TiO₂-Based Photocatalytic Building Material for Air Purification in Sustainable and Low-Carbon Cities: A Review

Abstract

TiO₂-based building materials possess air purification, self-cleaning, and sterilization functionalities, making them innovative green building materials with significant potential for future energy-saving and emission-reduction applications. However, the transition from laboratory-scale to practical applications poses substantial challenges in improving the photocatalytic efficiency and stability of TiO₂-based building materials. In recent years, researchers have made considerable efforts to enhance their efficiency and stability. This paper provides a concise overview of the photocatalytic principles employed in buildings for air purification, discusses preparation techniques for TiO₂-based building materials, explores strategies to improve their efficiency, outlines key factors influencing their performance in practical applications, analyzes limitations, and discusses future development trends. Finally, we propose recommendations for further research on photocatalytic buildings and their real-world implementation as a valuable reference for developing highly efficient and stable photocatalytic building materials. The aim of this paper is to guide the application of TiO₂-based photocatalysts in green buildings towards creating more efficient and stable low-carbon buildings that support sustainable urban growth.

1. Introduction

In recent years, human activities have caused a significant release of gaseous pollutants into the environment, including volatile organic compounds (VOCs), nitrogen oxides (NO_X), sulfur oxide (SO_X) and particulate matter. These pollutants have resulted in severe environmental issues [1]. They negatively impact human health by compromising the immune system which can lead to conditions such as skin diseases, asthma, chronic respiratory diseases, cardiovascular diseases, and cancer. Shockingly, air pollution has been linked to approximately 26,000 to 48,000 deaths in European countries alone [2]. Additionally, these pollutants also adversely affect plant growth and water and soil quality. They can lead to ecosystem collapse and contribute to climate change and global warming issues [3]. Given the gravity of the situation, extensive research and environmental remediation measures are being taken to address these concerns.

Photocatalytic technology is widely recognized for its potential to treat air pollutants [4]. This innovative technology works by converting harmful gases into harmless substances, effectively reducing air pollution’s harmful impacts [5]. Unlike conventional treatment methods, photocatalytic technology uses less energy and creates less secondary pollution. Additionally, photocatalysts are usually renewable and have low toxicity, providing better environmental sustainability [6]. Furthermore, photocatalysis has a wide range of applications and can treat various pollutants such as nitrogen dioxide, volatile organic compounds, formaldehyde, and benzene.

Based on these advantages, photocatalytic technology is widely used in various fields, such as construction materials, air purifiers, and wastewater treatment. Photocatalytic building materials are a new concept, and their potential to mitigate air pollution has received widespread attention [7]. Photocatalytic building materials are designed to convert harmful gases, like VOCs, NO_X and SO_X, into harmless substances through photocatalytic reactions. Additionally, they can decompose bacteria, viruses, and other microorganisms to improve indoor air quality. In recent years, many countries and regions have started using photocatalytic building materials to combat air pollution. For example, in 2003, photocatalytic building materials accounted for 60% of the Japanese market sales [7]. A range of photocatalytic building materials has been developed by adding TiO₂ nanoparticles to glass, ceramics, and mortar to achieve self-cleaning, antibacterial, and air-purifying materials [8]. In China, many high-rise buildings and public places including hospitals, schools, outdoor buildings, and roads have begun using photocatalytic building materials to purify air and improve the indoor environment.

TiO₂ has received widespread attention due to its simple preparation, high stability, low toxicity, and chemical inertness [9]. To date, a substantial body of research has been dedicated to summarizing the synthetic methods of TiO₂ nanomaterials and their applications in energy, environmental sustainability, healthcare, and construction sectors within the existing literature [6,10,11,12,13,14,15]. The growing prominence of sustainable urban development, carbon neutrality, and peak carbon emissions has heightened public concern regarding urban building energy consumption and air quality issues. Due to the exceptional compatibility of TiO₂ with construction materials, TiO₂-based photocatalytic building materials exhibit excellent photocatalytic performance and stability, effectively converting harmful airborne substances into harmless ones when exposed to light. This category of TiO₂-based photocatalytic building material holds immense potential in achieving air purification, automated cleaning, and energy consumption reduction. However, the literature available providing a comprehensive review of TiO₂-based construction materials in achieving air purification is limited, while the number of research articles in this field has been increasing in recent years. The present review aims to provide a comprehensive overview of the research progress, limitations, and future prospects concerning TiO₂-based photocatalytic building materials in air purification.

Firstly, this paper presents an overview of the photocatalytic mechanisms employed in the degradation of gaseous pollutants (VOCs, NO_X, SO_X) and sterilization. It emphasizes the preparation techniques for TiO₂ building materials and their impact on material physical and chemical properties, encompassing sol–gel, hydrothermal, spray drying, anodic oxidation, and microwave-assisted methods. Additionally, strategies to improve the catalytic efficiency of TiO₂-based building materials, along with strategies to address problems such as aggregation of TiO₂ nano-materials in suspension, poor adsorption ability, wide band gap, and high recombination rate, are discussed. Finally, based on current applications, the effectiveness of photocatalytic building materials is discussed including key factors affecting their catalytic efficiency as well as limitations and future development trends.

2. Working Principles and Properties of TiO₂-Based Photocatalytic Building Materials

In recent years, TiO₂ photocatalytic materials have garnered significant attention among researchers as a burgeoning building material. Within the construction industry, TiO₂-based building materials are extensively employed for air purification, deodorization, and sterilization owing to their distinctive photocatalytic properties. This section will primarily focus on elucidating the operating principles of photocatalytic building materials in pollution mitigation and carbon reduction.

2.1. The Basic Principal Mechanism of Photocatalysts

Semiconductor materials consist of orderly arranged crystals composed of many atoms or ions. The resulting dense accumulation of atoms facilitates the overlap of energy-matched adjacent atomic orbitals and the formation of distinct energy bands. Within semiconductor materials, there are two energy bands: an empty high-energy conduction band (CB) and a full low-energy electron valence band (VB), with a forbidden band located between the CB and VB [16]. When electrons are excited by light with energy equal to or greater than the band gap of a semiconductor (anatase TiO₂ ≥ 3.20 eV), photocatalytic reactions commence. As illustrated in Figure 1, this process entails the transition of an excited electron from the VB to the CB, leading to the generation of photo-generated electron–hole pairs. The environmental application of semiconductor photocatalysts is directly associated with the interfacial charge transfer mechanism occurring between the semiconductor surface and organic compounds [17]. The interface charge transfer process possesses the capability to directly catalyze the oxidation of or reduction in pollutants. Moreover, it exhibits the capacity to generate highly reactive oxidative species such as hydroxyl radicals and superoxide ions, thereby initiating subsequent oxidation or reduction reactions [17].

