Mechanisms of the Antibacterial Effects of TiO₂–FeO_x under Solar or Visible Light: Schottky Barriers versus Surface Plasmon Resonance

Abstract

This study reports the significant mechanistic difference between binary-oxide antibacterial films with the same composition but different microstructures. Binary TiO₂-FeO_x films were found to present a faster bacterial inactivation kinetics under visible light irradiation than each single oxide acting independently. The interaction between the film active surface species and the bacteria within the disinfection period was followed by X-ray photoelectron spectroscopy (XPS) and provided the evidence for a redox catalysis taking place during the bacterial inactivation time. The optical and surface properties of the films were evaluated by appropriate surface analytical methods. A differential mechanism is suggested for each specific microstructure inducing bacterial inactivation. The surface FeO_x plasmon resonance transferred electrons into the conduction band of TiO₂ because of the Schottky barrier after Fermi level equilibration of the two components. An electric field at the interface between TiO₂ and FeO_x, favors the separation of the photo-generated charges leading to a faster bacterial inactivation by TiO₂–FeO_x compared to the bacterial inactivation kinetics by each of the single oxides.

1. Introduction

Photocatalytic nanomaterials such as TiO₂ are receiving a great deal of attention owing to their potential applications in environmental remediation [1,2]. Nonetheless, the low efficiency of this class of materials under solar irradiation, absorbing <5% of the incident light, limits their performance in photo-induced processes. At the present time, there is a need to develop more efficient TiO₂ composite photocatalysts, active under visible light [3,4,5]. Hybrid nanostructured composite photocatalysts like binary oxides seem a fruitful solution. The present mini-review addresses hybrid, stable, adhesive antibacterial TiO₂–FeO_x films leading to visible light-driven bacterial inactivation. We describe briefly the properties of TiO₂ and Fe oxides (FeO_x) and address the preparation, evaluation, and properties of TiO₂–FeO_x films having the same composition but different microstructures.

TiO₂ is known to exhibit photocatalytic antimicrobial activity over a broad spectrum of microorganisms. The antimicrobial properties of TiO₂ are attributed to the high redox potential of reactive oxygen species (ROS) generated on TiO₂ under band-gap irradiation. Foster et al. [6], Yadav et al. [7], Kiwi et al. [8], Kubacka et al. [9], Verdier et al. [10], Al-Hazmi et al. [11], and others have recently reported a comprehensive review of the photocatalytic disinfection properties of TiO₂. The first report on bacterial sterilization by TiO₂ powders was published by Matsunaga et al. [12]. TiO₂ under band-gap irradiation photo-induces charge carriers that subsequently in the presence of O₂ lead to ROS with high oxidative potentials. These ROS are effective in pollutant abatement, bacterial inactivation, or both. When the oxidant used exceeds the natural antioxidant defenses in a bacterial strain, death cell follows through a complex set of redox reactions. Natural antioxidants like carotene, ascorbic acid, tocopherol inhibit lipid peroxidation or O-singlet effects [13] and the effects of ROS radicals such as HO₂^•− and ^•OH which are effective in biological inactivation.

Iron oxide thin film have extensive applications in semiconductor devices, magneto-optic memories, audio-video systems, computer chips, and memory storage devices. Iron oxide exists in three phases: α-Fe₂O₃, β-Fe₂O₃, γ-Fe₂O₃ [14]. α-Fe₂O₃ with a band gap of 2.2 eV absorbs in the visible up to 570 nm and its charges present a low hole diffusion length and short exciton lifetime (~10 ps). Iron oxide shows antibacterial behavior [15]. ROS generated by α-Fe₂O₃ induce physical damage by contact leading to bacterial reduction [16,17,18]. The genotoxicity of Fe oxides is currently investigated in medical research against cancer. The oxidative damage introduced by Fe, FeO_x, Fe₂O₃, and Fe₃O₄ nanoparticles (NPs) reduces or destroys cancer cells but concomitantly induces cellular injury or death in normal cells. Fe oxides lead to genetically regulated cell death (apoptosis) and to an increase of the ROS levels by damage within cells, followed by autophagy. The potential damage to tissues located behind cellular barriers needs to be considered when using Fe NPs for targeting tumors [19,20,21]. Super-paramagnetic iron oxide particles combined with certain chemicals are labelled SPIONS and increase ROS stress up to a factor of 10 during cancer cell treatment. Coupling SPIONS with X-ray radiotherapy amplifies the cytotoxicity on tumors and cancer cells. This is a synergistic strategy [22]. Because of the complexity of this field, this mini-review addresses only the interaction of binary oxides containing FeO_x NPs with simpler probes like bacteria and the details related to the bacterial damage and destruction.

