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Recent Progress in the Abatement of Hazardous Pollutants Using Photocatalytic Tio2-Based Building Materials

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Recent Progress in the Abatement of Hazardous Pollutants Using Photocatalytic Tio2-Based Building Materials nanomaterials Review Recent Progress in the Abatement of Hazardous Pollutants Using Photocatalytic TiO2-Based Building Materials 1, 2, 3 1 Anantha-Iyengar Gopalan y , Jun-Cheol Lee y, Gopalan Saianand , Kwang-Pill Lee , Prashant Sonar 4,5, Rajarathnam Dharmarajan 3, Yao-long Hou 6 , Ki-Yong Ann 7, Venkatramanan Kannan 8 and Wha-Jung Kim 1,* 1 Daegyeong Regional Infrastructure Technology Development Center, Kyungpook National University, Daegu 41566, Korea; [email protected] (A.-I.G.); [email protected] (K.-P.L.) 2 Department of Architecture, Seowon University, Cheongju 28674, Korea; [email protected] 3 Global Centre for Environmental Remediation (GCER), Faculty of Science, The University of Newcastle, Callaghan, New South Wales 2308, Australia; [email protected] (G.S.); [email protected] (R.D.) 4 School of Chemistry and Physics, Queensland University of Technology, 2 George Street, Brisbane, QLD 4001, Australia; [email protected] 5 Centre for Material Science, Queensland University of Technology, 2 George Street, Brisbane, QLD 4001, Australia 6 Department of Civil Engineering, Kyungpook National University, 80 Daehakro, Buk-gu, Daegu 41566, Korea; [email protected] 7 Department of Civil and Environmental Engineering, Hanyang University, Ansan 1588, Korea; [email protected] 8 Department of Physics, SCSVMV Deemed University, Kanchipuram 631561, India; [email protected] * Correspondence: [email protected]; Tel.: +82-53-950-6335 These authors contributed equally to this work. y Received: 7 August 2020; Accepted: 11 September 2020; Published: 16 September 2020 Abstract: Titanium dioxide (TiO2) has been extensively investigated in interdisciplinary research (such as catalysis, energy, environment, health, etc.) owing to its attractive physico-chemical properties, abundant nature, chemical/environmental stability, low-cost manufacturing, low toxicity, etc. Over time, TiO2-incorporated building/construction materials have been utilized for mitigating potential problems related to the environment and human health issues. However, there are challenges with regards to photocatalytic efficiency improvements, lab to industrial scaling up, and commercial product production. Several innovative approaches/strategies have been evolved towards TiO2 modification with the focus of improving its photocatalytic efficiency. Taking these aspects into consideration, research has focused on the utilization of many of these advanced TiO2 materials towards the development of construction materials such as concrete, mortar, pavements, paints, etc. This topical review focuses explicitly on capturing and highlighting research advancements in the last five years (mainly) (2014–2019) on the utilization of various modified TiO2 materials for the development of practical photocatalytic building materials (PBM). We briefly summarize the prospective applications of TiO2-based building materials (cement, mortar, concretes, paints, coating, etc.) with relevance to the removal of outdoor/indoor NOx and volatile organic compounds, self-cleaning of the surfaces, etc. As a concluding remark, we outline the challenges and make recommendations for the future outlook of further investigations and developments in this prosperous area. Keywords: building materials; photocatalytic; titanium dioxide; pollutant removal; self-cleaning Nanomaterials 2020, 10, 1854; doi:10.3390/nano10091854 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1854 2 of 47 1. Introduction In recent years, challenges regarding air pollution based on particulate matter (PM) and toxic gases that are arising from anthropogenic or external sources have become severe issues [1]. Research activities are being directed to resolve air pollution-related matters through the process of making practical remedial measures. Air pollution can occur both in indoor (e.g., household) contexts and in outdoor environments. Common hazardous compounds include NOx, SOx, CO, H2S, NH3, other nitrogen compounds (e.g., hydrogen cyanide), sulfur-containing compounds (organothiols), hydrocarbons, and a myriad of volatile organic compounds (VOCs) (benzene, toluene, methanol, etc.) which are of significant concern for environmental remediation [2]. The primary sources of these gaseous pollutants are anthropogenic. Mainly, VOCs based on alkanes, aromatics, esters, alkenes, carboxylic acids, and alcohols can exist in indoor environments, especially inside building spaces, originating from adhesives, carpeting, wood products, or cleaning products [3]. These indoor air pollutants can cause health issues, such as breathing problems, for people with compromised or