The reaction of the electron–hole composite is interdependent and competitive with the electron–hole reaction involving adsorbed material on the catalyst surface throughout the photocatalytic process. The total charge efficiency of interface charge migration is determined by two important competitive processes: (1) competition between carrier complexes and capture; (2) competition between captured carrier composites and interface charge migration. The transfer of captured carriers at the interface is a rate-determining step in the photocatalytic process which determines the quantum efficiency of TiO₂ photocatalysis.

During the oxidation or reduction reactions involving electron–hole pairs, the superoxide radical (•O₂⁻) is generated when photoelectrons react with O₂, while the hydroxyl radical (•OH) is produced when photoholes react with H₂O. These reactive oxygen species (ROS) decompose various pollutants in the atmosphere. The generation of ROS in the photocatalysis process can be depicted as follows:

$${\mathrm{TiO}}_2+\mathrm{hv}\to {\mathrm{TiO}}_2\left({\mathrm{e}}^{-}+{\mathrm{h}}^{+}{}_{\mathrm{VB}}\right)$$

(1)

$${\mathrm{TiO}}_2\left({\mathrm{h}}^{+}{}_{\mathrm{VB}}\right)+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{TiO}}_2+{\mathrm{H}}^{+}+•\mathrm{OH}$$

(2)

$${\mathrm{TiO}}_2\left({\mathrm{h}}^{+}{}_{\mathrm{VB}}\right)+{\mathrm{OH}}^{-}\to {\mathrm{TiO}}_2+•\mathrm{OH}$$

(3)

$${\mathrm{TiO}}_2\left({\mathrm{e}}^{-}{}_{\mathrm{CB}}\right)+{\mathrm{O}}_2\to {\mathrm{TiO}}_2+•{{\mathrm{O}}_2}^{-}$$

(4)

$$•{{\mathrm{O}}_2}^{-}+{\mathrm{H}}^{+}\to {\mathrm{HO}}_2•$$

(5)

$${\mathrm{HO}}_2•+{\mathrm{HO}}_2•\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2$$

(6)

$${\mathrm{TiO}}_2\left({\mathrm{e}}^{-}{}_{\mathrm{CB}}\right)+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{OH}}^{-}+•\mathrm{OH}$$

(7)

The generated ROS further react with the target contaminants and finally mineralize contaminants into non-toxic substances.

2.2. The Mechanism of Photocatalysts for Air Purification and Deodorization

The presence of air pollutants such as VOCs, NO_X and SO_X poses a risk to the environment and human health that cannot be ignored. In the following discussion, we will explore how TiO₂-based photocatalytic building materials work to degrade some of these gaseous pollutants.

The main indoor pollutants are VOCs, which are emitted from various sources such as building and combustion materials, electronic equipment, coal or oil combustion, indoor fuel gas, consumer products, and smoking [3]. These pollutants can damage the sensory system of human beings, causing various serious acute irritations and chronic diseases. This is especially true for workers who are exposed to air pollutants for a long time, and elderly people and young children with low immunity. Photocatalytic oxidation technology is a promising method for degrading VOCs. It can rapidly degrade organic pollutants in the atmosphere using hydroxyl radicals and superoxide radicals produced by photogenerated electron–hole pairs reacting with O₂ or H₂O. The photocatalytic mechanism of VOC degradation is depicted as follows:

$$\mathrm{HCHO}+•\mathrm{OH}\to •\mathrm{CHO}+{\mathrm{H}}_2\mathrm{O}$$

(8)

$$•\mathrm{CHO}+•\mathrm{OH}\to \mathrm{HCOOH}$$

(9)

$$•\mathrm{CHO}+•{\mathrm{O}}_2{}^{-}\to {\mathrm{HCO}}_3{}^{-}+{\mathrm{H}}^{+}\to \mathrm{HCOOOH}+\mathrm{HCHO}\to \mathrm{HCOOH}$$

(10)

$${\mathrm{HCOOH}}^{-}+{\mathrm{H}}^{+}\to {\mathrm{HCOO}}^{-}+•\mathrm{OH}\to {\mathrm{H}}_2\mathrm{O}+•{\mathrm{CO}}_2{}^{-}$$

(11)

$${\mathrm{HCOOH}}^{-}+{\mathrm{h}}^{+}{}_{\mathrm{VB}}\to {\mathrm{H}}^{{}_{+}}+•{\mathrm{CO}}_2{}^{-}$$

(12)

$$•{\mathrm{CO}}_2{}^{-}+•\mathrm{OH}+{\mathrm{h}}^{+}{}_{\mathrm{VB}}\to {\mathrm{CO}}_2+{\mathrm{OH}}^{-}$$

(13)

The active substances such as •OH, O₂•⁻, and electron holes attack the organic matter and mineralize it into CO₂, thus improving the air quality. NO_X, which mainly includes NO and NO₂, is commonly derived from anthropogenic activities such as vehicle exhaust gases and the industrial use of fossil fuels. These activities lead to various environmental problems such as photochemical smog, acid rain, and haze [18]. The greenhouse effect of NO₂ is 200~300 times greater than that of CO₂ and extremely harmful to the environment [19]. TiO₂-based photocatalysis is an environmentally friendly technology that can remove low concentrations of NO_X under solar or UV light irradiation. The photocatalytic degradation of NO_X utilizes the active substances produced by the photocatalyst to convert NO_X into NO₃⁻. The reaction pathway is as follows:

$$\mathrm{NO}+•\mathrm{OH}\to {\mathrm{HNO}}_2$$

(14)

$${\mathrm{HNO}}_2+•\mathrm{OH}\to {\mathrm{NO}}_2+{\mathrm{H}}_2\mathrm{O}$$

(15)

$${\mathrm{NO}}_2+•\mathrm{OH}\to {\mathrm{HNO}}_3$$

(16)

$$\mathrm{NO}+•{\mathrm{O}}_2{}^{-}\to {\mathrm{NO}}_3{}^{-}$$

(17)

$$\mathrm{NO}+•{\mathrm{O}}_2{}^{-}+{\mathrm{e}}^{-}\to {2\mathrm{NO}}_2$$

(18)

$${\mathrm{NO}}_2+{2\mathrm{OH}}^{-}\to {\mathrm{NO}}_3{}^{-}+\mathrm{NO}+{\mathrm{H}}_2\mathrm{O}$$

(19)

Equations (14)–(19) show that •OH and O₂•⁻ convert NO_X to NO₃⁻, thereby reducing the concentration of NO_X in the air.

