1.2. Photocatalytic TiO2 and its applications

photocatalytic performances of the abovementioned tiles. Moreover, the antibacterial properties of the tiles have been determined, using Escherichia coli as example. Life cycle assessments (LCAs) for the two processes were modeled for 1 m2 of tiles produced in Modena, Italy. The impact assessments revealed that jet spraying exhibited uniformly greater impacts than digital inkjet printing and that the principal impacts were in human toxicity, cancer effects, freshwater ecotoxicity, and climate change. Most of the impacts were associated with the energy required for the production processes. Further considerations revealed that jet spraying is projected to generate twice as much CO2 and 30% more

Keywords: TiO2, titania, digital inkjet printing, jet spraying, VOCs, NOx, E. coli, life cycle

The WHO (World Health Organization) estimates that 12.6 million deaths each year are attributable to unhealthy environments [1], confirming the urgent need for investment in strategies to reduce environmental risks in cities, homes, and workplaces. Of these deaths, 4.3 million (34%) and 3.7 million (29%) have been linked to exposure to indoor and outdoor pollution, respectively [2]. More broadly, it is considered that 92% of the population lives in areas that are exposed to respirable particulate matter of <2.5 μm in diameter (PM2.5) at an

WHO Air Quality Guidelines (AQG). The worst affected areas are China, the Indian subconti-

Particulate and gaseous air pollution results largely from industrial and motor vehicular sources, although these emissions are exacerbated by those generated by heating [3]. The combustion of solid biomass and impure liquid fuels is the main source of pollution leading to health problems, as evidenced by numerous recent studies. For example, in Ethiopia, the average concentration of respirable particulate matter of diameter of <PM2.5 reaches as high

limits for the US, and the average NO2 level is 97 ppb [4]. In Southeast Asia, the large-scale burning of biomass causes heavy emissions of combustion by-products, which photochemically transform to other more dangerous molecules [5], such as polycyclic aromatic hydrocarbons (PAHs) [6]. Europe is affected mainly by emissions of nitrogen oxides (NOx) [7]. Although outdoor pollution tends to be a focal point, indoor air in the US can be two to five times more harmful than outdoor air [8]. Almost 96% of houses in North America are reported to have at least one indoor air quality problem [9]. Cigarette smoke contains more than 7000 chemicals, of which 69 are known to be carcinogenic and many more to be poisonous [10]. Household items, such as cleaning products, air fresheners, and sanitation products, also contribute to indoor pollution by releasing chemicals. Consequently, there are environmental health imperatives to develop technologies that address these while satisfying other demands in the areas of energy efficiency, economic efficiency, utility, and

), which exceeds the

, indoor CO levels are greater than the regulatory

annual mean concentration of >10 μg/m3 (or a 24 h mean of >25 μg/m<sup>3</sup>

NOx than digital inkjet printing.

262 Titanium Dioxide - Material for a Sustainable Environment

assessment, impact assessments

nent, the Middle East, and northern Africa.

as a 24-h mean concentration of 280 μg/m<sup>3</sup>

1. Introduction

1.1. Pollution

esthetics.

Semiconductor photocatalysis involves the activation of a photocatalytic material, such as titania (TiO2), by the action of light to produce charge carriers that undergo redox reactions with water and oxygen molecules [11]. This property allows these materials to decompose organic contaminants ideally into CO2 and water, thus allowing applications in self-cleaning, anti-fogging, corrosion prevention, and air and water purification [12]. Although there are many scientific and technical requirements for effective photocatalytic performance [13], there are also numerous practical challenges that must be met. One of these is catalyst immobilization [14] because it generally must balance the requirements of achievement of a robust chemical bond to a substrate, the retention of sufficient exposed surface area, and the minimization of contamination from the substrate.

There has been an increase in the sales of TiO2-based products for self-cleaning and air and water purification owing to growing environmental concerns [15]. One of the key product developments was photocatalytic building materials, which appeared on the market at the beginning of the 1990s [16]. TiO2 can be applied either in exterior construction or in interior furnishing materials, including concrete, mortar, tiles, flooring, and glass. In the early 2000s, the sales of photocatalytic building materials accounted for ~60% of the total market for photocatalytic materials [17]. Recent market projections to 2020 foreshadow increasing demand in the construction sector for self-cleaning coatings and other technologies for improving air quality, leading to a global market for photocatalyst-based products of ~\$2.9 billion [18]. At present, TiO2-based photocatalyst products constitute 97.6% of the market, and this dominance is a result of TiO2's low cost, ready availability, and well-documented performance data.

Many studies have demonstrated the effectiveness of photocatalytic materials for pollution abatement of the volatile organic compounds (VOCs) and nitrogen oxides (NOx) that represent widespread risks to health. For example, concrete paving blocks with TiO2 added directly to the mix were able to remove 4.01 mg/h/m2 of NO [19]. However, similar work demonstrated that the efficiency of NOx photodegradation varied significantly according to preparation method and resultant microstructure, with some products achieving 40% photodegradation while others showed almost no effect [20]. A comparison between pure TiO2 and TiO2 embedded in neat hydrated cement revealed that the photocatalytic activity was three to 10 times lower for the latter, which was attributed to the agglomeration of the TiO2 powder [21]. TiO2 also has been applied as surface coatings on roofing tiles and corrugated sheet and found to achieve gaseous toluene removal efficiencies as high as 78 2%, although this was limited by gas concentration and residence time [22].

It is clear that the permanence and performance of TiO2-containing construction products depend significantly on the method used to immobilize the photocatalyst, whether in the bulk or on the surface. Other factors of importance include the type of substrate and the method of TiO2 application [23]. Hence, there has been a commensurate focus on the design of such building products and their manufacture in order to obtain maximal lifetimes and efficiencies. An additional consideration is the potential alteration of the photocatalyst by fouling, which can block the active sites and reduce or prevent photocatalysis [24]. Finally, there are also potential health issues associated with the formation of chemical intermediates [25] and the respirability of nanoparticulates [26].
