copper Nanowire-Based Flexible Transparent Conductors
Previous Article in Journal
Study on the Effect of SmFeN Content on the Wave-Absorbing Properties of SmFeN/YSZ Composite Thermal Barrier Coatings Prepared by Plasma Spraying
Previous Article in Special Issue
The Influence of Homogenous Magnetic Field Intensity on Surface Properties of Ni Thin Films Deposited from Citrate Baths and Their Role in Hydrogen Production
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Coatings
Volume 15
Issue 3
10.3390/coatings15030281
coatings-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Photocatalytically Induced Degradation of Nano-TiO2-Modified Paint Coatings Under Low-Radiation Conditions

by
Maciej Kalinowski
,
Karol Chilmon
*,
Justyna Kuziak
,
Paweł Łukowski
and
Wioletta Jackiewicz-Rek
Faculty of Civil Engineering, Warsaw University of Technology, 00-637 Warsaw, Poland
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(3), 281; https://doi.org/10.3390/coatings15030281
Submission received: 25 January 2025 / Revised: 20 February 2025 / Accepted: 25 February 2025 / Published: 27 February 2025
(This article belongs to the Special Issue Design of Nanostructures for Energy and Environmental Applications)
Browse Figures
Versions Notes

Abstract

:
Photocatalytic coatings incorporating nano-TiO2 have emerged as effective solutions for air purification, utilizing solar radiation to degrade airborne pollutants. However, the long-term stability of such coatings, particularly those based on organic binders, remains a concern due to their susceptibility to photocatalytic-driven degradation. This study investigates the effects of low-intensity UV-A irradiation (1–10 W/m2) on acrylic-based photocatalytic coatings’ structural integrity and air purification performance. The findings reveal that significant binder decomposition occurs even under low irradiation conditions—comparable to natural sunlight exposure in Northern and Central Europe during autumn and winter. The surface porosity increased from 2.28% to 9.09% due to polymer degradation, exposing more nano-TiO2 particles and enhancing NO removal efficiency from approximately 120 µg/hm2 to 360 µg/hm2 under UV-A irradiation (1 W/m2). However, this process also resulted in benzene emissions reaching approximately five ppb, raising concerns about secondary pollution and the potential release of nano-TiO2 due to polymer matrix disintegration. These findings highlight the need for optimized coating formulations that balance photocatalytic efficiency with long-term material stability, mitigating the environmental and health risks associated with secondary pollutant emissions.

1. Introduction

Air quality in urban environments has become a pressing issue due to its implications for environmental stability and public health [1,2,3,4]. Most common airborne pollutants (nitrogen oxides, sulfur oxides, carbon monoxide, volatile organic compounds, and others) are emitted into the atmosphere mainly due to various human activities, including industrial emissions, vehicular traffic, and energy production [5,6]. These pollutants contribute not only to environmental problems like acid rain, smog formation, and climate change but also to a range of adverse health effects [7,8]. Epidemiological studies have consistently linked exposure to urban air pollution with increased risks of cardiovascular and respiratory diseases.
Various approaches have been developed over the years to mitigate the impacts of these pollutants [9]. Legal frameworks, including emissions standards and air quality regulations, play an important role in reducing pollutant levels [10]. Simultaneously, advances in air purification technologies have directly sought to remove or neutralize harmful atmospheric substances [9]. Among these emerging technologies, photocatalytic materials have garnered significant interest due to their potential to degrade pollutants via light-induced chemical reactions [11,12,13,14,15].
Photocatalytic composites, primarily composed of nanometric titanium dioxide (nano-TiO2), a widely used photocatalyst due to its affordability, chemical inertness, and long-term stability [16], have shown significant potential for air purification, particularly in urban and built environments [17]. As nano-TiO2 is exposed to electromagnetic radiation of specific wavelengths, it produces highly reactive species, such as hydroxyl radicals (•OH) and superoxide anions (O2−•). These reactive oxygen species interact with airborne pollutants through redox reactions, breaking down hazardous compounds into more benign byproducts like water, carbon dioxide, or nitrate ions [18,19,20]. The photocatalytic properties of the nanomodified are influenced by several factors, including the characteristics of the semiconductor used (its crystal structure and particle size, among others [21]). In the case of nano-TiO2, three polymorphic forms can be distinguished: anatase, rutile, and brookite—while anatase can be characterized by higher photocatalytic potential, composites containing a mix of anatase and rutile demonstrate synergistically enhanced photocatalytic performance [22].
Integrating nano-TiO2 into building materials, such as paints, coatings, and concrete, can enhance the air-purification capabilities of urban infrastructure [23,24,25,26]. Photocatalytic surfaces contribute to pollutant degradation and provide additional benefits such as self-cleaning properties, expanding their serviceableness in sustainable urban design. Additionally, these materials offer a proactive and passive approach to improving urban air quality, as they operate solely with solar radiation. They target pollution at its source and enable continuous remediation under ambient light conditions [27,28].
However, the durability of organic coatings incorporating TiO2 micro- or nanoparticles remains a significant challenge. Reactive oxygen species generated on the photocatalyst’s surface, including •OH, HO2•, and O2•, are highly reactive and capable of degrading a wide range of organic and inorganic compounds. While these reactions enable self-cleaning and air purification functionalities, they also pose a risk to the stability of the polymeric matrix, potentially leading to structural degradation over time.
Allen et al. [29] investigated the impact of nano- and micro-sized TiO2 pigments on polymer degradation and stability, revealing that nano-TiO2 significantly accelerated oxidation in polyethylene, leading to the formation of hydroperoxide and carbonyl groups during processing and long-term thermal aging. In paint films, nano-TiO2 exhibited higher photooxidation activity, causing more significant degradation of the polymer binder than pigmentary-grade TiO2. Further insights from Dao et al. [30] highlighted that nano-anatase TiO2 (A-TiO2) exacerbated the photodegradation of water-based acrylic polymer coatings, as evidenced by increased weight loss and chemical structural changes following UV exposure. Their findings indicate that nano-A-TiO2 accelerates the degradation of alkane (C–H) groups and progressively reduces the transparency of coatings during prolonged aging. The study further revealed that oxygen radicals generated through photocatalytic reactions initiate chain scission within the polymeric network, resulting in increased coating surface roughness and a gradual loss of mechanical integrity. Coatings with higher A-TiO2 concentrations (up to 4 wt%) exhibited more pronounced degradation due to the intensified photocatalytic activity.
However, neither Dao nor Allen provided any information on the UV-A irradiation applied to the tested coatings, making it impossible to determine whether the photoinduced degradation of coatings containing nano-TiO2 occurs only under high UV-A irradiance or can also take place under low irradiance conditions. The standard accelerated aging procedures for paint coatings recommend exposing samples to relatively high UV-A irradiance, such as 60 W/m2, specified in ISO 16474-2:2013 [31]. On the other hand, researchers employ a wide range of UV-A irradiance levels in accelerated aging studies. For instance, Wang et al. [32] investigated the UV-A-driven degradation of organic coatings under 12 W/m2 irradiance at 340 nm. In another study, Mizielińska et al. [33] examined the influence of accelerated UV-A aging at 1.55 W/m2 on the antimicrobial properties of coatings containing ZnO nanoparticles. The significant variation in irradiance levels and exposure durations across studies makes direct comparisons of results challenging.
The degradation of the polymer matrix due to UV-A also has implications for the efficiency of photocatalytic reactions. As the polymeric network deteriorates, the progressive exposure of embedded TiO2 nanoparticles enhances their accessibility to light and reactants, thereby increasing the coatings’ air purification and self-cleaning performance. Dao et al. [30] observed that with continued degradation, the erosion of the polymer matrix results in the unveiling of previously encapsulated photocatalyst grains, leading to a higher active surface area available for pollutant degradation. This effect, while beneficial for photocatalytic efficiency [34], also signifies a trade-off between functional performance and material longevity.
To mitigate this problem, coating TiO2 particles with protective layers is commonly suggested to limit their direct interaction with the polymer matrix and reduce photocatalytic degradation. This encapsulation strategy is widely employed in the paint industry, where TiO2 primarily functions as a pigment, and its high photoreactivity is undesirable, as it can degrade the surrounding binder. In conventional paints, surface treatments with silica, alumina, or zirconia effectively suppress the oxidative effects of TiO2 while maintaining its pigmentary properties and dispersibility [35]. However, in the case of photocatalytic coatings, this approach presents a significant challenge. Unlike conventional paints, photocatalytic coatings rely on exposed TiO2 surfaces to drive air purification and self-cleaning processes through light-induced reactions. Encapsulating TiO2 in protective coatings may hinder its ability to generate reactive oxygen species (ROS), thereby reducing the overall efficiency of pollutant degradation and compromising the primary functionality of these coatings.
Instead of limiting TiO2 exposure, manufacturers of photocatalytic paints often advocate for UV exposure to activate the coating. This process involves the controlled degradation of the polymeric matrix, gradually unveiling the embedded TiO2 particles and enhancing their reactivity [36,37]. This controlled degradation maximizes photocatalytic efficiency, allowing coatings to reach their peak performance in NOx removal, VOC degradation, and self-cleaning applications. While this approach offers a clear functional advantage, it raises concerns regarding coating longevity, mechanical durability, and unintended environmental effects, such as the potential release of TiO2 nanoparticles and other harmful by-products of photolitically driven oxidation of organic matrixes. Therefore, optimizing photocatalytic coatings remains an ongoing challenge, requiring a balance between enhancing air purification capabilities and maintaining material stability to ensure long-term effectiveness in real-world conditions.