SO_X primarily refers to SO₂, which is produced by the combustion of sulfur-containing fossil fuels and is extremely harmful to human beings and ecosystems. SO_X in the air is one of the causes of acid rain which could erode building surfaces, accelerate material ageing, inhibit crop growth, and change the acidity and alkalinity of the soil [20]. TiO₂-based photocatalysts effectively degrade SO_X under solar or UV light excitation, thereby limiting their impact on the environment. The mechanism for photocatalytic degradation of SO₂ is as follows:

$${\mathrm{SO}}_2+•\mathrm{OH}\to {\mathrm{HSO}}^{-}$$

(20)

$${\mathrm{SO}}_2+•{\mathrm{O}}_2{}^{-}\to {\mathrm{SO}}_4{{}^2}^{-}$$

(21)

$${\mathrm{SO}}_2+{\mathrm{h}}_{\mathrm{VB}}{}^{+}+{\mathrm{H}}_2^{}\mathrm{O}\to {\mathrm{H}}_2{\mathrm{SO}}_4+{\mathrm{H}}^{+}$$

(22)

As shown in Equations (20) and (22), SO₂ is converted into SO₄²⁻ through free radicals and photogenerated holes.

2.3. The Photocatalytic Mechanism of Disinfection

Airborne bacteria and viruses are a significant source of disease, necessitating disinfection and sterilization measures. In recent years, photocatalytic disinfection technology has been widely used. This technology generates reactive oxygen species (ROS) through light-excited photocatalytic materials, thereby disrupting the auto-sensing signals of colonies and impeding biofilm formation. As depicted in Figure 2a, ROS attack bacteria by inducing cell membrane rupture; they penetrate the cell and target intracellular components such as ATP, SOD, and CAT, leading to reduced concentrations of these substances. Additionally, ROS react with DNA and cause strand breaks. Prolonged exposure to light results in cytoplasmic loss of and damage to DNA, rendering cells incapable of self-repair and ultimately fragmenting them. ROS exert analogous effects on viruses by targeting viral RNA strands resulting in their fragmentation and subsequent virus inactivation (Figure 2b).

3. The Status of TiO₂-Based Photocatalytic Building Materials

In recent years, significant advancements have been made in the field of TiO₂ photocatalytic construction materials. TiO₂ has gained considerable attention as a photocatalyst for degrading organic pollutants due to its chemical stability, cost-effectiveness, and compatibility with various building materials [14]. TiO₂ can react with metal oxides present in cement to form stable compounds like CaTiO₃, which possesses a band gap energy of 3.5 eV and an isoelectric point pH of 3 [22]. Table 1 presents several investigations on incorporating TiO₂-based nanomaterials into building materials for the purpose of removing airborne pollutants.

Researchers have developed various TiO₂ catalysts with diverse morphological designs, encompassing nanoparticles, nanotubes, hollow fibres, and mesoporous materials, to enhance their photocatalytic performance [8]. Recent studies have extensively investigated the correlation between TiO₂’s crystallinity, crystal phase, crystal size, surface area, pore structure, pore size and photocatalytic and adsorption capacities [23]. The utilization of photocatalysts in building materials commenced in the early 1990s, and since then, TiO₂-modified building materials have been employed for a multitude of purposes such as environmental pollution remediation, self-cleaning and self-disinfection [7]. TiO₂ is an adaptable material that can serve as both a photocatalyst and as a structural component. Consequently, it has gained widespread use in various exterior and interior building materials including cement mortars, exterior wall tiles, paving blocks, glass, and PVC fabrics. Photocatalytic building materials based on TiO₂ offer substantial advantages and exhibit immense potential across a wide range of applications. These materials are compatible with conventional building materials, like cement, without altering any of their original properties. Moreover, they are effective even under ambient atmospheric conditions with weak solar radiation [12]. An increasing number of findings substantiate the viability of this technology within the construction industry. Cárdenas et al. [24] conducted a study that demonstrates that the percentage of TiO₂ added directly correlates with enhanced photocatalytic activity in cement paste when utilizing highly efficient TiO₂ nanoparticles. The results of their study revealed that the cement paste containing 5% TiO₂ exhibited the highest rate of NO_X removal. Furthermore, Cheung et al. [25] developed photocatalytic building materials by incorporating TiO₂ with local waste materials such as cement, crushed glass, and sand to degrade NO. Interestingly, they found that materials with lower density and higher porosity were more effective for NO degradation when mixed with less cement. Additionally, Demeestere et al. [26] reported remarkable photocatalytic toluene-removing activity in roofing tiles and corrugated sheets containing TiO₂. They suggested that integrating photocatylic TiO₂ into building materials could significantly aid air purification.

In summary, due to its high photocatalytic activity, compatibility with traditional building materials, and effectiveness in ambient atmospheric environments under weak solar radiation, TiO₂ holds great promise for photocatalytic applications in buildings. However, the efficiency of low-cost TiO₂-based building materials remains unsatisfactory under visible or solar light irradiation due to TiO₂’s wide band gap (3.2 eV). Therefore, it is crucial to develop narrow-band-gap and efficient photocatalysts for practical applications.

Table 1. The selected literature on air pollutant removal by TiO₂-based building materials.

Building Material	Method	Light Source	Efficiency	Reference/Year
cement mortar	Mixing with cement mortar	UV	The degradation rate of NOX can reach 40.0%	2009 [27]
cement mortar	Mix with mortar (2 and 5 wt%)	UV	NO (400 ppb) removal rate: 90 μ·mol/(m2·h) Toluene (200 ppb) removal: 100%	2011 [28]
ceramic tiles	Photocatalyst brushing on the top surface of tiles	UV	Toluene (17–35 ppbv) removal rate up to 512 μ·g/(m2·h)	2008 [26]
cement mortar	Mix with mortar (1–10% wt%)	UV	Formaldehyde (20 ppm) removal rate up to 65%	2011 [29]
portland cement	Mix with cement slurry (0.5–5 wt%.)	UV	NOX (1 ppmv) removal amount: 120 μmol/m2, 65 h	2012 [24]
Wall paint and plaster	Mixing with 2 wt% TiO2	UV	NOX (400 ppb) conversion range ranges from 80% of 50 days samples to 30% of 1-year samples	2011 [30]
cement mortar	Mixed cement (0.5–2.5 wt%)	Simulated sunlight	The removal rate of NO (1 ppm) can reach 15%	2014 [31]
cement mortar	Mixing with cement mortar	UV	The degradation efficiency of NOX (1000 ppb) can reach 60.4%	2015 [32]
cement mortar	Combine photocatalytic materials with building materials using mixing and spraying methods, respectively	UV	NO (1000 ppb) removal condition: Material for spraying method: 220 μ·Mol/(m2·Å·h), mixed material: 80 μ·mol/(m2·h)	2017 [33]
cement mortar	Mix with cement mortar (0.5~2.5 wt%)	UV Sunlight Visible light	The highest conversion rates of NO (500 ppb) are 38% (P25), 15% (P25), and 5.5% (Fe TiO2 and V-TiO2), respectively	2017 [34]
cement mortar	Mixing with cement mortar (1–10 wt%)	UV	NO (1 ppm) removal rate: 72%	2017 [35]
Concrete and gypsum	Coating deposited on the test concrete wall	Sunlight	Efficient removal of NOX from polluted air.	2017 [36]
White cement (WC) and ordinary Portland cement paste	Mixed cement slurry (2–5 wt%)	UV	NO (1000 ppb) removal condition: OPC is 380 μ·mol/(m2·h) and WC at 500 μ·mol/(m2·h)	2018 [37]

Table 1. The selected literature on air pollutant removal by TiO₂-based building materials.