Fe(III)-modified titania and Fe(III)–Ti(IV) binary oxides have received little attention as photocatalyst films compared to their parent single metal oxides, Fe₂O₃ and TiO₂. This is due to the difficulty of obtaining a pure mono-phase TiO₂. In addition, the effect of different preparative conditions on films prepared by sol–gel as well as their surface properties, catalytic and photocatalytic activity have been the object of very few studies [23,24,25,26,27]. In colloidal formulations up to 10%, FeO_x can be added and will disperse well in the TiO₂ lattice. The addition of higher percentages of FeO_x leads to Fe-phase segregation. Very little work has been reported on the detailed microstructure of TiO₂–FeO_x stable, uniform, adhesive films. This moved us to investigate the films microstructure effect on the bacterial inactivation kinetics. Surface properties and reaction mechanism leading to bacterial killing is reported by the binary-oxide composites.

2. TiO₂–FeO_x Surfaces Leading to Bacterial Inactivation under Solar Light with a Faster Kinetics Compared to Either TiO₂ or FeO_x Films

Dispersions of FeCl₃ and TiO₂ Degussa P25 were prepared using FeCl₃ (100 mg L⁻¹) and TiO₂ (5 g L⁻¹) and were irradiated to photo-corrode polyethylene (PE) and introduce negatively charged oxidative sites able to bind both oxides by exchange–adsorption and electrostatic interaction. The photo-corrosion of the TiO₂ powder surfaces was carried out under UV irradiation for 15 h. After UV irradiation, the films were sonicated in aqueous solution for 10 min to remove loosely bound oxide particles, washed, and dried for 10 min at 80 °C. This operation was repeated two times, and the films were dried at 60 °C. Stable TiO₂–FeO_x films were obtained by this procedure.

Figure 1 shows the kinetics of bacterial inactivation under low-intensity solar irradiation for TiO₂, FeO_x, and TiO₂–FeO_x. It is readily seen that the binary composite induced a faster bacterial inactivation kinetics compared to each of the single oxides evaluated separately [28]. The main reactions leading to bacterial inactivation on TiO₂ are suggested below in Equations (1)–(5) [2,29]:

$$\mathrm{bacteria}+\left[\mathrm{Ti}{\mathrm{O}}_2-\mathrm{PE}\right]\mathrm{light}\to \left[{\mathrm{Ti}\mathrm{O}}_2^{*}-\mathrm{PE}\right]\mathrm{bacteria}\to \left[{\mathrm{bacteria}}^{*}\dots \mathrm{Ti}{\mathrm{O}}_2-\mathrm{PE}\right]{\mathrm{cbe}}^{-}$$

(1)

$${\mathrm{TiO}}_2\left({\mathrm{cbe}}^{-}\right)-\mathrm{PE}+{\mathrm{O}}_{2\mathrm{ads}}\to {}^{•}{\mathrm{O}}_2{{}^{-}}_{\mathrm{ads}}{\mathrm{E}}_0-0.16\mathrm{NHE}$$

(2)

$${\mathrm{TiO}}_2({\mathrm{cbe}}^{-})-\mathrm{PE}+{\mathrm{O}}_2+{\mathrm{H}}^{+}\to \mathrm{H}{\mathrm{O}}_2{}^{•}{\mathrm{E}}_0-0.05\mathrm{NHE}$$

(3)

$$\mathrm{Ti}{\mathrm{O}}_2({\mathrm{vbh}}^{+})-\mathrm{PE}+{\mathrm{OH}}_{\mathrm{ads}}^{-}{\to }^{•}\mathrm{OH}{\mathrm{E}}_0-1.90\mathrm{NHE}$$

(4)

$$\mathrm{Ti}{\mathrm{O}}_2({\mathrm{vbh}}^{+})-\mathrm{PE}+{\mathrm{H}}_2{\mathrm{O}}_{\mathrm{ads}}{\to }^{•}{\mathrm{OH}}_{\mathrm{ads}}+{\mathrm{E}}^{+}$$

(5)