Research Significance

This study investigated the mechanism of photoinduced degradation in acrylic-based paint modified with photocatalysts under low UV-A irradiance (1–10 W/m2). To examine this, the authors analyzed morphological changes in the coatings, variations in air purification efficiency, and benzene emission resulting from polymer degradation during exposure. This study aimed to clarify how low-intensity UV-A exposure affects photocatalytic coatings’ structural integrity and functional performance by monitoring these parameters. Air purification performance was assessed as a function of the total energy supplied to the coating surface (Wh/m2) during UV-A exposure, providing a universal reference for future research and enabling meaningful comparisons across different experimental conditions.

2. Materials and Methods

2.1. Materials

Photocatalytic paint was prepared using the following formula (Table 1). Two nano-TiO2 powders were used as photocatalytic materials—A: P25 (Shanghai, China), a first-generation photocatalyst photoactive primarily in the UV-A spectrum, and B: K7000 (Leverkusen, Germany), a second-generation carbon-doped photocatalyst with extended activation bandwidth to the visible light spectrum.
Both photocatalytic materials were characterized by spherical/regular morphology (Figure 1). Phase composition, crystallite size, and the specific surface of both photocatalytic additives are presented in Table 2.
Photocatalytic paint was prepared using components commonly used in paint manufacturing. Except from photocatalytic additives, the composite consisted of inorganic micro filler (fine marble powder, Kremer Pigments, New York, NY, USA) and inorganic pigments (titanium (IV) oxide and iron (II) oxide, Kremer Pigments, New York, NY, USA). The organic binder used in the study consisted of three different constituents: acrylic polymer—orgal P900 (Kremer Pigments, New York, NY, USA), polyvinyl alcohol—moviol PVA 4-88 (Kremer Pigments, New York, NY, USA), and water-soluble polycarboxylate—orotan 731K (Kremer Pigments, New York, NY, USA). To properly disperse and stabilize the inorganic constituents of the composite within the organic binder, as well as modify the viscosity of the paint to enable its efficient application on various surfaces, additional admixtures were introduced into the composite: dispersing agent—sodium metaphosphate (Chempur, Piekary Śląskie, Poland), viscosity-modifying agent—tylose (Kremer Pigments, New York, NY, USA), defoamer (Entschaumer, Kremer Pigments, New York, NY, USA), stabilizing agent—polypropylene glycol (Chempur, Piekary Śląskie, Poland), and film-forming agent—texanol (Kremer Pigments, New York, NY, USA).
Before the coating was applied to the mineral base, all ingredients were mixed according to the procedure presented in Figure 2. After preparation, coatings were applied on non-photocatalytic cementitious mortar samples of 140 mm × 160 mm × 40 mm. Mortars were prepared with the use of CEM I 42.5R (of properties verified according to PN-EN 196-1 [38] and PN-EN 196-3 [39], normalized fine aggregate 0/2 (PN-EN 196-1 [38]), and tap water (EN 1008 [40]). The total water-to-cement ratio was 0.50. Before applying the coating, samples were cured for 28 days in the curing chamber (RH > 95%, temp = 20 ± 2 °C). The coating was applied to the surface using a paint roller with dimensions 140 mm × 160 mm.