Building Material	Method	Light Source	Efficiency	Reference/Year
cement mortar	Mixing with cement mortar	UV	The degradation rate of NOX can reach 40.0%	2009 [27]
cement mortar	Mix with mortar (2 and 5 wt%)	UV	NO (400 ppb) removal rate: 90 μ·mol/(m2·h) Toluene (200 ppb) removal: 100%	2011 [28]
ceramic tiles	Photocatalyst brushing on the top surface of tiles	UV	Toluene (17–35 ppbv) removal rate up to 512 μ·g/(m2·h)	2008 [26]
cement mortar	Mix with mortar (1–10% wt%)	UV	Formaldehyde (20 ppm) removal rate up to 65%	2011 [29]
portland cement	Mix with cement slurry (0.5–5 wt%.)	UV	NOX (1 ppmv) removal amount: 120 μmol/m2, 65 h	2012 [24]
Wall paint and plaster	Mixing with 2 wt% TiO2	UV	NOX (400 ppb) conversion range ranges from 80% of 50 days samples to 30% of 1-year samples	2011 [30]
cement mortar	Mixed cement (0.5–2.5 wt%)	Simulated sunlight	The removal rate of NO (1 ppm) can reach 15%	2014 [31]
cement mortar	Mixing with cement mortar	UV	The degradation efficiency of NOX (1000 ppb) can reach 60.4%	2015 [32]
cement mortar	Combine photocatalytic materials with building materials using mixing and spraying methods, respectively	UV	NO (1000 ppb) removal condition: Material for spraying method: 220 μ·Mol/(m2·Å·h), mixed material: 80 μ·mol/(m2·h)	2017 [33]
cement mortar	Mix with cement mortar (0.5~2.5 wt%)	UV Sunlight Visible light	The highest conversion rates of NO (500 ppb) are 38% (P25), 15% (P25), and 5.5% (Fe TiO2 and V-TiO2), respectively	2017 [34]
cement mortar	Mixing with cement mortar (1–10 wt%)	UV	NO (1 ppm) removal rate: 72%	2017 [35]
Concrete and gypsum	Coating deposited on the test concrete wall	Sunlight	Efficient removal of NOX from polluted air.	2017 [36]
White cement (WC) and ordinary Portland cement paste	Mixed cement slurry (2–5 wt%)	UV	NO (1000 ppb) removal condition: OPC is 380 μ·mol/(m2·h) and WC at 500 μ·mol/(m2·h)	2018 [37]

4. Preparation of TiO₂ Photocatalytic Building Materials

The production of TiO₂ photocatalytic construction materials involves multiple stages, including the selection of suitable TiO₂ nanoparticles, fabrication of the construction materials, and integration of the nanoparticles into the materials. Various methodologies have been extensively reported in the scientific literature for creating TiO₂ photocatalytic construction materials, encompassing sol–gel, hydrothermal, and spray drying techniques.

4.1. Sol–Gel Method

The sol–gel technique is a widely employed method for the fabrication of TiO₂ photocatalytic building materials. This approach involves synthesizing TiO₂ nanoparticles through the hydrolysis of TiO₂ salts in a solvent while incorporating stabilizers to prevent nanoparticle aggregation. Subsequently, the solution is blended with construction materials like concrete or gypsum and allowed to dry. In terms of enhancing material properties, this method enables precise control over the size, shape, and composition of the TiO₂ nanoparticles. Such control leads to an improved surface-area-to-volume ratio and enhanced charge separation efficiency within the material, thereby increasing photocatalytic activity. This increased activity allows for the effective decomposition of both organic and inorganic pollutants under light exposure, leading to improved air quality and self-cleaning performance. Furthermore, this method ensures uniformity in nanoparticle distribution throughout the building material, guaranteeing consistent photocatalytic performance across its surface. The impact of this technique on the physicochemical properties primarily manifests itself in two aspects: (1) Controlled composition—by adjusting precursor materials and conditions during the sol–gel process, the chemical composition is fine-tuned, which can induce changes in properties like the band gap energy and the crystal structure that influence photocatalytic efficiency as well as material durability. (2) Surface morphology—employing the sol–gel method facilitates the formation of porous structures that increase the surface area available for photocatalytic reactions and contaminant adsorption.

In terms of equipment cost, the sol–gel method requires simple equipment compared to other techniques. Typically, this includes reactors, heating units, and mixing equipment that are not extremely expensive compared to other advanced nanomaterial synthesis methods [38]. However, it should be noted that while the sol–gel method offers evident advantages in enhancing the performance of photocatalytic building materials, there are concerns regarding the stability of the nanoparticles prepared using this approach. Over time, TiO₂ nanoparticles produced through this method may experience agglomeration or dissolution which could potentially compromise the long-term performance and durability of these materials [39]. Consequently, researchers must employ appropriate strategies to address these issues.

4.2. Hydrothermal Method

The synthesis of TiO₂ nanostructures by the hydrothermal method has advantages such as suitable crystallization temperature, environmental friendliness, controllable reaction conditions, low energy consumption, and cost-effectiveness. These advantages make it one of the most widely used methods [40]. Hydrothermally synthesized TiO₂ nanoparticles typically exhibit a higher specific surface area, which provides abundant active sites for photocatalytic reactions and enhances the catalytic performance. Furthermore, by adjusting the hydrothermal conditions (including temperature, pressure, precursor concentration), alkali solutions used, and TiO₂ precursors selected, it is possible to modulate the crystal structure, particle size distribution and agglomeration ratio of TiO₂ nanostructures to tailor their material properties and optimize photocatalytic efficiency [40,41]. The equipment required for hydrothermal synthesis includes an autoclave or high-pressure reactor along with associated laboratory instrumentation and heating units. Although there may be variations in the initial costs of this equipment depending on specific requirements, they are considered affordable compared to other advanced synthesis methods. Moreover, the operating costs including energy consumption and maintenance are usually lower than those associated with more energy-intensive methods.

Although hydrothermal methods can yield TiO₂ nanoparticles with high surface area and controllable properties at manageable equipment costs, their long-term stability remains a potential concern. Factors such as agglomeration and dissolution may impact the durability and performance of the building materials.

