## 1 Introduction  
The formation of microbial colonies inside the wet or damp indoor environments is one of the major contributing elements to the deterioration of indoor air quality [1, 2]. These microorganisms have the potential to produce contaminants like spores, allergens and toxins affecting the health of the occupants [3, 4]. This kind of continuous exposure can lead to various health concerns like respiratory or skin diseases [5, 6]. Several studies in the past few decades have looked at the alteration of building materials and paints to overcome the challenges of indoor microbial contamination [7, 8]. Most of the physicochemical techniques and usual cleaning procedures are inactive to completely eliminate the microbes. Photocatalysis is one of the most promising, simple and economically viable technology to address indoor air microbial contamination [9]. Existing photocatalyst-based coatings suffer two important disadvantages, they require artificial UV light sources to generate reactive oxygen species (ROS) and secondly these coatings remain inactive in dark atmosphere as the ROS generated in light declines with time [10]. TiO\({}_{2}\) has been recognized as a traditional semiconductor photocatalyst, which has been widely reported for its anti-microbial coatings [11; 12]. The formation of uniform aqueous slurry and the feasibility of deposition on the various substrates make TiO\({}_{2}\) a lucrative candidate [13; 14]. However, this material suffers from its own set of limitations. Among the three phases of TiO\({}_{2}\), anatase is reported to be the most photocatalytically active phase [15]. However, the phase stability of anatase is very crucial as it has a tendency to rutilation at elevated temperatures. Apart from this, low visible light absorption because of the wide band gap of TiO\({}_{2}\) (3.2 eV) is incompatible to receive visible light for practical applications [16]. Elemental doping, morphological tailoring in the form of nanotubes and fibers, making composites out of new 2-D materials like graphene, C\({}_{3}\)N\({}_{4}\), are some of the efficient techniques to overcome these limitations [17; 18; 19; 20]. Earlier studies on heterojunctions of anatase-rutile and anatase-brookite for photocatalytic applications have been reported. Although anatase is the photocatalystly active phase, the heterojunctions formed in the mixed phase samples showed superior photocatalytic activity under visible light [21; 22]. It is attributed to the delayed charge recombination initiated by the electron-hole separation effect [22]. Elemental doping is an effective strategy to tune the band gap of TiO\({}_{2}\) by introducing new energy levels in between and by alteration of conduction band minima (CBM) and valence band maxima (VBM) [23; 24]. In this regard, cationic dopants served to extend the visible light absorption and also enhanced the high temperature stability of the anatase phase TiO\({}_{2}\). Copper is one of the interesting dopants added to TiO\({}_{2}\) at the different mole and weight percent. Cu-doped TiO\({}_{2}\) was evaluated for various photocatalytic applications like Hydrogen production [25; 26; 27; 28], CO\({}_{2}\) conversion [29], degradation of dyes and organic contaminants. [PERSON] et al. reported that Cu existed in the form of both Cu\({}^{+}\) and Cu\({}^{2+}\) states in Cu-doped TiO\({}_{2}\) thin films. These nano-architectures showed higher activity for degradation of organic dye molecules under visible light illumination [30]. Similarly, [PERSON] et al., demonstrated high CO\({}_{2}\) conversion to hydrocarbon fuels using Cu incorporated TiO\({}_{2}\) nanotube arrays [31]. In another study, authors explored the electronic and structural stability of Cu-doped TiO\({}_{2}\) synthesized in different acidic mediums (HNO\({}_{3}\) and H\({}_{2}\)SO\({}_{4}\)). The doped sample calcined at 600 \({}^{\circ}\)C displayed high photocatalytic activity for phenol degradation [32].