2.2. Methods

The removal efficiency of NOx was evaluated using a procedure developed by the authors based on the ISO 22197 standard [41]. Three cuboid mortar samples were prepared, each coated with a designed photocatalytic layer. Before the air purification tests, the samples underwent a preparatory phase to ensure the cleanliness of the photoactive surface. This process involved washing the surface with distilled water, scrubbing to remove contaminants, and drying the samples at 60 °C for two hours. Measurements of NOx removal efficiency were conducted at least two hours after the completion of the preparation process.
During the testing phase, the prepared cementitious samples were placed inside a sealed glass reaction chamber with a volume of 4 dm³ (20 × 20 × 10 cm), ensuring that the photoactive surface faced the light source (Figure 3). A controlled gas (humidified zero air with NO) flow rate of 2 L/min was maintained within the chamber. The nitrogen oxide concentration at the chamber inlet was stabilized at 100 ± 5 ppb using a multi-gas calibrator CGM200 (MCZ Umwelttechnik, Bad Nauheim, Germany) to reflect NOx concentrations measured in urban settings in Warsaw, Poland. The experimental conditions were kept constant, with a relative humidity of 40 ± 5% and a temperature of 20 ± 2 °C. The tests were conducted in an air environment.
Two low-intensity LED light sources were utilized to replicate the lighting conditions typical of autumn and winter in Poland. The first source emitted UV-A radiation at a wavelength of 365 nm with an intensity of 1.0 W/m2, while the second source emitted visible light (VIS) with a global radiation intensity of 150 W/m2, devoid of any radiation below 400 nm. Nitric oxide (NO) and nitrogen oxide (NOx) concentrations were measured during the tests using a Teledyne API T200 chemiluminescence detection analyzer (Teledyne API, San Diego, CA, USA). Additionally, volatile organic compound (VOC) concentrations were quantified using a gas chromatograph analyzer (Synspec GC955, Groningen, The Netherlands). This instrument utilizes a gas chromatograph equipped with a built-in preconcentration system and a photoionization detector (PID) for analysis. The analysis involves collecting an air sample, concentrating it using the preconcentration system, separating the VOCs via gas chromatography, and detecting them with the PID.
The experimental procedure was conducted in a series of designed stages (Figure 4). To evaluate the photocatalytic performance under varying spectral conditions, the light sources were alternated among three configurations: visible light only, UV-A light only, and a combination of visible and UV-A light. Each irradiation phase lasted 30 min, followed by a 30-min interval without illumination.
During each irradiation phase, the mass amounts of nitric oxide (NO) introduced into the reactor, NO removed by the photocatalytic sample, and nitrogen dioxide (NO2) produced during the photocatalytic process were systematically measured. These data were used to calculate the rates of NOx removal and NO2 generation, expressed as the absolute change in NOx concentration in micrograms per hour per square meter of the photocatalytic coating (µg/hm2) under the different irradiation conditions. Furthermore, the selectivity of NOx removal (S) was determined using Equation (1), which accounts for the relative mass quantities of NO and NO2, as calculated from the corresponding highlighted areas in Figure 4.
S = 1 − g e n e r a t e d   N O 2 r e m o v e d   N O
The experimental protocol was divided into three distinct stages. In the first stage, air purification tests were conducted on samples with a preliminary UV-A exposure of 1 W/m2 for one hour. In the second stage, the same samples were subjected to UV-A irradiation at 1 W/m2 for 24 h before testing. The third stage involved extended UV-A irradiation of the samples at the elevated intensity (10 W/m2) for 24 h. This phased approach allowed for the analysis of the impact of cumulative UV-A exposure on the photocatalytic performance of the investigated photocatalytic coatings.
A modified experimental protocol was developed to evaluate the potential volatile organic compound (VOC) emissions—benzene—resulting from the irradiation of the photocatalytic coating. A sample with the photocatalytic coating layer oriented toward the radiation source (using parameters identical to those employed in the NOx purification analysis) was placed within the sealed reactor chamber of the specified dimensions. No additional pollutants were introduced into the system; only humidified zero air was used, with a maintained airflow rate of 2 L/min. After conducting baseline measurements, the visible and UV-A light sources were activated (emitting the same radiation intensity as previously described) for 400 min. Subsequently, the light sources were deactivated, and measurements of benzene concentrations continued for approximately 200 additional minutes to assess the emission dynamics post-irradiation.

3. Results

The photocatalytic efficiency in nitric oxide (NO) removal, NO2 generation, and final NO2 removal varied depending on irradiation conditions and intensity of preliminary exposition of photoactive coated surfaces to UV-A radiation before air purification tests (Figure 5).
With an increase in the preliminary exposure to UV-A radiation, changes in the characteristics of photocatalytic reaction dynamics were observed. Its scope varied between the investigated irradiation conditions under which photocatalytic reactions occurred (visible light only, UV-A light only, and combined light (visible and UV-A)). In the case of visible light (irradiation of 150 W/m2), the average NO removal rate increased from approx. 161 µg/hm2 to approx. 345 µg/hm2. Simultaneously, a decrease in the NO2 generation was detected, reducing from approx. 225 µg/hm2 to approx. 116 µg/hm2. In the case of exposure to UV-A light (irradiation of 1 W/m2), the average NO removal rate increased from approx. 120 µg/hm2 to approx. 360 µg/hm2. Simultaneously, NO2 generation was detected, increasing from approximately 13 µg/hm2 to approx. 35 µg/hm2. In the combined light irradiation conditions (visible light—irradiation of 150 W/m2, UV-A light—irradiation of 1 W/m2), the average NO removal rate increased from approx. 206 µg/hm2 to approx. 388 µg/hm2. Simultaneously, a decrease in the NO2 generation was detected, reducing from approx. 129 µg/hm2 to approx. 96 µg/hm2.
The two investigated factors, NO removal rate and NO2 generation rate, reflect the amount of NO that underwent either only the primary photocatalytic oxidation (conversion of NO to NO2) or the primary and secondary reactions (NO2 to NO₃−). The final NO2 removal rate was calculated (Figure 5). This rate represents the amount of introduced NO that underwent both oxidation reactions minus the NO2 generated through the photocatalytic process (NO that underwent only the primary reaction). It was found that greater preliminary exposure to UV-A irradiation enhanced the overall effectiveness of the photocatalytic process, regardless of the irradiation conditions during the air purification tests. In the case of visible light (irradiation of 150 W/m2), the average final NO2 removal rate increased from approx. 22 µg/hm2 to approx. 414 µg/hm2. The average final NO2 removal rate for UV-A irradiation increased from approx. 170 µg/hm2 to approx. 517 µg/hm2, and in the case of combined irradiation conditions, the average final NO2 removal rate increased from approx. 187 µg/hm2 to approx. 500 µg/hm2.
The relationship between the amount of NO removed and NO2 generated varied depending on the irradiation conditions during the air purification tests and the prior exposure of the photocatalytic coating to UV-A irradiation (Table 3). It was observed that the selectivity of the photocatalytic process improved with increased preliminary UV-A exposure. This effect was particularly pronounced under visible light conditions, where selectivity increased significantly from approximately −0.40 to 0.66 following initial UV-A irradiation of the samples. An increase in photocatalytic selectivity indicates a reduction in NO2 generation during the process, as more NO molecules undergo both photocatalytic reactions. This enhancement improves the overall efficiency and quality of the photocatalytic air purification process. The highest selectivity was detected in the case of UV-A irradiation conditions, staying above 0.80 regardless of preliminary exposure of samples to UV-A radiation.