4.3. Spray Drying Method

The spray drying method is employed for the synthesis of TiO₂ nanoparticles by atomizing a precursor solution in a high-temperature drying chamber. Subsequent evaporation of the solvent by hot air results in the formation of nanoparticles, which are then collected and blended with the construction material. This technique offers precise control over various nanoparticle properties, including phase composition, crystal size, and surface area. Such tunability enables the customization of nanoparticles according to specific material requirements and the optimization of their photocatalytic characteristics [42]. Moreover, dry TiO₂ nanoparticles produced through spray drying can easily be incorporated into building materials, ensuring uniform dispersion throughout the material, and thereby achieving stable and reliable photocatalytic activity across its entire surface area. Despite being an efficient and scalable approach for synthesizing TiO₂ nanoparticles, spray drying has certain limitations. One major drawback lies in its strong dependence on factors such as viscosity, surface tension, and concentration of the parent ionic solution. Improper formulation may lead to poor-quality nanoparticles or even failure of nanoparticle formation [42]. Additionally, although the initial equipment investment is low, this method requires significant energy directly impacting operational costs. Therefore, these factors should be considered alongside specific application requirements when deciding whether to employ spray drying methods—ultimately striking a balance between performance effectiveness, cost efficiency, and stability.

4.4. Anodic Oxidation Method

Anodising technology is an innovative approach that utilizes TiO₂ to produce photocatalytic building materials. This method involves the electrochemical oxidation of metallic titanium or alloys in an electrolyte solution, resulting in the formation of a porous TiO₂ layer. The resultant TiO₂ layer can be applied as a coating for various building materials such as steel, aluminium, and glass. The anodic oxidation technique yields a highly porous TiO₂ layer with a large surface area and exceptional porosity [43,44]. This unique structure provides many active sites for photocatalytic reactions, thereby significantly enhancing the photocatalytic activity of the building material. By adjusting the morphology and crystallinity of the TiO₂ layer, its performance can be finely tuned to maximize efficiency. Furthermore, the resulting TiO₂ coating exhibits strong adhesion and durability to ensure long-term effectiveness against environmental stresses while maintaining its photocatalytic properties. With moderate equipment costs and controllable operating expenses, this method presents itself as a viable option for both research and industrial applications. Ensuring proper maintenance guarantees stability and reliability during periods of equipment use. However, careful consideration must be given to substrate compatibility and potential corrosion issues to ensure the successful implementation of this method [44]. Moving forward, researchers will continue exploring and refining this technique to fully harness its benefits in developing efficient and durable photocatalytic building materials tailored for specific applications.

4.5. Microwave-Assisted Method

The microwave-assisted method is a contemporary technique for synthesizing TiO₂ nanoparticles and incorporating them into building materials. This approach utilizes microwave radiation to heat and activate titanium precursors and solvents in enclosed containers, resulting in the production of TiO₂ nanoparticles. These particles are subsequently blended with building materials such as cement or ceramics and then cured under appropriate conditions. The TiO₂ nanoparticles generated by the microwave-assisted method exhibit exceptional crystallinity, purity, and have a narrow size distribution, while being uniformly dispersed as spherical nanoparticles within the building materials. This uniform dispersion enhances the consistency and stability of photocatalytic properties across the material surface [45]. Moreover, researchers can precisely control reaction parameters to tailor the characteristics of TiO₂ nanoparticles according to specific material requirements. Such control extends to factors like particle size and crystallinity, enabling optimization of photocatalytic efficiency on a case-by-case basis [46]. Although specialized equipment is necessary for generating and controlling microwave radiation, the initial investment required is generally modest. Microwave reactors along with associated equipment are reasonably priced options available for both research laboratories and industry applications. Furthermore, microwave-assisted methods typically consume less energy compared to other approaches, thereby ensuring manageable overall operating costs. However, there are certain limitations associated with this methodology, including requirements for special equipment and microwave-absorbing materials as well as the formation of hot spots and thermal gradients within the reaction vessels that may pose challenges during scaling up industrial applications.

5. Strategies for Improving TiO₂’s Photocatalytic Efficiency

TiO₂, a low-cost and environmentally friendly photocatalyst, plays a vital role in the preparation of photocatalytic building materials. However, there are three main challenges that need to be addressed when using TiO₂: (1) rapid aggregation in suspension due to small particle size, leading to reduced effective surface area and catalytic efficiency as well as restricted adsorptive power for photocatalytic oxidation reactions; (2) limited utilization of visible light in the solar spectrum due to its wide band gap (>3.0 eV); (3) high recombination rate of electron–hole pairs limiting its photocatalytic ability. This section provides a comprehensive review of current strategies employed in addressing the TiO₂ aggregation problem and expanding its response to light, aiming to establish a solid foundation for future development and large-scale industrial application of TiO₂-based photocatalysis technology.

5.1. Strategies for Reducing Aggregation of TiO₂

The presence of TiO₂ nanoparticles with smaller particle diameters results in a higher specific surface area and an increased number of active sites, thereby accelerating the photocatalytic reaction rate of TiO₂-based photocatalysts [47]. However, due to their relatively small diameter size, TiO₂ particles are prone to aggregation in suspension, leading to a rapid reduction in photocatalytic efficiency [48]. Therefore, researchers have immobilized TiO₂ onto various supporting materials such as activated carbon [49], clay [48] and silicon [50] to mitigate aggregation during the reactivation processes, enhance the adsorption capacity, and increase the photocatalytic efficiency. Among these supporting materials, silica (SiO₂) is commonly found in building materials due to its high chemical stability and low cost; thus, it holds great promise for preparing TiO₂-based nanocomposites [50]. Ghdini et al. [51] synthesized an environmentally friendly and readily available TiO₂-SiO₂ photocatalyst with a high surface area using an incipient wetness impregnation method. They demonstrated that under UV excitation, the TiO₂-SiO₂ photocatalyst degraded ethylbenzene. Their findings suggest that multifunctional TiO₂-SiO₂ photocatalysts could be combined with building materials to improve air quality or reduce indoor pollution levels. Zhou et al. [52] employed the sol–gel method to synthesize a highly porous TiO₂-SiO₂-based photocatalyst with both high specific surface area and high adsorption capacity. They observed that this composite material degraded toluene under UV irradiation, indicating that the SiO₂ carrier combined with the porous TiO₂ catalysts exhibits excellent adsorption capacity which promotes subsequent photocatalytic reactions through positive synergistic effects.

FAS12-loaded UV-responsive microcapsules were successfully synthesized by Chen et al. [53] using Pickering emulsion polymerization (Figure 3). The microcapsules were readily dispersed into waterborne polysiloxane latex, enabling their application through spray onto various surfaces including aluminium plates, sheets, glass, polypropylene, and wood. This resulted in the achievement of superhydrophobicity by virtue of the UV-induced rupture of the microcapsules and subsequent release of FAS12.