Bacterial inactivation by FeO_x under solar simulated light irradiation could be obtained as noted in Equations (6)–(9):

$$[\mathrm{PE}-\mathrm{Fe}{\mathrm{O}}_{\mathrm{x}}]\mathrm{h}\mathsf{\upsilon }\to [\mathrm{bacteria}*\dots \mathrm{Fe}{\mathrm{O}}_{\mathrm{x}}]-\mathrm{PE}\to [{\mathrm{bacteria}}^{+•}+\mathrm{Fe}{\mathrm{O}}_{\mathrm{x}}]-\mathrm{PE}+{\mathrm{cbe}}^{-}$$

(6)

$$\mathrm{Fe}{\mathrm{O}}_{\mathrm{x}}+\mathrm{hv}\to \mathrm{Fe}{\mathrm{O}}_{\mathrm{x}}({\mathrm{vbh}}^{+})$$

(7)

$${\mathrm{vbh}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_{\mathrm{ads}}\to {}^{•}{\mathrm{OH}}_{\mathrm{ads}}+{\mathrm{H}}^{+}$$

(8)

$${\mathrm{cbe}}^{-}+{\mathrm{H}}_2{\mathrm{O}}_{\mathrm{ads}} \to {\mathrm{OH}}^{-}+{\mathrm{H}}^{+}$$

(9)

The acceleration of bacterial inactivation in Figure 1 by the TiO₂–FeO_x photocatalyst can be rationalized in terms of the intervention of FeO_x, injecting e⁻ into TiO₂, as noted below:

$${\mathrm{FeO}}_{\mathrm{x}}+\mathrm{light}\to {\mathrm{FeO}}_{\mathrm{x}}{(\mathrm{e}}^{-}{)+\mathrm{FeO}}_{\mathrm{x}}{(\mathrm{h}}^{+})$$

(10)

$${\mathrm{FeO}}_{\mathrm{x}}\left({\mathrm{e}}^{-}\right)+{\mathrm{TiO}}_2\to {\mathrm{FeO}}_{\mathrm{x}}+{\mathrm{TiO}}_2\left({\mathrm{e}}^{-}{}_{\mathrm{trapping}\mathrm{sites}}\right)$$

(11)

$${\mathrm{FeO}}_{\mathrm{x}}{(\mathrm{e}}^{-})+{\mathrm{O}}_2\to {\mathrm{FeO}}_{\mathrm{x}}+{\mathrm{O}}_2{}^{-}$$

(12)

$${}^{•}{\mathrm{O}}_2{}^{-}+{\mathrm{H}}^{+}\to {\mathrm{HO}}_2{}^{•}\to \mathrm{ROS}$$

(13)

$${\mathrm{TiO}}_2{(\mathrm{e}}^{-}{}_{\mathrm{trapped}\mathrm{sites}})+{\mathrm{O}}_2\to {\mathrm{TiO}}_2+{\mathrm{O}}_2{}^{-}\to \mathrm{ROS}$$

(14)

$${\mathrm{TiO}}_2\left({\mathrm{h}}^{+}\right)+\mathrm{bacteria}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}+\text{inorganic residues}$$

(15)

The mechanism of the reaction between TiO₂–FeO_x–PE and the bacteria under visible light proceeded noted in Equation (1). Under visible light, FeO_x (mainly Fe₂O₃), as it will be described in the paragraph below, would transfer the photo-generated electrons in the conduction band (cbe⁻) to the lower-lying TiO₂ sites, since FeO_x presents a conduction band (cb) positioned at potential energy values 0.4–0.6 eV, below the anatase trapping states [30]. Leytner et al. [31] identified the electron-trapping sites in anatase positioned at ~0.8 eV below the anatase (cb) by time-resolved photo-acoustic spectroscopy (TRPAS). Gray et al. [32] used electron paramagnetic resonance (EPR) spectroscopy and reported anatase trapping sites located ~0.5–0.8 eV below the anatase (cb). The mechanism for the interfacial charge transfer (IFCT) between Fe₂O₃ and TiO₂ on sol–gel films is suggested in Figure 2.