4. Discussion

Photocatalytic composites have emerged as promising materials for mitigating air pollution in urban environments by effectively degrading air pollutants [42,43]. Their ability to perform photocatalytic reactions is underpinned by an interplay of physicochemical processes that govern the rate and efficiency of pollutant breakdown. These processes are initiated when the material is exposed to electromagnetic radiation within specific wavelength ranges, typically UV-A or visible light, activating the photocatalytic compounds embedded in the composite matrix.
The efficiency of photocatalysis in these composites is influenced by several interdependent factors [3]. The inherent surface properties of the composite, such as porosity, roughness, and chemical composition, significantly affect the adsorption and subsequent reaction kinetics between free radicals and pollutants [44,45]. Environmental conditions, including light intensity, humidity, and the presence of competing substances, further influence the overall photocatalytic performance by impacting the generation and reactivity of free radicals [3,46,47]. These factors collectively regulate the dynamic interactions between reactive oxygen species and target pollutants. Surface properties are crucial in regulating the duration and intensity of these interactions, as they affect the adsorption–desorption equilibrium and the availability of active sites for chemical reactions. Since photocatalytic reactions primarily occur on the composite’s photoactive surface, a high nano-TiO2 concentration and uniform distribution in the near-surface layer enhance the interaction frequency between pollutant particles and photocatalytically generated free radicals [48]. However, as the mass content of the nano-photocatalyst increases, particle agglomeration becomes more likely. This leads to an uneven distribution on the photoactive surface, which can negatively impact the composite’s photocatalytic performance [49,50].
This study focused on the performance of photocatalytic coatings, explicitly examining their potential degradation under low-radiation conditions due to free radical generation during the photocatalytic process and its subsequent impact on air purification efficiency. It was found that increasing preliminary exposure to UV-A irradiation before air purification tests significantly enhanced the effectiveness of NO2 removal. This improvement suggests alterations in the surface properties of the photocatalytic coating, likely due to increased exposure of photocatalytic nano-TiO2 particles to the external environment.
This hypothesis was confirmed through scanning electron microscopy and energy-dispersive spectroscopy (SEM/EDS) analysis (Figure 6), which revealed significant changes in the photoactive surface characteristics before and after preliminary UV-A irradiation. Microcracks and disruptions in the continuity of the photocatalytic layer were observed, indicating an increase in the overall photocatalytic surface area. These changes likely resulted in greater exposure of nano-TiO2 particles to external pollutants, enhancing the coating’s photocatalytic performance.
Previous research by the authors established that surface characteristics, including the porosity of the photocatalytic layer, play a critical role in determining the air purification efficiency of photocatalytic composites [14]. Increased porosity expands the photoactive surface area for photocatalytic reactions, improving air purification performance. A similar phenomenon was observed in this study: pre-irradiation with UV-A partially degraded the organic components in the coating, altering its surface characteristics and improving its air purification performance.
For the quantitative analysis of the coating degradation caused by UV-A exposure, SEM images of microregions at 1000× magnification (Figure 6a,b) were binarized, assuming that the black areas represented pores. Notably, the surface porosity in the analyzed microregions of coatings exposed to 265 Wh/m2 increased more than fourfold, from 2.28% to 9.09%, due to the formation of microcracks (Figure 7).
For the investigated photocatalytic coating, it was hypothesized that the observed alterations in surface properties of the composite were driven by photocatalytically induced degradation of the organic binder. This decomposition process likely played a central role in modifying the structural and functional attributes of the coating, which in turn influenced its performance during air purification applications. The breakdown of the organic binder, initiated by the generation of reactive oxygen species (ROS) during the photocatalytic process, is believed to contribute to several interrelated phenomena. The degradation of the binder increases the coating’s surface porosity and roughness, exposing more nano-TiO2 particles to the environment. This enhances the photocatalytic activity of the photoactive layer by providing additional active sites for pollutant degradation.
However, the decomposition of the binder could also lead to potential secondary effects. The decomposition of the binder could contribute to increased volatile organic compound (VOC) emissions during the photocatalytic process, which may arise from the release of binder degradation byproducts. These VOC emissions can interact with ambient nitrogen oxides (NOx) to promote the formation of near-surface ozone, a secondary pollutant that poses environmental and health risks [51,52,53]. The elevated ozone generation near the coating surface may offset some air purification benefits by contributing to localized air quality deterioration.
The binder degradation also contributes to the loss in the structural integrity of the coating, as evidenced by the microcracks and discontinuities observed in SEM imagery of the photoactive surface after its exposure to UV-A irradiation. While these changes can enhance the photocatalytic surface area, they may also reduce the long-term durability and adhesion of the coating to its substrate, necessitating additional considerations for practical implementation in urban environments. Also, as the matrix of the composites loses continuity, the risk of nano-TiO2 emissions from the photoactive surface due to external loads could be increased. The authors investigated the issue of photocatalytic cementitious composites, where it was found that proper compatibility and embedding of nano-TiO2 grains within the cement matrix reduces particle emissions originating from an abrasion load [54]. However, in the case of photocatalytic coatings, one can imagine that the load required to mobilize partially bounded nano-TiO2 grains within the cement matrix would be significantly smaller due to the partial decomposition of the binder.
An additional experimental protocol was introduced to assess benzene emissions, representing volatile organic compounds (VOCs), during the irradiation phase of the photocatalytic coating (Figure 8). A sample of the photocatalytic coating was placed in a reaction chamber and subjected to combined light irradiation conditions, the same as in the NOx air purification procedure. The concentrations of benzene were monitored and analyzed.
The results indicated that approximately five ppb of benzene was emitted on average as a direct consequence of binder decomposition under the low irradiation conditions (UV-A—1 W/m2 and visible light—150 W/m2). While the decomposition of the coating binder facilitated the degradation of harmful air pollutants (NOx), it released secondary contaminants due to the breakdown of organic components within the coating itself. The release of benzene, a known hazardous air pollutant with carcinogenic properties, is particularly concerning in this context.
The investigated degradation impacts the positive influence on air quality of photocatalytic coatings made with organic binders. Enclosed spaces often have limited ventilation, which can lead to the accumulation of VOCs like benzene, potentially compromising indoor air quality. Prolonged exposure to even low levels of VOCs can pose significant health risks, including respiratory irritation and long-term effects such as an increased risk of cancer. Additionally, the presence of VOCs indoors may react with other indoor air components, such as ozone, to form secondary pollutants [51]. In outdoor settings, the impact of VOC emissions from photocatalytic coatings is compounded by their interaction with ambient nitrogen oxides (NOx) under sunlight, which can contribute to forming near-surface ozone and photochemical smog [52,53]. This is particularly problematic in urban areas, where high NOx concentrations and intense sunlight provide favorable conditions for secondary pollutant formation. The emission of benzene and other VOCs could inadvertently undermine the air purification benefits of photocatalytic coatings by promoting localized air quality degradation (Figure 9).
To address these challenges, several mitigation strategies can be considered. For indoor applications, coatings with lower organic binder content or those incorporating non-degradable alternatives may help reduce VOC emissions. Additionally, integrating photocatalytic coatings with air purification systems capable of adsorbing or degrading VOCs could enhance their safety and effectiveness. Optimizing the formulation of photocatalytic coatings to minimize binder decomposition while maintaining high pollutant removal efficiency is essential for outdoor use. For instance, utilizing silicone-modified acrylic resins [55] as the polymer matrix, incorporating fluoroacrylate monomers [56], or introducing hindered amine light stabilizers (HALS) [57] into the polymeric resin can enhance the resistance of the polymer matrix to photooxidation. Additionally, using the rutile form of TiO2 instead of anatase can further mitigate the degradation of the organic matrix. Environmental conditions such as light intensity, temperature, and humidity should also be carefully considered to tailor the coating’s performance to specific locations. Regulatory measures, including limits on VOC emissions from construction materials, may further ensure that photocatalytic coatings contribute positively to air quality without introducing significant secondary pollution.
While photocatalytic coatings hold considerable promise for air purification, their unintended consequences must be thoroughly understood and addressed to maximize their benefits in indoor and outdoor environments.