In addition to inhibiting TiO₂ agglomeration, SiO₂ can also regulate the crystallization of TiO₂ during high-temperature calcination [54]. It is well known that the catalytic properties of TiO₂ are influenced by its crystalline structure. Anatase TiO₂ exhibits higher photoactivity compared to other crystalline TiO₂ structures due to its remarkable specific surface area [55]. Silica can control the crystallization process of the amorphous TiO₂ layer into anatase nanocrystals and restrict the growth size of anskite grains during the high-temperature calcination process [54]. However, most TiO₂/SiO₂ nanocomposites only exhibit photocatalytic activity under UV irradiation, limiting their potential applications in the building industry. Therefore, combining weak visible-light-induced TiO₂-based photocatalysts with metal elements may offer a promising approach for effectively utilizing the entire solar light spectrum. Zhao et al. [56] synthesized highly porous ternary composite aerogels consisting of TiO₂/SiO₂/Ag using a facile sol–gel method combined with a supercritical drying technique. They observed that the addition of Ag particles reduced recombination between photoelectrons and holes while enhancing visible-light photocatalytic activity in the TiO₂/ SiO₂/Ag ternary composite material. Raza et al. [57] successfully synthesized highly porous ternary composite aerogels consisting of TiO₂/SiO₂/Ag using a facile sol–gel method combined with a supercritical drying technique. They observed that the addition of Ag particles reduced recombination between photoelectrons and holes while enhancing visible-light photocatalytic activity in the TiO₂/ SiO₂/Ag ternary composite material.

To obtain TiO₂-based visible-light photocatalysts with high adsorption ability and excellent photocatalytic efficiency, researchers combined nanocarbon materials with TiO₂. Carbon nanotubes (CNTs), as a typical one-dimensional nanostructure, have garnered significant attention in the development of new photocatalytic materials due to their electrical properties, chemical properties, and mechanical characteristics [58,59]. The deposition of TiO₂ on the surface of carbon nanotubes was found to enhance the light absorption efficiency of photocatalysts. When light shines into the hollow tubes and nanosheets of TiO₂, it undergoes multiple reflections before eventually being absorbed, thereby improving the photon capture efficiency. Nguyen et al. [60] synthesized nanohybrid TiO₂/CNT materials through the hydrolysis method and compared them with pure TiO₂ or CNTs; the resulting nanohybrid exhibited superior photocatalytic performance for methylene blue degradation. Wang et al. [61] discovered that combining TiO₂ with CNTs could minimize recombination of photogenerated electrons and holes.

In addition to carbon nanotubes, graphene is also a prevalent carbon-based carrier that facilitates the even distribution of TiO₂ nanoparticles in liquids and enhances the separation and transmission of electron–hole pairs in TiO₂ [47]. An rGO-based TiO₂ composite improved TiO₂’s photocatalytic capability due to rGO’s ability to impede electron–hole pair recombination [62]. Similarly, Xue et al. [63] confirmed that graphene significantly enhances photogenerated electron–hole pair separation and transport while reducing recombination. Although these nanocomposites exhibit greater potential for application than pure TiO₂, incorporating TiO₂ into the mesopores of carbon materials drastically reduces their specific surface area, resulting in limiting their ability to adsorb pollutants. Furthermore, as most carbon materials are in powder form and lack magnetic properties, recycling non-magnetic powder carbon-material-based-TiO₂ composites is challenging, thereby limiting their industrial applications [64]. Currently, there are few practical applications for carbon-based TiO₂ nanocomposites; most are still at the fundamental development stage. Therefore, achieving high photocatalytic efficiency with carbon-based TiO₂ remains highly challenging.

5.2. Strategies for Improving the Photocatalytic Efficiency of TiO₂

Anchoring TiO₂ to silica, carbon nanotubes, graphene and other carriers can mitigate TiO₂ particle agglomeration. However, this approach may partially cover the carrier’s specific surface area, leading to a decrease in their photocatalytic efficiencies [65]. To achieve nanocomposites with enhanced photocatalytic efficiencies, researchers have employed metal ions or non-metallic elements for modifying TiO₂ and obtaining metal/non-metal-doped nanocomposites. Previous studies demonstrated that doping metal atoms onto TiO₂ can significantly enhance its efficiency and broaden its excitation band. Among various metal dopants, Fe is particularly promising due to its low cost and environmental friendliness [66]. Liu et al. [67] synthesized Fe(III)-doped visible-light-driven TiO₂ and observed superior visible-light absorption compared to pure TiO₂. Furthermore, they found that the high quantum efficiency of Fe(III)-doped TiO₂ was maintained by surface-grafted Fe(III) ions.

Liu et al. [68] developed a novel TiO₂ photocatalyst that exhibits visible light activity through the coupling of Ti(IV) and Fe(III) nanoclusters on the surface of TiO₂. Figure 4 illustrates the photodegradation mechanism of this innovative photocatalyst, highlighting that visible light irradiation generates holes and electrons. Specifically, Ti(IV) nanoclusters on the TiO₂ surface accumulate photogenerated holes, while Cu(II) and Fe(III) nanoclusters accumulate photogenerated electrons. Consequently, this design facilitates a slow recombination process for electron–hole pairs, thereby enhancing the solar energy conversion efficiency. Yu et al. [11] reported on the synthesis of a nanocomposite material comprising polyaniline-wrapped manganese-doped TiO₂ (PANi/Mn-TiO₂). They observed that the incorporation of Mn facilitated efficient charge transfer processes, leading to exceptional visible light photocatalytic activity exhibited by the catalyst. Furthermore, the presence of polyaniline as a photosensitizer effectively promoted electron migration and induced robust oxidation/reduction reactions within the nano-sized TiO₂ particles, thereby enhancing their overall photocatalytic performance. Poudel et al. [69] demonstrated that the photocatalytic activity of titanium dioxide can be enhanced through Ag modification. Ag@TiO₂ NFs nanosheets were successfully synthesized using solvothermal and photoreduction methods. The research findings revealed that, in comparison to unmodified titanium dioxide, Ag@TiO₂ NFs exhibited significantly augmented antibacterial effects against Gram-positive Staphylococcus aureus and Gram-negative Escherichia coli. The incorporation of metal into catalysts not only leads to a reduction in the optical bandgap, but also has the potential to influence the structure and properties of thin films. Awatef et al. [70] demonstrated that the addition of chromium to TiO₂ resulted in a significant decrease in the optical bandgap from 3.31 eV to 1.89 eV, while concurrently enhancing film density compared to undoped counterparts. Furthermore, by adjusting the concentration of chromium dopants, it becomes feasible to modulate the conductivity of TiO₂ thin films from n-type to p-type, thereby offering advantageous prospects for surface reactions on TiO₂ films.

