# Improvement of Buildings' Air Quality and Energy Consumption Using Air Purifying Paints
## 1 Introduction
To that end, the photocatalytic reactions between the radical species and binder molecules not only produce and emit VOCs (e.g., formaldehyde, acetaldehyde, and acrolein) under UV irradiation but they also affect paint durability, have a negative impact on indoor air quality, and limit the paint's mechanical and photocatalytic properties. One of the current solutions to overcome this problem is to reduce or replace the organic binder content with a mineral one [20]. The latter could lead to a significant decrease in total VOC emission under given experimental conditions. Taking under consideration these promising results, an innovative inorganic and organic paint, which is activated under Vis light irradiation (instead of the usually used UV, which significantly affects the VOC emission), has been developed in the frame of the current study. As far as the evaluation tests of the above-mentioned photocatalytic materials are concerned, most studies have focused on NOx (NO, NO\({}_{2}\)) and VOC removal using UV lights for outdoor applications [24; 25; 26; 27; 28; 29]. Meanwhile, the evaluation was performed at relatively high concentrations (well above those usually encountered outside) [30; 31; 32; 33; 34; 35]. Visible light-driven photocatalytic materials studies are limited [36; 37; 38; 39; 40; 41]. Studying the state of the art regarding photocatalytic paints, the variety of results on the air pollutant degradation is obvious. This large variability is mainly due to the employment of different photoreactor configurations, the operating conditions, the air pollutants to be tested, and the components of the photocatalytic paint. Thus, the direct comparison among these systems could not be valid and a common protocol should be followed in order to obtain comparable results, e.g., following the European standard EN16980-1:2021 [42]. TiO\({}_{2}\) is a wide-band-gap semiconductor, thus requiring ultraviolet irradiation, which corresponds only to 4% of the solar spectrum. In order to obtain activity in the visible region, it is essential to modify the semiconductor materials by several techniques such as noble metal (such as Au, Pt, and Pd) or non-metal doping (such as C, N, and S); modification with transition metals has been employed in order to extend light absorption into the visible region [40]. Modifications such as altering surface properties by modified surface chemistry or introducing co-catalysts can facilitate the charge separation of electron-hole pairs and enhance the overall photocatalytic efficiency. In the current study, many powder formulae were synthesized using different synthetic pathways, concentration of metal dopants, and different particle size. The most promising photo-powder was used in the paint production process with an ultimate scope to produce paint that aims to photocatalytically improve IAQ by degraded indoor air pollutants. A plethora of scientific methods have been investigated for the synthesis of TiO\({}_{2}\) by employing different synthesis techniques including sol-gel, solvothermal, chemical precipitation, micro-emulsion, chemical vapor deposition, etc. In the present work, TiO\({}_{2}\) particles were synthesized via three different synthetic routes (sol-gel route, hydrothermal, and precipitation). In particular, we focus on the sol-gel method, which is the most commonly used technique for the preparation of TiO\({}_{2}\) nanoparticles due to low-cost, low-temperature processing, and good shaping ability. The pollutants that were chosen as targets were NO and toluene. The reason is that they are typical indoor air pollutants, which could be emitted from various indoor sources. Among them are building materials, tobacco smoke, cooking, furniture, and fireplaces. In order to simulate the real indoor air conditions and avoid reactions with paint organic contaminants, which might lead to harmful emitted by-products, visible light was used as the source for the photocatalytic reaction in the current study.