5. Conclusions

Photocatalytic coatings incorporating nano-TiO2 have gained significant attention as a passive air purification technology capable of degrading pollutants under natural light exposure. However, their long-term performance depends not only on their photocatalytic efficiency but also on their structural stability, particularly in organic-based formulations. This study examined the effects of prolonged low-intensity UV-A exposure (1–10 W/m2) on the degradation of acrylic-based photocatalytic coatings, simulating real-world conditions in autumn and winter months in Northern and Central Europe. This research focused on how binder decomposition influences air purification performance and potential secondary emissions.
Based on the results of this research, it was found that:
  • Significant organic binder decomposition was observed even under low irradiation conditions (UV-A irradiance in the range of 1–10 W/m2), corresponding to light conditions during autumn and winter in regions such as Northern and Central Europe. This finding raises concerns regarding organic-based photocatalytic coatings, as the research demonstrated that acrylic-based coatings could undergo continuous degradation throughout the year. As shown in this study, such degradation may lead to the emission of VOCs into the atmosphere and potentially the release of nano-TiO2 due to the disintegration of the polymer matrix.
  • Prolonged exposure to low-intensity UV-A radiation (1–10 W/m2) led to the progressive degradation of the organic binder, increasing surface porosity from 2.28% to 9.09%. This transformation enhanced the accessibility of nano-TiO2 particles, improving the photocatalytic efficiency of NOx removal by facilitating more significant interaction between pollutants and active sites.
  • The organic binder tended to decompose due to the reactive species generated during photocatalytic reactions, with the extent of degradation being strongly linked to the duration of UV-A irradiation exposure.
  • The partial decomposition of the binder resulted in increased VOC emissions (up to 5 ppb), which could serve as a precursor to the formation of near-to-surface ozone.