Although TiO₂ modified with transition metals can be activated by visible light irradiation, the quantum efficiencies of these catalysts under such conditions are still unsatisfactory. Furthermore, transition-metal-modified TiO₂ typically exhibits poor thermal stability and is susceptible to light corrosion. In contrast, nonmetal decoration can create a narrower band gap in TiO₂ as an electron donor or acceptor, thereby enhancing its visible light absorption capability. This approach has proven more successful than metal decoration. Given nitrogen’s small ionization energy, stability, and comparable atomic size to oxygen [71], nitrogen incorporation is more suitable for improving TiO₂’s photocatalytic efficiency compared to other nonmetals. Chen et al. [72] synthesized N-doped TiO₂ and immobilized it on the surface of an asphalt road to evaluate its photodegradation ability towards vehicle emissions. They observed that the N-doped TiO₂ asphalt road exhibited higher efficiency in photodegrading vehicle emissions under visible-light irradiation compared to pure TiO₂ asphalt road samples. This finding demonstrates that N-doped TiO₂ offers a valuable avenue for preparing photocatalytic materials for asphalt roads with high visible-light-induced photocatalytic activity. Jun et al. [73] reported synthesizing N-doped TiO₂ through pyrolysis of co-precipitated trithiocyanuric acid with TiO₂ at 500 °C. The results suggested that nitrogen doping into the lattice structure of TiO₂ reduces the band gap energy by altering the band structure, thus enhancing its visible light absorption capacity and accelerating superoxide radical formation. Barolo et al. [74] found that N-doped TiO₂ exhibits moderate photocatalytic activity under visible light illumination. The synergistic effect between visible (approximately 400 nm) and near-infrared light enables this photocatalytic material to generate surface electrons and holes.

Although N-doped TiO₂ exhibits visible-light photocatalytic ability, researchers have identified several challenges that limit its practical application: (1) The nitrogen doping concentration is relatively low. (2) Nitrogen doping into the lattice of TiO₂ generates more oxygen vacancies compared to pure TiO₂, which promotes the recombination of photogenerated electron–hole pairs. (3) The stability of N-doped TiO₂ is often unsatisfactory [75]. To address these issues and enhance the separation of photogenerated electrons and holes in TiO₂, co-doping with two different ions has gained significant attention to resist the recombination of the photogenerated electron–hole pairs. Vaiano et al. [76] discovered that co-doping Fe and N into TiO₂ narrows its band gap, resulting in significantly enhanced visible-light-induced photodegradation for Acid Orange 7 azo dye. Under LED irradiation for 60 min, decolorization and mineralization rates reached 90% and 83%, respectively. Li et al. [77] reported that Fe-N co-doped TiO₂/GF exhibited superior photocatalytic efficiency compared to single doping with Fe-TiO₂/GF or N-TiO₂/GF, suggesting a synergistic effect between N and Fe on TiO₂’s structure that inhibits the recombination of photogenerated electron–hole pairs. Liu et al. [78] synthesized a composite material consisting of carbon nanosheets doped with both Fe and N using an in situ self-template strategy (Figure 5). They observed excellent ORR activity, high selectivity, and admirable stability in the Fe-N-CNS composites.

Hayati et al. [79] successfully achieved the deposition of N and Fe elements onto the surface of functionalized single-walled carbon nanotubes. The resulting nanocomposites exhibited complete photodegradation of sulfathiazole under ultrasonic irradiation at pH 7.0, demonstrating an excellent removal rate for COD and TOD in real wastewater samples. These findings highlight this novel catalyst’s exceptional photocatalytic efficiency for decontaminating recalcitrant compounds and pharmaceutical wastewater. However, achieving satisfactory photocatalytic performance for industrial applications remains a challenge, necessitating the development of TiO₂-based photocatalysts with high specific surface area, low charge recombination rates, superior photocatalytic activity, and excellent stability.

6. Application Status and Prospects of TiO₂-Based Building Materials

Numerous practical applications have demonstrated the feasibility of integrating photocatalytic technology with building materials to achieve pollution and carbon reduction in buildings. In the preceding chapters, we discussed the preparation methods of photocatalytic building materials and the modification strategies employed for enhancing their photocatalytic performance based on TiO₂. This chapter focuses on presenting practical examples of photocatalytic building materials to further analyze their effectiveness and identify critical factors influencing the application of modified functional materials. Finally, we discuss the limitations and future trends associated with implementing photocatalytic construction, aiming to deepen understanding of photocatalytic construction materials while providing a reference for the subsequent development of functional building materials.

6.1. Application Status and Key Influencing Factors of Photocatalytic Building Materials

After successful laboratory-scale tests, photocatalytic building materials have been implemented in real-world applications, including office buildings, museums, and transportation infrastructures.

Table 2. The application of TiO₂ in a building facade or roof.

Building Name	Location	Building Material	Benefits	Difficulties
Palazzo Italia	Milan, Italy	TiO2-based photocatalytic coating on façade	Purifies air, reduces carbon emissions, energy-efficient design, use of renewable energy sources	Cost of installation and maintenance
Jubilee Church	Rome, Italy	TiO2-coated façade	Reduces air pollution, improves air quality by breaking down harmful pollutants	Limited effectiveness in high-traffic areas
Palazzo Lombardia	Milan, Italy	TiO2-coated façade	Purifies air, reduces energy consumption by reflecting sunlight and reducing need for air conditioning	Cost of installation and maintenance
Bullitt Center	Seattle, USA	TiO2-coated roof	Purifies air, reduces air pollution by breaking down harmful pollutants	Limited effectiveness in high-traffic areas
Denby Dale Passivhaus	Yorkshire, UK	TiO2-coated façade	Purifies air, reduces air pollution, reduces energy consumption for heating and cooling	Cost of installation and maintenance
Edificio Malecon	Mexico City, Mexico	TiO2-coated façade	Reduces air pollution, improves air quality, self-cleaning properties, reduces energy consumption	Cost of installation and maintenance
Haze-Free Tower	Beijing, China	TiO2-coated façade	Reduces air pollution, improves air quality, enhances aesthetics, self-cleaning properties	Limited effectiveness in high-traffic areas
Queen’s Building	Bristol, UK	TiO2-coated façade	Purifies air, reduces air pollution, self-cleaning properties	Limited effectiveness in shaded areas
Nanjing Green Lighthouse	Nanjing, China	TiO2-coated façade	Purifies air, reduces energy consumption, improves air quality, self-cleaning properties	Cost of installation and maintenance
LaFargeHolcim Headquarters	Switzerland	TiO2-coated façade	Reduces air pollution, self-cleaning properties, improves energy efficiency	Limited effectiveness in high-pollution areas

presents a range of demonstration projects utilizing photocatalytic building materials. After undergoing long-term field testing to ensure their durability and efficacy, these materials have been utilized in various countries such as China, Germany, and the Netherlands. For instance, Elegant Decoration, a construction company based in Berlin, has provided TiO₂-coated tiles for the Toledo Specialist Hospital in Mexico City. Long-term field tests confirmed that the photocatalytic façade significantly reduced pollution levels in the surrounding air. Similarly, Rome’s Jubilee Church features a TiO₂ coating on its façade which successfully decomposes harmful pollutants like NO_X and VOCs into harmless compounds while maintaining the church’s white appearance and preventing vegetation growth. Furthermore, due to its location within a heavily polluted area of China, the Nanjing Yangtze River Bridge’s north toll station employed a nanoTiO₂-loaded photocatalytic material to an area of approximately 6000 m². After a few months of monitoring pollutant levels inside these buildings, researchers observed an impressive removal efficiency exceeding 80% for NO_X.