It is interesting, at this point, to look into the oxidation states of the FeO_x species before and after bacterial inactivation. Figure 3 presents the changes in the Fe oxidation states of PE–FeO_x within the 120 min bacterial inactivation reported in Figure 1, trace (1). Figure 3 shows that the initial Fe(III)/Fe₂O₃ at 712.2 eV decreased from ~80.0% at time zero to ~53.0% after bacterial inactivation. Concomitantly, an increase in the Fe₃O₄ at 713.6 eV and Fe(II) at 709.7 eV was observed. The X-ray photoelectron spectroscopy (XPS) shifts were referenced by the values found in reference [33]. Therefore, during the bacterial inactivation period, redox reactions occurred on the catalyst surface during bacterial inactivation. After bacterial inactivation, the XPS peak positions for Fe(III), FeO(II/III), and Fe(II) peaks were: 711.4, 708.6, and 713.8 eV, respectively [34]. The three Fe-oxides in Figure 3 intervened with a different potential during bacterial reduction as shown in Figure 3. The surface concentration of the elements were observed to remain fairly stable within the period of bacterial inactivation (see

Table 1. Surface atomic percentages concentration determined by XPS on PE–FeO_x sputtered for 60 s before and after bacterial reduction under solar irradiation (52 mW cm⁻²).

Element	Before	After
Fe2p	7.87	7.39
O1s	31.11	35.27
C1s	61.02	57.34
N2p	0.9	1.19

).

3. Sputtering of TiO₂–Fe₂O₃ Microstructure to Accelerate the Bacterial Inactivation Kinetics: Process Optimization

Films prepared by sol–gel are not perfectly reproducible, are not robust, are weakly adhesive, and lack uniformity. This is a serious hindrance for antibacterial applications over long periods, for which film stability is a primary consideration. Work by Kelly et al. [35,36] addressed the preparation of stable antibacterial films by sputtering metals and oxides. Sputtering active metals and oxides leads to highly oxidative species, leading to bacterial death. A magnetron sputtering unit is shown in Figure 4. The amounts of the point defects in the aggregates/crystallites films on the substrate are considerably higher than in similar films prepared by sol–gel because of the higher energies during the sputtering process [37]. Defects in oxide crystals and nuclei are located at energies within the band gaps, giving raise to intermediate mid-gap energy states mediating the photo-excited electron transition from the cb band to the valence band (vb) band [38]. Alternatively, they can act as recombination centers depending on their concentration. The rapid inclusion during sputtering, of Fe and O atoms during the sputtering time into the TiO₂ film lattice creates O vacancies and interstitial defects. [39]. The higher activity of the sputtered films is in part attributed to an increase in the defects in the crystallites. Oxides like TiO₂ and FeO_x have been extensively reported in the literature presenting four type defects: (a) O vacancies, (b) Ti or Fe vacancies, (c) O interstitials, and (d) Ti or Fe interstitials [40,41]. The work in the field of bacterial inactivation films need a more advanced catalysts design and preparation leading to a faster kinetics and a higher absorption in the visible range. Also, a higher reusability threshold is needed for antibacterial films for large scale applications. Section 4 below illustrates how the microstructure of TiO₂–FeO_x profoundly affects the bacterial inactivation kinetics.

4. Optical and Surface Properties of Co-Sputtered and Sequentially Sputtered TiO₂–FeO_x Films Active in Bacterial Inactivation

Figure 5 shows the diffuse reflection spectroscopy (DRS) spectra in Kubelka–Munk units for TiO₂–PE, FeO_x–PE, (a) sequentially sputtered TiO₂/FeO_x–PE films, and (b) co-sputtered TiO₂–FeO_x–PE films. The sputtering times noted in the caption of Figure 5 were optimized to find the most suitable ratio TiO₂/FeO_x for a film leading to bacterial inactivation kinetics. The TiO₂–FeO_x composite absorbs in the visible region >400 nm, inducing TiO₂–FeO_x charge transfer bands [42]. The light absorption in the spectral region between 400 and 500 nm in Figure 5 is attributed to IFCT between TiO₂ and FeO_x. The weak absorption >500 nm is due to the short-lived Fe d–d inter-band transitions. The electron pair-deficient oxygen vacancy was suggested to be able to react with Ti⁴⁺-ions to form Ti³⁺ centers, by Serpone et al. [43]. The amount of vacancies was reported to be one-half of the Fe(III) found in the TiO₂ (Ti⁴⁺) network [44,45,46,47,48,49].