Author Contributions

Conceptualization, M.K., J.K. and K.C.; methodology, M.K., K.C. and J.K.; software, K.C. and M.K.; validation, M.K., K.C. and J.K.; formal analysis, P.Ł. and W.J.-R.; investigation, M.K., K.C., J.K., P.Ł. and W.J.-R.; resources, J.K., K.C. and M.K.; data curation, M.K. and K.C.; writing—original draft preparation, M.K. and K.C.; writing—review and editing, M.K., K.C., J.K., P.Ł. and W.J.-R.; visualization, K.C. and M.K.; supervision, P.Ł. and W.J.-R.; project administration, W.J.-R.; funding acquisition, W.J.-R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the project TECHMATSTRATEG-III/0013/2019-01 of the NCBiR (National Centre for Research and Development).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are included in the paper and are available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Guo, M.-Z.; Chen, J.; Xia, M.; Wang, T.; Poon, C.S. Pathways of Conversion of Nitrogen Oxides by Nano TiO2 Incorporated in Cement-Based Materials. Build. Environ. 2018, 144, 412–418. [Google Scholar] [CrossRef]
  2. Seo, D.; Yun, T.S. NOx Removal Rate of Photocatalytic Cementitious Materials with TiO2 in Wet Condition. Build. Environ. 2017, 112, 233–240. [Google Scholar] [CrossRef]
  3. Staub de Melo, J.V.; Trichês, G. Evaluation of the Influence of Environmental Conditions on the Efficiency of Photocatalytic Coatings in the Degradation of Nitrogen Oxides (NOx). Build. Environ. 2012, 49, 117–123. [Google Scholar] [CrossRef]
  4. Nahorski, Z.; Holnicki, P.; Kałuszko, A. Towards Air Quality Protection in an Urban Area—Case Study. Atmosphere 2024, 15, 1106. [Google Scholar] [CrossRef]
  5. Ballari, M.M.; Hunger, M.; Hüsken, G.; Brouwers, H.J.H. NOx Photocatalytic Degradation Employing Concrete Pavement Containing Titanium Dioxide. Appl. Catal. B Environ. 2010, 95, 245–254. [Google Scholar] [CrossRef]
  6. Holnicki, P.; Kałuszko, A.; Nahorski, Z. A Projection of Environmental Impact of a Low Emission Zone Planned in Warsaw, Poland. Sustainability 2023, 15, 16260. [Google Scholar] [CrossRef]
  7. Cakaj, A.; Lisiak-Zielińska, M.; Khaniabadi, Y.O.; Sicard, P. Premature Deaths Related to Urban Air Pollution in Poland. Atmosphere 2023, 301, 119723. [Google Scholar] [CrossRef]
  8. Holnicki, P.; Tainio, M.; Kałuszko, A.; Nahorski, Z. Burden of Mortality and Disease Attributable to Multiple Air Pollutants in Warsaw, Poland. Int. J. Environ. Res. Public Health 2017, 14, 1359. [Google Scholar] [CrossRef]
  9. Mata, T.M.; Martins, A.A.; Calheiros, C.S.C.; Villanueva, F.; Alonso-Cuevilla, N.P.; Gabriel, M.F.; Silva, G.V. Indoor Air Quality: A Review of Cleaning Technologies. Environments 2022, 9, 118. [Google Scholar] [CrossRef]
  10. European Union. Directive 2008/50/EC of the European Parliament and of the Council of 21 May 2008 on Ambient Air Quality and Cleaner Air for Europe. Off. J. Eur. Union 2008, L 152, 1–44.
  11. Chilmon, K.; Kalinowski, M.; Jackiewicz-Rek, W. Influence of Coarse Aggregate Exposure on Air Purification Efficiency in Photocatalytic Cement Composites. Buildings 2024, 14, 3639. [Google Scholar] [CrossRef]
  12. Parasuraman, V.; Sekar, P.P.; Lee, H.; Sheraz, M.; Ly, H.N.; Azizar, G.A.B.; Hong, J.W.; Lee, W.R.; Kim, S. Photocatalytic Self-Cleaning Eco-Friendly Paint: A Unique Approach for Efficient Indoor Air Pollutant Removal and Surface Disinfection. Constr. Build. Mater. 2024, 412, 134671. [Google Scholar] [CrossRef]
  13. Boyjoo, Y.; Sun, H.; Liu, J.; Pareek, V.K.; Wang, S. A Review on Photocatalysis for Air Treatment: From Catalyst Development to Reactor Design. Chem. Eng. J. 2017, 310 Pt 2, 537–559. [Google Scholar] [CrossRef]
  14. Chilmon, K.; Kalinowski, M.; Jackiewicz-Rek, W. Effect of Cement Substitution with Mineral Fillers on NOx Air-Purification Efficiency and Photocatalytic Reaction Selectivity of Nano-TiO2-Modified Cementitious Composites. Materials 2024, 17, 5775. [Google Scholar] [CrossRef] [PubMed]
  15. Chen, Z.; Yin, H.; Wang, C.; Wang, R.; Peng, Y.; You, C.; Li, J. Environmental Science & Technology. Environ. Sci. Technol. 2021, 55, 9285–9292. [Google Scholar] [CrossRef] [PubMed]
  16. Seremak, W.; Jasiorski, M.; Baszczuk, A.; Winnicki, M. Durability Assessment of Low-Pressure Cold-Sprayed TiO2 Photocatalytic Coatings: Photocatalytic and Mechanical Stability. Surf. Coat. Technol. 2025, 497, 131740. [Google Scholar] [CrossRef]
  17. Coelho, S.; Ferreira, J.; Rodrigues, V.; Lopes, M. Source Apportionment of Air Pollution in European Urban Areas: Lessons from the ClairCity Project. J. Environ. Manag. 2022, 320, 115899. [Google Scholar] [CrossRef]
  18. Amakiri, K.T.; Angelis-Dimakis, A.; Chatzisymeon, E. Photocatalytic Degradation of Phenol and Polycyclic Aromatic Hydrocarbons in Water by Novel Acid Soluble Collagen-Polyvinylpyrrolidone Polymer Embedded in Nitrogen-TiO2. Chem. Phys. 2025, 589, 112485. [Google Scholar] [CrossRef]
  19. Nakata, K.; Fujishima, A. TiO2 Photocatalysis: Design and Applications. J. Photochem. Photobiol. C Photochem. Rev. 2012, 13, 169–189. [Google Scholar] [CrossRef]
  20. Fujishima, A.; Rao, T.N.; Tryk, D.A. Titanium Dioxide Photocatalysis. J. Photochem. Photobiol. C Photochem. Rev. 2000, 1, 1–21. [Google Scholar] [CrossRef]
  21. Kalinowski, M.; Chilmon, K.; Jackiewicz-Rek, W.; Rakowski, B. The Influence of Selected Material Variables of Photocatalytic Cementitious Composites on the Self-Cleaning Properties and Air Purification Efficiency from NOx Pollutants. Sustainability 2023, 15, 853. [Google Scholar] [CrossRef]
  22. Li, C.; Wang, J.; Guo, H.; Ding, S. Low Temperature Synthesis of Polyaniline–Crystalline TiO2–Halloysite Composite Nanotubes with Enhanced Visible Light Photocatalytic Activity. J. Colloid Interface Sci. 2015, 458, 1–13. [Google Scholar] [CrossRef] [PubMed]
  23. Jiménez-Relinque, E.; Hingorani, R.; Rubiano, F.; Grande, M.; Castillo, Á.; Castellote, M. In Situ Evaluation of the NOx Removal Efficiency of Photocatalytic Pavements: Statistical Analysis of the Relevance of Exposure Time and Environmental Variables. Environ. Sci. Pollut. Res. 2019, 26, 36088–36095. [Google Scholar] [CrossRef] [PubMed]
  24. Folli, A.; Strøm, M.; Madsen, T.P.; Henriksen, T.; Lang, J.; Emenius, J.; Klevebrant, T.; Nilsson, Å. Field Study of Air Purifying Paving Elements Containing TiO2. Atmos. Environ. 2015, 107, 44–51. [Google Scholar] [CrossRef]
  25. Park, H.-J.; Hossain, S.M.; Choi, K.; Shon, H.-K.; Kim, J.-H. A Study on the Evaluation Methods of Nitrogen Oxide Removal Performance of Photocatalytic Concrete for Outdoor Applications. Catalysts 2022, 12, 846. [Google Scholar] [CrossRef]
  26. Russell, H.S.; Frederickson, L.B.; Hertel, O.; Ellermann, T.; Jensen, S.S. A Review of Photocatalytic Materials for Urban NOx Remediation. Catalysts 2021, 11, 675. [Google Scholar] [CrossRef]
  27. Kalinowski, M.; Chilmon, K.; Jackiewicz-Rek, W. Carbon-Negative Nano-TiO2-Modified Photocatalytic Cementitious Composites: Removal of Airborne Pollutants (NOx and O3) and Its Impact on CO2 Footprint. Coatings 2024, 14, 1607. [Google Scholar] [CrossRef]
  28. Bui, V.K.H.; Nguyen, T.N.; Tran, V.V.; Hur, J.; Kim, I.T.; Park, D.; Lee, Y.-C. Photocatalytic Materials for Indoor Air Purification Systems: An Updated Mini-Review. Environ. Technol. Innov. 2021, 22, 101471. [Google Scholar] [CrossRef]
  29. Allen, N.S.; Edge, M.; Ortega, A.; Sandoval, G.; Liauw, C.M.; Verran, J.; McIntyre, R.B. Degradation and stabilisation of polymers and coatings: Nano versus pigmentary titania particles. Polym. Degrad. Stab. 2004, 85, 927–946. [Google Scholar] [CrossRef]