underdeveloped immune systems [4,5]. On the other hand, fossil fuel burning by motor vehicles (including cars, buses, trucks, etc.) and power plants (including coal-fired and natural gas plants) can expel a mixture of pollutants, such as tiny particles or PM, NOx, and VOCs [6–9]. Besides, secondary pollutants also originate indirectly from the air when primary pollutants react or interact with each other or with other environmentally existing gaseous molecules. For example, chemical interactions between SOx and NOx can produce ground level ozone, and this can happen in the upper atmosphere too, leading to photochemical smog and acid rain [10]. These outdoor and secondary pollutants can cause environmental and human health issues. Gaseous pollutants are mainly controlled or removed by a few significant techniques: absorption, adsorption, chemical oxidation, and incineration. These techniques may be employed either singly or in combination, which, in turn, can be decided based on the type of pollutant. Gas adsorption is a surface phenomenon where gas molecules are adsorbed or are attracted to the surface and held on the surface of the solid. These approaches for the depollution process, however, are constrained with drawbacks [11]. For example, while activated carbon [12,13] adsorption can lead to secondary pollution, incineration and catalytic oxidation are expensive and consume more energy. Photocatalysis is a rapidly evolving technology for the treatment of air pollutants and is considered as one of the most effective methods to remediate environmental gaseous pollutants. Photocatalysis refers to the conversion of photon energy into chemical energy and the acceleration of photoreactions in the presence of a photocatalyst. Photocatalysis offers several advantages over the conventional catalytic processes that involve time-consuming steps, elevated temperatures, and pressures. Typically, photocatalysis, or photocatalytic oxidation (PCO), has been effectively employed for the removal of low concentrations [i.e., part per billion (ppb) level] of gaseous pollutants, with distinct advantages that include ambient temperature operations, minimum energy consumption, and effectiveness for various contaminants with innocuous final products [14]. Several metal-oxide based semiconductors, such as titanium dioxide (TiO2)[15–18], zinc oxide (ZnO) [19–27], copper oxide (CuO) [28–31], zirconium oxide or zirconia (ZrO2), cerium oxide (CeO2)[29], tungsten oxide (WO3), and tin oxide (SnO2), have been successfully demonstrated as efficient photocatalysts for pollutant removal [32–34] and utilized in several other applications (energy, environment, and health) [35–37]. The semiconductor photocatalytic process includes irradiation of the semiconductor surface with a light having energy greater than the semiconductor’s bandgap. This triggers the excitation of valence band (VB) electrons and transfers to the conduction band + (CB) to create electron-hole pair (e−-h ). These charge carriers subsequently participate in reactions between the pollutant molecules and radicals generated on the surface of the semiconductor [38,39]. The rapid development and convergence of the critical scientific components, such as catalysis science, nanoscience, and material characterizations, form the basis for significant advances in the development of new photocatalysts, photocatalytic reaction mechanisms, and the structure-photo activity relationship between photocatalysts and target pollutants [40–44]. More interestingly, the structural features of Nanomaterials 2020, 10, 1854 3 of 47 Nanomaterials 2020, 10, x FOR PEER REVIEW3 of 50 photocatalysts can be further fine-tuned to derive enhanced photocatalytic performances for the performances for the depollution process [42]. Particularly, semiconductor photocatalysts have been depollution process [42]. Particularly, semiconductor photocatalysts have been proved to exhibit proved to exhibit significant efficiency for the degradation of both organic and inorganic pollutants. significant efficiency for the degradation of both organic and inorganic pollutants. TiO2 is one of the popular commercial photocatalysts which is prevalently used for the photo- TiO2 is one of the popular commercial photocatalysts which is prevalently used for the degradation of many pollutants. TiO2 offers major advantageous characteristics, such as ease of photo-degradation of many pollutants. TiO2 offers major advantageous characteristics, such as synthesis and versatility of application. TiO2-based photocatalysis has proven to be a very promising ease of synthesis and versatility of application. TiO2-based photocatalysis has proven to be a very advanced oxidation process for the depollution and cleaning of indoor and outdoor air, exhibiting promising advanced oxidation process for the depollution and cleaning of indoor and outdoor air, unique advantages
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