The roughness (R_g) of the co-sputtered TiO₂–FeO_x–PE films was ~24 nm, as determined by atomic force microscopy (AFM). A value of ~11 nm was found for the sequentially sputtered TiO₂/FeO_x–PE films. The co-sputtered films showed FeO_x nano-particle sizes of 15–30 nm and TiO₂ nano-particle sizes of 10–15 nm. The particle size, diffusion, and mass transport determine particle growth and surface roughness [50]. The sequential sputtered films showed FeO_x NPs sizes of 20–40 nm. The bigger size of the sequentially sputtered NPs compared to the co-sputtered films is attributed to an easier collective diffusion in the FeO_x top-most layers of the latter samples [51]. The deposition of the co-sputtered TiO₂–FeO_x–PE films and the sequentially sputtered TiO₂/FeO_x–PE films was carried in a similar way. Bacterial inactivation mediated by the co-sputtered TiO₂–FeO_x–PE sample was completed within 60 min compared with the 120 min required by the sequentially sputtered samples. Both samples were sputtered for 2 min. Both samples were close in specific surface area (SSA) and that the difference in the bacterial inactivation times could not be ascribed to a difference in the surface area between the samples. The photo-sensitizing role of FeO_x unexpectedly led to similar inactivation times under solar simulated light and under visible light in both samples (404 nm cut-off filter).

5. Evidence by XPS of Bacterial Inactivation Inducing Differentiated Redox Interactions with TiO₂–FeO_x Samples

Figure 6a presents the changes in the Fe oxidation states for sequentially sputtered TiO₂/FeO_x–PE films. The initial Fe₂O₃ was seen to increase from ~70% at time zero to ~80% after 30 min at the expense of Fe₃O₄ and FeO. The Fe oxide peaks were referenced by the values reported in reference [33]. Figure 6b shows the changes of FeO_x and TiO₂ oxidation states in the co-sputtered films within the disinfection time. The initial 60% Fe₂O₃ percentage remained constant during the disinfection time, and the Fe₃O₄ and FeO percentages were conserved up to 60 min, while TiO₂ (Ti⁴⁺) slightly increased with a concomitant decrease of Ti³⁺-oxidation state. Electrostatic attraction occurs between the negatively charged Escherichia coli at pH 6–7 and the slightly positive TiO₂–FeO_x–PE surface. The interaction between reactants at distances below 4–8 Å is accompanied by a strong polarization at these short distances [52,53]. Fe₂O₃ presents a cb at +0.1 eV and a vb at +2.2 eV [1,2]. The valence band holes (vbh⁺) interact with the adsorbed –OH surface groups but do not have a potential high enough to lead to the formation of ^•OH radicals, since the transformation ^•OH–OH⁻ requires 1.90 eV. The HO₂^• radicals oxidize bacteria undergoing concomitantly HO₂^•–HO₂⁻ reduction at 0.75 eV. This is a significant lower potential compared to that required by the transformation ^•OH–OH⁻. The HO₂^• decomposes at pH > 4.8, driving the pH to acidic values, as shown below in Equation (16):

$${\mathrm{HO}}_2{}^{•}+{\mathrm{Fe}}^{3+}\to {\mathrm{Fe}}^{2+}+{\mathrm{O}}_2+{\mathrm{H}}^{+}$$

(16)

Figure 7a presents the XPS for the sequentially sputtered films. Figure 7a shows the atomic percentage composition as a function of the etching depth for Fe, Ti, and O. The etching of the film surface was carried out by sputtering Ar ions of 5 kV. These Ar ions are able to reach a depth of ~50 nm (~250 layers). The TiO₂ under-layers in Figure 7 were only detected up to 30 nm and reached an atomic concentration of 60% after sputtering Ar ions for 45 nm. The O-enrichment level was stable at 25%–30% up to 65 nm. Figure 7b shows that the surface atomic percentage of the Ti and Fe layers was similar within 50 nm (250 atomic layers). The amount of O in the surface was close to those of TiO₂ and FeO_x at ~30%. Figure 7a,b show the drastic differences in the microstructure of the TiO₂–FeO_x films for the sequential and co-sputtered samples.