  30. Dao, P.H.; Nguyen, T.V.; Nguyen, T.A.; Doan, T.Y.O.; Hoang, T.H.; Le, T.T.; Nguyen-Tri, P. Acrylic polymer/TiO2 nanocomposite coatings: Mechanism for photo-degradation and solar heat reflective recovery. Mater. Chem. Phys. 2021, 272, 124984. [Google Scholar] [CrossRef]
  31. ISO 16474-2:2013; Paints and Varnishes—Methods of Exposure to Laboratory Light Sources—Part 2: Xenon-Arc Lamps. International Organization for Standardization (ISO): Geneva, Switzerland, 2013.
  32. Wang, A.; Li, Y.; Zhang, D.; Li, X. A comparative study on UV degradation of organic coatings for concrete: Structure, adhesion, and protection performance. Prog. Org. Coat. 2020, 149, 105892. [Google Scholar] [CrossRef]
  33. Mizielińska, M.; Łopusiewicz, Ł.; Mężyńska, M.; Bartkowiak, A. The Influence of Accelerated UV-A and Q-SUN Irradiation on the Antimicrobial Properties of Coatings Containing ZnO Nanoparticles. Molecules 2017, 22, 1556. [Google Scholar] [CrossRef] [PubMed]
  34. Pill, D.; Wiesen, P.; Kleffmann, J. Temperature Dependencies of the Degradation of NO, NO2, and HONO on a Photocatalytic Dispersion Paint. Phys. Chem. Chem. Phys. 2021, 23, 9418–9427. [Google Scholar] [CrossRef] [PubMed]
  35. Guo, J.; Van Bui, H.; Valdesueiro, D.; Yuan, S.; Liang, B.; Van Ommen, J.R. Suppressing the Photocatalytic Activity of TiO2 Nanoparticles by Extremely Thin Al2O3 Films Grown by Gas-Phase Deposition at Ambient Conditions. Nanomaterials 2018, 8, 61. [Google Scholar] [CrossRef] [PubMed]
  36. Bonil, L.; Rojas, S.; Buesa, J.; Pérez-Rodríguez, F.J.; Domingo-Calap, P.; Sánchez, G. Virucidal Properties of Photocatalytic Coating on Glass against a Human Coronavirus. Microbiol. Spectr. 2022, 10, e00269-22. [Google Scholar] [CrossRef]
  37. Han, R.; Coey, J.; O’Rourke, C.; Mills, A. Flexible, disposable photocatalytic plastic films for the destruction of viruses. J. Photochem. Photobiol. B 2022, 234, 112551. [Google Scholar] [CrossRef] [PubMed]
  38. PN-EN 196-1:2016-07; Methods of Testing Cement—Part 1: Determination of Strength. PKN—The Polish Committee for Standardization: Warsaw, Poland, 2016.
  39. PN-EN 196-3:2016; Methods of Testing Cement—Part 3: Determination of Setting Times and Soundness. PKN—The Polish Committee for Standardization: Warsaw, Poland, 2016.
  40. EN 1008:2002; Mixing Water for Concrete-Specification for Sampling, Testing and Assessing the Suitability of Water, Including Water Recovered from Processes in the Concrete Industry, as Mixing Water for Concrete. European Committee for Standardization: Brussels, Belgium, 2002.
  41. ISO 22197-1:2016; Fine Ceramics (Advanced Ceramics, Advanced Technical Ceramics)—Test Method for Air-Purification Performance of Semiconducting Photocatalytic Materials. Part 1: Removal of Nitric Oxide. ISO (The International Organization for Standardization): Geneva, Switzerland, 2016.
  42. Yoo, S.-W.; Lee, J.-W.; Park, B.; Choi, Y.C. Photocatalytic NOx Degradation Performance of TiO2-Nanofiber-Spray-Coated Foam Composite According to Saturated Conditions. Constr. Build. Mater. 2022, 358, 129414. [Google Scholar] [CrossRef]
  43. Casagrande, E.A.; Repette, W.L.; Hotza, D. Effect of Environmental Conditions on Degradation of NOx Gases by Photocatalytic Nanotitania-Based Cement Mortars after Long-Term Hydration. J. Clean. Prod. 2020, 274, 123067. [Google Scholar] [CrossRef]
  44. Xu, M.; Clack, H.; Xia, T.; Bao, Y.; Wu, K.; Shi, H.; Li, V. Effect of TiO2 and Fly Ash on Photocatalytic NOx Abatement of Engineered Cementitious Composites. Constr. Build. Mater. 2020, 236, 117559. [Google Scholar] [CrossRef]
  45. Şahin, O. Assessment of the Long-Term Photocatalytic Performance of Cementitious Systems Incorporating Different Mineral Admixtures. Constr. Build. Mater. 2022, 354, 129215. [Google Scholar] [CrossRef]
  46. Ahmad, R.; Ahmad, Z.; Khan, A.U.; Mastoi, N.R.; Aslam, M.; Kim, J. Photocatalytic Systems as an Advanced Environmental Remediation: Recent Developments, Limitations and New Avenues for Applications. J. Environ. Chem. Eng. 2016, 4 Pt A, 4143–4164. [Google Scholar] [CrossRef]
  47. Wang, H.; Wu, Z.; Zhao, W.; Guan, B. Photocatalytic Oxidation of Nitrogen Oxides Using TiO2 Loading on Woven Glass Fabric. Chemosphere 2007, 66, 185–190. [Google Scholar] [CrossRef]
  48. Jia, Z.-M.; Zhao, Y.-R.; Shi, J.-N. Adsorption Kinetics of the Photocatalytic Reaction of Nano-TiO2 Cement-Based Materials: A Review. Constr. Build. Mater. 2023, 370, 130462. [Google Scholar] [CrossRef]
  49. Lakshminarasimhan, N.; Kim, W.; Choi, W. Effect of the Agglomerated State on the Photocatalytic Hydrogen Production with in Situ Agglomeration of Colloidal TiO2 Nanoparticles. J. Phys. Chem. C 2008, 112, 20451–20457. [Google Scholar] [CrossRef]
  50. Folli, A.; Pochard, I.; Nonat, A.; Jakobsen, U.H.; Shepherd, A.M.; Macphee, D.E. Engineering Photocatalytic Cements: Understanding TiO2 Surface Chemistry to Control and Modulate Photocatalytic Performances. J. Am. Ceram. Soc. 2010, 93, 3360–3369. [Google Scholar] [CrossRef]
  51. Kanayankottupoyil, J.; John, K. Characterization and Source Apportionment of Ambient VOC Concentrations: Assessing Ozone Formation Potential in the Barnett Shale Oil and Gas Region. Atmos. Pollut. Res. 2025, 16, 102327. [Google Scholar] [CrossRef]
  52. Derwent, R.G.; Utembe, S.R.; Jenkin, M.E.; Khan, M.A.H.; Shallcross, D.E. Investigating the Role of Organic Compounds in Intercontinental Ozone Transport: Reactivity Scales and Global Warming Potentials (GWPs). Atmos. Environ. 2023, 306, 119817. [Google Scholar] [CrossRef]
  53. Badia, A.; Vidal, V.; Ventura, S.; Curcoll, R.; Segura, R.; Villalba, G. Modelling the Impacts of Emission Changes on O3 Sensitivity, Atmospheric Oxidation Capacity, and Pollution Transport over the Catalonia Region. Atmos. Chem. Phys. 2023, 23, 10751–10774. [Google Scholar] [CrossRef]
  54. Witkowski, H.; Jarosławski, J.; Szkop, A.; Chilmon, K.; Kalinowski, M.; Jackiewicz-Rek, W. The Potential Risk of Nanoparticulate Release from Photocatalytic Pavement Concrete Surface Due to a Simulated Abrasion Load—An Experimental Study. Materials 2024, 17, 3022. [Google Scholar] [CrossRef] [PubMed]
  55. Zhang, D.; Liu, J.; Li, Z.; Shen, Y.; Wang, P.; Wang, D.; Hu, X. Preparation and properties of UV-curable waterborne silicon-containing polyurethane acrylate emulsion. Prog. Org. Coat. 2021, 160, 106503. [Google Scholar] [CrossRef]
  56. Malshe, V.C.; Sangaj, N.S. Fluorinated acrylic copolymers: Part I: Study of clear coatings. Prog. Org. Coat. 2005, 53, 207–211. [Google Scholar] [CrossRef]
  57. Hodgson, J.L.; Coote, M.L. Clarifying the Mechanism of the Denisov Cycle: How Do Hindered Amine Light Stabilizers Protect Polymer Coatings from Photo-oxidative Degradation? Macromolecules 2010, 43, 4573–4583. [Google Scholar] [CrossRef]
Coatings 15 00281 g001
Figure 1. TEM micrographs of agglomerated nano-TiO2 crystallites of regular/spherical grain morphology. Samples were prepared on copper grids covered with carbon film (microscope: TEM Tecnai TF 20 X-TWIN; EDAX, 200 kV). (a) first generation nano-TiO2; (b) second generation nano-TiO2.
Figure 1. TEM micrographs of agglomerated nano-TiO2 crystallites of regular/spherical grain morphology. Samples were prepared on copper grids covered with carbon film (microscope: TEM Tecnai TF 20 X-TWIN; EDAX, 200 kV). (a) first generation nano-TiO2; (b) second generation nano-TiO2.
