## 1 Introduction  
The development of industry and the progress of traffic have brought with them many negative influences, such as air, noise, and water pollution. In particular, air pollution causes damage to human health, causing many people to suffer from allergy or respiratory diseases [1], which promotes researchers' interest in the filtering of air pollutants. The issue of polluted air is commonly addressed using air filter materials that are divided into mechanical and nonmechanical types. As for nonmechanical types, polymer electret filter materials garner the most attention because of their high efficiency and low pressure drop [2; 3; 4; 5; 6].  
Polymer electret filter materials are extensively used in the air filtration field, and the most effective polymer is poly(tetrafluoroethylene) (PTFE). However, PTFE has a high cost and challenging production issues, which makes polypropylene (PP) a staple material for air filtration products in the market. In addition, there are two approaches to strengthen the electret efficacy. One approach is to add dielectric-constant reinforcing materials to the filters, thereby storing much more space charges [7] and thus obtaining higher surface voltage. The reinforcing materials are lithium niobate [8], silicon dioxide [9], nanoscale graphite platelets [10], and barium titanate [7; 11; 12]. The other approach is to add a nucleating agent that increases the crystallinity of polymers and to obtain greater crystalline and amorphous regions to house more space charges. The nucleating agents are magnesium stearate [13], stearate [14], modified rosin [14], polycarbonate [15], tourmaline [16], DMDBS (3:2, 4-bis(3,4-dimethyldibenzylidene) sorbitol) [17], and NA11 (sodium 2,2'-methylene-bis(4,6-di-terbutylphenyl)-phosphate) [17].  
Titanium dioxide (TiO\({}_{2}\)) is one of the materials commonly used in paints and cosmetics [18]. Because of its high dielectric constant, TiO\({}_{2}\) is also used in semiconductors and solar cells. [PERSON] et al. found that TiO\({}_{2}\) had excellent charge storage capacity and a great potential as a supercapacitor [19]. [PERSON] et al. used TiO\({}_{2}\) as the core to be synthesized with titanium compounds, producing materials that had a greater dielectric constant and specific energy when compared to poly(vinylidene fluoride) (PVDF) [20]. Due to the aforementioned upsides of TiO\({}_{2}\), [PERSON] et al. added TiO\({}_{2}\) to polypropylene (PP) in the production of composite films. It was found that the surface potential was attenuated in a shorter time when the TiO\({}_{2}\) ratio was increased, which might be attributed to the fact that a higher TiO\({}_{2}\) ratio had a positive influence on the conductivity [21]. Similar findings were found in the study by [PERSON] et al., where TiO\({}_{2}\) was incorporated with polyimide to form nanocomposite membranes. They found that a TiO\({}_{2}\) ratio exceeding 5 wt% resulted in a significant improvement in the conductivity [22]. Because one feature of photocatalysis is to purify air, TiO\({}_{2}\) has been commonly investigated in previous studies on air filter materials [23; 24; 25; 26; 27]. For example, [PERSON] et al. strengthened the decomposition of benzene and toluene by means of this particular feature of TiO\({}_{2}\)[23]. Because the majority of studies focused on the photocatalysis purifying feature of TiO\({}_{2}\) where air filtration was concerned and the electret materials were made in the form of membranes or composites, this study incorporates TiO\({}_{2}\) with PP, thereby forming an air filter featuring a fiber morphology. Afterward, the fiber morphology, surface potential, and static electricity filtration efficiency of electret melt-blown fabrics were evaluated, examining the effects of the presence of TiO\({}_{2}\).