6. The Role of the Microstructure Controlling the Bacterial Inactivation Mechanism: Critical Issues

The schematic intervention of co-sputtered FeO_x–TiO₂ leading to bacterial inactivation is suggested in Figure 8a. The mixed TiO₂–FeO_x–PE film led to charge separation and involved quasi-Fermi equilibration between the two oxides. The FeO_x electron transfer to low-lying TiO₂ trapping states is shown in Figure 8a. The co-sputtered TiO₂–FeO_x–PE films induced a faster bacterial inactivation compared to the sequentially sputtered TiO₂/FeO_x–PE. A decrease in the film FeO_x electron-hole recombination rate occurred in the co-sputtered TiO₂-FeO_x films. Some steps involving the transfer of Fe₂O₃ cb (e⁻) into the TiO₂ trapping sites are noted below:

$$\mathrm{Fe}{\mathrm{O}}_{\mathrm{x}}+\mathrm{visible}\mathrm{light}\to {\mathrm{FeO}}_{\mathrm{x}}\left({\mathrm{e}}^{-}\right)+{\mathrm{FeO}}_{\mathrm{x}}\left({\mathrm{h}}^{+}\right)$$

(17)

$${\mathrm{FeO}}_{\mathrm{x}}\left({\mathrm{e}}^{-}\right)+{\mathrm{TiO}}_2\to {\mathrm{FeO}}_{\mathrm{x}}+{\mathrm{TiO}}_2\left({\mathrm{e}}^{-}{}_{\text{trapping-sites}}\right)$$

(18)

$${\mathrm{TiO}}_2\left({\mathrm{h}}^{+}\right)+\mathrm{bacteria}\to \mathrm{inorganic}/\mathrm{organic}\mathrm{residues}$$

(19)

In the sequentially sputtered TiO₂/FeO_x–PE film as shown in Figure 8b, the FeO_x topmost layers absorbed the visible light reaching the sample surface [36,39,54]. The incorporation of Fe oxides into the crystal lattice of wide band-gap semiconductors (such as TiO₂) improved the photocatalytic activity of TiO₂ in the visible-light region because of the surface plasmon resonance (SPR) effect of FeO_x [55,56]. The surface plasmon resonance arises from the collective oscillations of electrons on the surfaces of metals and oxides, and these surface transfer electrons into the conduction band of TiO₂ because of the Schottky barrier. The Schottky barrier between FeO_x and TiO₂ arises as a consequence of the Fermi level equilibration between TiO₂ and FeO_x. This favors the separation of the photo-generated charges, as noted in Figure 9 [57,58].

Figure 10a shows that the TiO₂ nanocomposites irradiated with solar energy photons, having energy higher than the TiO₂ band gap, photo-excited electrons from the valance band to the conduction band, leaving holes in the valance band. Figure 10b shows the IFCT at the TiO₂-FeO_x heterojunction under visible light. The close contact between FeO_x NPs and TiO₂ in the sputtered films acts as an electron sink to promote the reduction of oxygen on their surfaces. Subsequently, the holes in the valence band of TiO₂ migrated, inducing bacterial oxidation. A local electric field developed by the SPR of FeO_x in contact with TiO₂. The increased charge separation, due to the FeO_x NPs sputtered on TiO₂, increased the lifetime of the TiO₂ charge-carriers. This was due to FeO_x partly substituting the lattice Ti⁴⁺ sites in TiO₂, which modifies the visible light absorbance of TiO₂. Takeuchi [59] has recently reported metal implantation on TiO₂ films, increasing the film photocatalytic activity. Figure 8b consisting only of FeO_x NPs (in the TiO₂/FeO_x–PE film) is a film made up by a single component. In this case, a faster charge recombination of the photo-induced charges occurs, limiting the amount of charges available for the photocatalytic reactions leading to bacterial inactivation [60].

7. Conclusions

This work describes the modification strategies of TiO₂ to prepare more performing binary oxides employable in photocatalysis. Basic concepts related to the surface modification of TiO₂ by FeO_x are discussed. Further, this review suggests basic mechanisms for photo-chemical processes as a function of the film microstructure. The heterojunction between FeO_x and TiO₂ promotes a directional electron flow in the co-sputtered films, leading to a faster bacterial inactivation. FeO_x and TiO₂ deposition in the films follow a random distribution. The photochemical intervention in bacterial inactivation processes were a function of the sputtering time and applied sputtering energy. The redox reactions taking place during bacterial inactivation were monitored by XPS within the disinfection time. The co-sputtered FeO_x–TiO₂–PE films were shown to lead to a faster bacterial inactivation kinetic. These films show the potential to prevent biofilm formation under sun or visible light. This mini-review may be useful to orient the work on low-cost, stable TiO₂-films for pollutants degradation and bacterial inactivation with enhanced absorption in the visible region.