Despite the promising treatment outcomes observed in these demonstration projects, there are critical issues that necessitate attention in the practical application of photocatalytic building materials. The foremost concern pertains to the stability of the building’s photocatalytic function. Insufficient structural stability during the utilization of photocatalytic building materials leads to material deactivation. Additionally, a substantial accumulation of intermediate products such as VOC by-products or airborne dust occurs on the surfaces of these materials during air pollutant degradation, resulting in catalyst contamination and subsequent negative impacts on their photocatalytic function. Furthermore, limited information is available regarding the impact and quantification of the by-products produced. The environmental consequences stemming from intermediate by-products generated during pollutant degradation within photocatalytic buildings remain unknown, warranting further research for a comprehensive understanding of their environmental implications.

In real-world applications, the utilization of photocatalytic building materials necessitates considering a range of factors that include selecting the catalyst, optimizing the installation environment, and adapting the building structure to ensure stable and efficient operation. Therefore, it is crucial to select photocatalytic building materials with highly active and stable catalysts while optimizing the catalyst’s structure and morphology to enhance degradation efficiency and stability. To prevent the accumulation of intermediates and by-products, multifunctional catalysts such as TiO₂ composites with other catalysts can be explored to improve the catalytic activity and selectivity. Additionally, appropriate installation environments are essential for effective light absorption and exposure to air by the photocatalytic building materials. The positioning within the building (e.g., light openings, vents, walls) also significantly influences the effectiveness of these materials. During installation, reaction rate and light intensity should be considered to achieve optimal degradation during contact between pollutants and light under specific conditions. Lastly, it is important to note that the effective application of photocatalytic building materials requires compatibility with the building’s structure. Properties like light transmission, mechanical strength, and durability need careful consideration during the design phase. Coatings or veneer materials must also be coordinated with photocatalytic substances for both degradation efficacy as well as aesthetic appeal.

6.2. Future Perspective and Related Problem Discussions

Using photocatalytic building materials to prevent air pollution holds great promise, as environmental awareness increases and technology advances. With the acceleration of urbanization, urban air pollution poses a significant threat to both public health and the quality of life. Therefore, incorporating photocatalytic building materials into urban construction and planning can effectively enhance air quality. However, researchers are still exploring more efficient and stable photocatalytic building materials to enhance the prevention and control of air pollution. Both government entities and enterprises should promote and implement photocatalytic building materials to maximize their environmental benefits. For the successful implementation of photocatalytic building materials in real-world environments, it is crucial to overcome the following technical challenges:

• Enhanced stability of the photocatalytic materials: The stability of the photocatalytic materials is crucial for their practical use, necessitating a comprehensive investigation into the effects of prolonged exposure to light and environmental factors on their lifetime. Humidity, temperature, pH, pollutants, and microorganisms can significantly impact the stability of these materials. Additionally, it is important to consider that photocatalytic materials may also induce degradation in the substrates or binders they are attached to, leading to reduced mechanical strength and durability [80]. Therefore, it is imperative to develop strategies aimed at augmenting both the stability of photocatalytic materials themselves and that of their associated substrates or binders to ensure long-term performance.
• Photocatalytic reaction rate: The photocatalytic reaction rate is a crucial factor influencing the real-world use of photocatalytic building materials. It is imperative to ensure that the reaction rate is sufficiently rapid to degrade the desired harmful substances. Therefore, it becomes necessary to explore diverse mechanisms governing photocatalytic reactions to enhance the reaction rate. Several factors such as the light intensity, wavelength, catalyst loading, surface area, morphology, crystallinity, doping, and co-catalysts may influence the reaction rate [4]. Additionally, the type and concentration of pollutant along with other interfering substances also impacts the reaction rate. Consequently, optimizing these parameters becomes indispensable for achieving high efficiency and selectivity in photocatalysis.
• Selectivity of photocatalytic materials: The selectivity of photocatalytic materials refers to their capacity for the targeted oxidation or reduction of specific pollutants in the presence of other substances [81]. Achieving high selectivity is crucial for enhancing efficiency and minimizing unwanted by-products or secondary pollution during photocatalysis. However, most existing photocatalytic materials exhibit low selectivity and tend to react with a variety of organic and inorganic compounds present in the air [82]. This can lead to reduced photocatalytic activity and increased energy consumption. Therefore, a key challenge lies in designing and modifying photocatalytic materials with enhanced selectivity.
• The economics of photocatalytic materials encompass their cost-effectiveness and feasibility in terms of production and application. Factors influencing the cost include the type and quantity of raw materials, synthesis method, fabrication process, scale-up potential, and maintenance expenses. The benefits are contingent upon the photocatalytic material’s performance, durability, environmental impact, and social acceptance [58]. Therefore, it is crucial to evaluate and optimize the economic aspects of photocatalytic materials so they can be widely adopted and used for air pollution prevention.

7. Conclusions and Future Perspectives

This work reviews recent progress in the photodegradation of atmospheric pollutants through TiO₂-based building materials and discusses the photodegradation mechanism of VOCs, NO_X, and SO_X by photocatalysts. Numerous studies have confirmed the ability of photocatalytic technology to decompose gaseous pollutants. In particular, the photodegradation efficiency of low-concentration pollutants is acceptable. Furthermore, strategies to improve TiO₂ aggregation and broaden its ability to absorb visible light in practical applications are discussed. These include synthesizing composite nanomaterials with silica dioxide, carbon nanotubes, graphene, and other materials to reduce TiO₂ agglomeration. As well as modifying TiO₂ with metals or non-metals to enhance its photocatalytic ability and visible light absorption. Currently, research has made considerable progress on TiO₂ photocatalysts. However, the catalytic efficiency of TiO₂-based building materials under visible light irradiation in practical applications still falls short. Moreover, the by-product-generating pathways remain unclear, general evaluation criteria are lacking, and there are limited simulation tools. The durability of the photocatalysts is also unknown and their widespread commercial application is far from being realized. To expedite the commercialization process, there is a pressing need for more basic research work to overcome existing catalyst shortcomings. It is suggested that the research priorities should focus on developing new photocatalysts that exhibit excellent pollutant removal efficiency under solar-light irradiation. This should be carried out with minimal or no low toxicity byproduct production. Additionally, these new photocatalysts should be compatible with building materials while improving the building material’s service life. This review provides a reference for optimizing existing methods and exploring new strategies, aiming to design better building materials with photocatalytic capability to achieve efficient air purification function.