Coatings 15 00281 g001
Coatings 15 00281 g002
Figure 2. Mixing protocol for the preparation of photocatalytic coatings. Low- and high-speed mechanical homogenization was performed using a high-performance mechanical dissolver (2.20 kW, proLAB, Gliwice, Poland) equipped with a rotating dispersing disk. Sonification was performed with the use of a SONICS Vibra-Cell ultrasonic processor homogenizer (VCX 750, Sonics & Materials, Newtown, CT, USA), employing ultrasonic cavitation (amplitude = 115 μm, frequency = 20 kHz) for adequate dispersion of photocatalytic material within the matrix of the coating.
Figure 2. Mixing protocol for the preparation of photocatalytic coatings. Low- and high-speed mechanical homogenization was performed using a high-performance mechanical dissolver (2.20 kW, proLAB, Gliwice, Poland) equipped with a rotating dispersing disk. Sonification was performed with the use of a SONICS Vibra-Cell ultrasonic processor homogenizer (VCX 750, Sonics & Materials, Newtown, CT, USA), employing ultrasonic cavitation (amplitude = 115 μm, frequency = 20 kHz) for adequate dispersion of photocatalytic material within the matrix of the coating.
Coatings 15 00281 g002
Coatings 15 00281 g003
Figure 3. Air purification test setup.
Figure 3. Air purification test setup.
Coatings 15 00281 g003
Coatings 15 00281 g004
Figure 4. Air purification from NOx test procedure.
Figure 4. Air purification from NOx test procedure.
Coatings 15 00281 g004
Coatings 15 00281 g005
Figure 5. Average values of (a) NO removal rate of investigated photocatalytic coatings in different irradiation conditions after varying preliminary exposure to UV-A irradiation, (b) NO2 generation rate of investigated photocatalytic coatings in different irradiation conditions after varying preliminary exposure to UV-A irradiation, and (c) final NO2 removal rate of investigated photocatalytic coatings in different irradiation conditions after varying preliminary exposure to UV-A irradiation.
Figure 5. Average values of (a) NO removal rate of investigated photocatalytic coatings in different irradiation conditions after varying preliminary exposure to UV-A irradiation, (b) NO2 generation rate of investigated photocatalytic coatings in different irradiation conditions after varying preliminary exposure to UV-A irradiation, and (c) final NO2 removal rate of investigated photocatalytic coatings in different irradiation conditions after varying preliminary exposure to UV-A irradiation.
Coatings 15 00281 g005
Coatings 15 00281 g006
Figure 6. (a) The photoactive surface of the investigated coating before preliminary exposure to UV-A radiation, along with a magnified view of the highlighted area. (b) The photoactive surface of the investigated coating after the preliminary exposure to UV-A radiation (265 Wh/m2) with visible cracks, and loss of continuity, along with a magnified view of the highlighted area. (c) EDS mapping of TiO2 grains on the photoactive surface depicted in (a). (d) EDS mapping of TiO2 grains on the photoactive surface depicted in (b).
Figure 6. (a) The photoactive surface of the investigated coating before preliminary exposure to UV-A radiation, along with a magnified view of the highlighted area. (b) The photoactive surface of the investigated coating after the preliminary exposure to UV-A radiation (265 Wh/m2) with visible cracks, and loss of continuity, along with a magnified view of the highlighted area. (c) EDS mapping of TiO2 grains on the photoactive surface depicted in (a). (d) EDS mapping of TiO2 grains on the photoactive surface depicted in (b).
Coatings 15 00281 g006
Coatings 15 00281 g007
Figure 7. Binarized images illustrating the surface porosity of the coating: (a) before and (b) after UV-A exposure.
Figure 7. Binarized images illustrating the surface porosity of the coating: (a) before and (b) after UV-A exposure.
Coatings 15 00281 g007
Coatings 15 00281 g008
Figure 8. Change in the benzene concentrations due to organic binder degradation of the investigated photocatalytic coating.
Figure 8. Change in the benzene concentrations due to organic binder degradation of the investigated photocatalytic coating.
Coatings 15 00281 g008
Coatings 15 00281 g009
Figure 9. Mechanism of photocatalytically induced degradation of organic-based coating under low radiation conditions.
Figure 9. Mechanism of photocatalytically induced degradation of organic-based coating under low radiation conditions.
Coatings 15 00281 g009
Table 1. The composition of photocatalytic paint investigated in the study.
Table 1. The composition of photocatalytic paint investigated in the study.
Components of Photocatalytic PaintTypeComponent Content, % m.c. *
Nano-TiO2 photocatalyst (A)first-generation nano-TiO25.00
Nano-TiO2 photocatalyst (B)carbon-doped second-generation nano-TiO25.00
Distilled water-10.30
Dispersing agentsodium metaphosphate, 1%10.00
Defoamersilicone oil1.00
Organic viscosity-modifying agentmethylhydroxycellulose, 2%13.52
Stabilizing agentpolypropylene glycol4.00
Organic bindermix of acrylic polymer, water-soluble polycarboxylate, and polyvinyl alcohol17.20
Inorganic micro fillerfine marble powder < 32 µm27.00
Inorganic pigmenttitanium (IV) oxide and iron (II) oxide5.48
Organic film-forming agenttexanol, 3-hydroxy-2,2,4-trimethylpentyl isobutyrate1.50
* mass content.
Table 2. The content of the crystalline phases, size of the crystallites, and specific surface area of the photocatalysts used in this study.
Table 2. The content of the crystalline phases, size of the crystallites, and specific surface area of the photocatalysts used in this study.
Nano-TiO2Specific Surface Area, m2/gPhase, %Crystallite Size, nm
AnataseRutileAnataseRutile
A—first generation53.887133354
B—second generation246.8100010-
Table 3. Average selectivity of NOx-based photocatalytic reactions for coatings investigated in this study, calculated according to Equation (1).
Table 3. Average selectivity of NOx-based photocatalytic reactions for coatings investigated in this study, calculated according to Equation (1).
Preliminary Exposure to UV-A Irradiation Before Air Purification Test, Wh/m2Average Selectivity, -
Visible LightUV-A LightCombined Light (Visible and UV-A)
1−0.400.890.37
250.380.920.59
2650.660.900.75
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Kalinowski, M.; Chilmon, K.; Kuziak, J.; Łukowski, P.; Jackiewicz-Rek, W. Photocatalytically Induced Degradation of Nano-TiO2-Modified Paint Coatings Under Low-Radiation Conditions. Coatings 2025, 15, 281. https://doi.org/10.3390/coatings15030281

AMA Style

Kalinowski M, Chilmon K, Kuziak J, Łukowski P, Jackiewicz-Rek W. Photocatalytically Induced Degradation of Nano-TiO2-Modified Paint Coatings Under Low-Radiation Conditions. Coatings. 2025; 15(3):281. https://doi.org/10.3390/coatings15030281

Chicago/Turabian Style

Kalinowski, Maciej, Karol Chilmon, Justyna Kuziak, Paweł Łukowski, and Wioletta Jackiewicz-Rek. 2025. "Photocatalytically Induced Degradation of Nano-TiO2-Modified Paint Coatings Under Low-Radiation Conditions" Coatings 15, no. 3: 281. https://doi.org/10.3390/coatings15030281

APA Style

Kalinowski, M., Chilmon, K., Kuziak, J., Łukowski, P., & Jackiewicz-Rek, W. (2025). Photocatalytically Induced Degradation of Nano-TiO2-Modified Paint Coatings Under Low-Radiation Conditions. Coatings, 15(3), 281. https://doi.org/10.3390/coatings15030281

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop