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Article

Low-Temperature Hot-Water Treatment as a Green Strategy to Enhance the Self-Cleaning and Antibacterial Performance of Sputtered TiO2 Thin Films

by
Manel Boukazzoula
1,
Djamila Maghnia
2,
Frank Neumann
3 and
Oualid Baghriche
1,*
1
Laboratory of Innovative Techniques of Environment Preservation, Department of Chemistry, University Mentouri-Constantine 1, Constantine 25000, Algeria
2
Department of Biology, University Oran 1 Ahmed Ben Bella, Oran 31000, Algeria
3
Fraunhofer Institute for Surface Engineering and Thin Films IST, Bienroder Weg 54E, 38108 Braunschweig, Germany
*
Author to whom correspondence should be addressed.
Photochem 2026, 6(1), 4; https://doi.org/10.3390/photochem6010004
Submission received: 5 November 2025 / Revised: 15 December 2025 / Accepted: 22 December 2025 / Published: 6 January 2026
(This article belongs to the Topic Supported Photocatalysts: Nano-Architecture vs. Reactivity)
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Abstract

Titanium dioxide (TiO2) thin films were deposited by DC magnetron sputtering and subsequently treated in hot water at 50, 70, and 95 °C for 72 h to investigate the influence of low temperature on their structural optical and functional properties. XRD analysis revealed a progressive transformation from amorphous to anatase phase with increasing treatment temperature, accompanied by an increase in crystallite size from 5.2 to 15.1 nm. FT-IR spectroscopy confirmed enhanced surface hydroxylation and contact angle measurements showed a decrease from 77.4° to 19.7°, indicating a significant improvement in superior wettability. The transmittance spectroscopy revealed a slight narrowing of the optical band gap from 3.34 to 3.21 eV, consistent with improved visible-light absorption. Photocatalytic tests using the Resazurin indicator demonstrated that the film treated at 95 °C exhibited the highest activity, achieving a bleaching time of 245 s three times faster than treated at 50 °C and twice as fast as treated at 70 °C. Under low-intensity solar irradiation, the same sample achieved complete E. coli inactivation within 90 min. These improvements are attributed to increased crystallinity, surface hydroxyl density, and enhanced ROS generation. Overall, this study demonstrates that mild hot-water treatment is an effective, substrate-friendly route to enhance TiO2 film wettability and multifunctional performance, enabling the fabrication of self-cleaning and antibacterial coatings on fragile materials such as plastics and textiles.

1. Introduction

TiO2 (titanium dioxide) has received extensive interest over the last several years for environmental and antibacterial applications because of its strong oxidative ability, stability, non-toxicity, and low cost [1,2]. Its ability to generate reactive oxygen species (ROS) when illuminated makes TiO2 an active photocatalyst for removing organic pollutants and bacterial inactivation [3,4]. Due to its photocatalytic reactivity, TiO2 is widely used for self-cleaning, antifogging, and photocatalytic coatings for water treatment and antimicrobial surfaces [5,6]. Many physical and chemical depositions methods have been used to prepare TiO2 films containing dip-coating, spin-coating, sol–gel, spray pyrolysis, and deposition by electrophoretic deposition [7,8,9,10]. However, these conventional solution-based methods often suffer from non-uniform surface morphology, poor adhesion, and non-reproducibility, and they necessitate heat treatment above 350 °C to reach crystalline anatase phase TiO2 [11,12]. These high-temperature treatments limit their application on fragile or flexible substrates such as polymers, plastics, and textiles, which are damaged by thermal heat. In contrast, compared to colloidal coating, magnetron sputtering provides a useful and low-temperature physical vapor deposition route that allows to control film thickness, composition, and uniformity while promoting strong adhesion to the substrate [13,14]. However, sputtered TiO2 films are often amorphous as-deposited and therefore necessitate a subsequent crystallization treatment to improve their photocatalytic performance [15,16,17]. Conventional annealing treatments such as calcination may not be appropriate for temperature-sensitive substrates, motivating the examination of low-temperature crystallization strategies. An attractive alternative are hot-water-assisted or mild hydrothermal treatments, which have appeared as promising ecofriendly alternatives to high-temperature calcination as a means to promote the amorphous-to-anatase transition of TiO2 nanotubes and nanopowders [18,19,20]. This aqueous treatment enables the development of hydroxylated anatase phases via dissolution–precipitation processes, leading to a significant increase in the density of surface –OH groups and enables improved wettability and photocatalytic performance [21]. For instance, the hydrothermal treatment (≤100 °C) has been shown to improve crystallinity anatase density and in turn photocatalytic activity performance under visible-light irradiation. Although sputtered TiO2 films have been extensively studied, reports addressing the role of post-deposition hot-water treatment in optimizing their performance for environmental applications remains insufficiently explored.
In the current study, TiO2 thin films were deposited using DC magnetron sputtering, and we then used hot-water treatments at different temperatures. For post-treatment, we investigated their microstructure, optical absorption, crystallographic properties, and photocatalytic activity. This study contributes to the knowledge of modifying sputtered TiO2 at low temperatures, while providing a sustainable pathway for producing self-cleaning and antibacterial coatings that are applicable to fragile substrates such as plastics and textiles. We consider that the current study represents an additional step toward the development of optimized smart materials based on titanium oxide use on a large scale.

2. Experimental

2.1. Preparation Samples and Characterization Methods

The quartz slide (GVB GmbH, Nordstern-Park 2, Herzogenrath, Germany) substrates were previously cleaned by sonication in acetone, ethanol, and deionized water (10 min for each step), and then dried in a N2 stream. The TiO2 films were sputtered on quartz slides from a Ti-target (99.999% purity, Lesker Ltd., Hastings, East Sussex, UK) with 2 inches in diameter at 308 mA and applying a bias voltage of 586 V (108 W) in an atmosphere of O2 of 0.45 Pa. The target-substrate distance was fixed at 100 mm. The TiO2 films sputtered on Si-wafers were analyzed by profilometry to determine the TiO2 thickness as a function of sputtering time (Alphastep500, TENCOR, Milpitas, CA, USA); deposition times up to 5 min for TiO2 lead to the film thicknesses of 40 nm. Further details on the deposition conditions, as well as structural and morphological analyses, can be found in our previous work [22].
During film deposition, the substrate temperature was maintained below 40 °C using a substrate holder (EpiCentre, San Jose, CA, USA) equipped with a resistance heater and a temperature controller system, making the process suitable for temperature-sensitive substrates such as plastic. After deposition, the samples were immersed into deionized water at neutral pH. They were then either left untreated or subjected to hot-water treatment at 50, 70, and 95 °C for 72 h, and designated as S0, S1, S2, and S3, respectively. This treatment is notable to promote hydroxylation, surface rearrangement, and the growth of nanostructured anatase phases [19,23].
The crystallographic phase was carried out by means of an X’Pert diffractometer (Cu Kα radiation, λ = 1.5409 Å, Philips, Delft, The Netherlands). The transmittance spectra were recorded using a Perkin Elmer Lambda 950 UV/VIS spectrometer (PerkinElmer, Waltham, MA, USA) to evaluate the optical properties and estimate the band gap. The Fourier transform infrared spectroscopy (FTIR) spectra were recorded on an INVENIO-R 329 spectrometer (Bruker Optics, Ettlingen, Germany). The contact angle was measured by means of a Data Physics OCA 35 instrument (DataPhysics Instruments GmbH, Filderstadt, Germany) following the Sessile’s method for the analysis of water droplets by this technique.

2.2. Photocatalytic and Antibacterial Properties

Photocatalytic activity of the samples (S0, S1, S2, and S3) was inspected using the Resazurin (Rz) (Sigma–Aldrich, Baden-Württemberg, Germany) indicator ink method, as adapted from Mills et al. [24,25]. The samples were cleaned with ethanol and coated with ink by hand using a wire-wound rod (RK Print, K-bar #3). Photocatalytic measurements were then made by UV-A illumination (λmax = 352 nm, ~2 mW·cm−2) using a Blak-Ray® XX-15 lamp purchased from Cole-Parmer (UVP, Vernon Hills, IL, USA). As the Rz layer was irradiated, it gradually changed color from blue to pink, which showed photocatalytic reduction in Resazurin to Resorufin on the TiO2 surface. During this irradiation process, digital pictures were taken with an Ion Copycat handheld document scanner (ION, Cumberland, RI, USA). The resulting RGB (red, green, blue) values from the images were used to quantify the surface color change and assess the photocatalytic performance.
The antibacterial properties were carried as reported previously [24]. Escherichia coli K12 ATCC23716 bacteria were obtained from the Deutsche Sammlung von Mikroorganismen and Zellkulturen GmbH (DSMZ) ATCC23716, Braunschweig, Germany. The 2 mL culture aliquots with an initial concentration of ~5.106 colony-forming unit per milliliter (CFU mL−1) in NaCl/KCl (pH 7) were placed on the samples. The samples were places on Petri dish provided with a lid to prevent evaporation. The samples were irradiated in the cavity of suntest solar simulator (Atlas, Heraeus, Hanau, Germany) cavity tuned at 50 mW/cm2, which was additionally equipped with a cut-off filter to block wavelengths below 310 nm. No germicidal wavelength were employed. Sampling was carried out by taking the reaction mixture prior to illumination and at 30 min intervals up to 6 h. For each sampling point, E. coli survival was determined as log(CFU/CFU0), where CFU0 is the initial colony at time 0 and CFU the count at time t. Replica samples were incubated at 37 °C for 24 h at the end of each bacterial inactivation cycle. No bacterial re-growth was noticed. All experiments were carried out at 22 ± 2 °C.

3. Results and Discussion

3.1. X-Ray Diffraction Analysis

Figure 1 exhibits the X-ray diffraction pattern of untreated S0 and treated samples, S1, S2, and S3, at different temperatures. The XRD pattern of the untreated sample S0 exhibits low intensity and a broad shoulder at around 24–27°, indicating partial crystallization or low crystallinity. However, the crystallinity of the sputtered TiO2 films significantly improved with increased heating up to 95 °C for 72 h (sample S3), as evidenced by the sharper and more intense diffraction peaks at 2θ = 25.4°, 37.9°, 48.1°, 54.1°, and 62.7°, corresponding to the (101), (004), (200), (105), and (204) planes, respectively. These peaks are characteristic of the anatase TiO2 phase (inset Figure 1). Exposure of the samples to hot water at 50 °C, 70 °C, and 95 °C led to the formation and growth of anatase crystallites with average crystallite sizes of 5.23 nm, 8.05 nm, and 15.1 nm, respectively. As calculated using the Scherrer equation, D = 0.89λ/(β cos θ), where λ is the X-ray wavelength, β is the full-width half-maximum (FWHM) of the (101) peak, and θ is the Bragg diffraction angle. The XRD intensity ration [ I 004 ] [ I ( 101 ) ] were also calculated and are reported in Table 1. Both D values and the rations increased by increasing thermal exposure, confirming the significance of hot-water treatment on the growth of anatase TiO2 crystallites. The anatase phase is mostly desirable due to its superior photoactivity in photocatalytic applications [26,27].

3.2. UV–Vis Spectroscopy Analysis and Band Gap Calculation

The transmittance spectra of the samples are shown in Figure 2. The UV–Vis transmission spectra of treated samples for different temperature exhibit a progressive rise in visible transmittance near 550 nm and a slight redshift of the absorption edge with increasing temperature. The untreated sample S0 exhibits lower transparency ≈ 49% and a wider optical band gap of 3.34 eV, whereas the samples treated S1, S2, and S3 show higher transmittance values of 76.97, 81.03, and 83.3% as well as narrower optical band gaps of 3.28, 3.26, and 3.21 eV, respectively (inset Figure 2). These values are slightly above the bulk anatase band gap of 3.20 eV but lower than the typical band gap of amorphous titania at ~3.40 eV [28,29], confirming that hot-water assistance drives a change from amorphous to anatase phase. Consequently, hot-water assistance enhances visible transparency while simultaneously promoting photocatalytic activity under low-intensity, solar-simulated illumination, as confirmed by the subsequent photocatalytic activity on the surface and bacterial inactivation results.

3.3. Fourier Transform Infrared Spectroscopy Analysis (FT-IR)

Figure 3 presents the FT-IR spectra of sputtered TiO2 thin films at different temperatures for 72 h. The untreated sample S0 spectra exhibit the characteristic Ti-O stretching and Ti-O-Ti bending vibrations within the wavenumber range of 820–650 cm−1 [30]. A strong absorption peak at 508 cm−1 is assigned to Ti-O-Ti bond vibrations. The sharp band observed at 1697 cm−1 corresponds to the stretching vibration of hydroxyl groups, while the broad absorption in the 2600–3750 cm−1 region is attributed to O-H stretching of adsorbed water molecules [31]. Upon prolonged heating to 95 °C, as observed for sample S3, the intensity of water absorption bands increases, while the Ti-O-Ti vibration bands become less prominent. This indicates that the thermally treated, sputtered TiO2 film surface becomes increasingly hydroxylated and water-enriched with increased temperature treatment. The enhanced hydroxylation, reflected by stronger O-H bands in the FT-IR spectra, suggests a higher surface density of hydroxyl groups, which act as active sites for oxygen species generation, thereby enhancing the photocatalytic performance of the TiO2-coated samples.

3.4. Contact Angle Behavior of Thermally Treated TiO2 Films

Figure 4a presents photographs of water droplets on both untreated and thermally treated samples, while Figure 4b illustrates the variation in the contact angle as a function of temperature treatment. The surface wettability was calculated by measuring the contact angle, which provides insight into the hydrophilic or hydrophobic nature of the surface. TiO2 is well known to decrease hydrophobicity due to the formation of surface hydroxyl groups [21,32]. The values of the contact angles for samples S0, S1, S2, and S3 are 77.4°, 65.3°, 44.5°, and 19.7°, respectively. These results clearly exhibit that the water contact angle decreases progressively with increasing treatment temperature, indicating a gradual enhancement of surface hydrophilicity. Subsequently, the enhancement of surface hydroxyl groups, as consistent IR results, favors the formation of hydroxyl radicals (●OH), which are crucial in the photocatalytic degradation process. Therefore, the superior wettability of S3 directly relates to its superior photocatalytic activity compared to the other samples.

3.5. Photocatalytic Activity and Bacterial Inactivation

Figure 5a presents digital images of the ink-coated samples S0, S1, S2, and S3 recorded under UV-A irradiation (2 mW cm−2) at various exposure times ranging from 0 to 840 s, showing the progressive photodegradation behavior of the samples. A plot of variation in the RGB (red)t values extracted from the images in Figure 5a are illustrated in Figure 5b. The values of time to bleach (ttb) for samples S1, S2, and S3 derived from Figure 5b are 620, 400, and 245 s, respectively. These results clearly indicate that the sample S3 exhibited the highest photocatalytic activity among all samples. In particular, the time to bleach (ttb) of sample S3 is approximately three times shorter than that of sample S1 and twice as fast as that of sample S2. This enhanced performance is mainly related to the formation of the anatase phase with lowest band gap, which typically demonstrates superior photocatalytic properties than the amorphous phase due to its enhanced charge separation and higher surface reactivity, as noted in earlier studies [27,33].
Figure 6 presents the E. coli inactivation kinetics at the interfaces of sputtered TiO2 samples prepared under different conditions. In the absence of TiO2 under Osram Lumilux lamp irradiation, no bacterial inactivation was observed. The untreated sample S0 presents the almost-negligible disinfection (Figure 6. trace 1). For samples S1 and S2, the E. coli inactivation kinetics are enhanced by high-temperature treatment, progressively leading to bacterial inactivation (Figure 6. trace 2 and 3). In contrast, sample S3 shows the highest bacterial inactivation efficiency reaching complete disinfection within 90 min, demonstrating the positive effect of thermal modification on antibacterial performance (Figure 6. trace 4).
In fact, the samples (S1, S2, and S3) exhibit similar bacterial interaction behavior under low-intensity solar irradiation. For sample S3, two different phases were observed: an initially slow phase due to the gradual attack of the bacterial cell membrane by photogenerated reactive oxygen species (ROS), leading to membrane damage [34,35,36]. Then, a fast inactivation phase occurred within approximately 45 min, corresponding to cell wall damage and coenzyme A oxidation, which disturbs respiratory leading to cell death [37,38,39]. Similar behavior was observed for samples S1 and S2, with delayed phases of 90 and 120 min, respectively. Therefore, this successive process explains why the photocatalytic activity kinetics are faster than the bacterial inactivation kinetics, as the initial oxidative attack on the cell membrane needs a longer induction time before complete cell damage occurs.
The sample S3 showed the highest photocatalytic efficiency due to several properties. As revealed in a previous analysis, this sample presents a high-crystallinity anatase phase and wettability with lowest band gap, which could directly affect ROS production and, in turn, photocatalytic performance.
Figure 7a exhibits the time to bleach (ttb) and E. coli inactivation behavior of the sample S3 under different light intensities: 100, 50, and 25 mW/cm2. The time to bleach and bacterial inactivation were observed to have shortened under 100 mW/cm2 compared to 50 and 25 mW/cm2. Figure 7b shows the recycling of the sample S3 against photocatalyst activity and bacterial inactivation over five independent cycles under solar-simulated light (50 mW/cm2). This proves the stability of the prepared material and its sustainable use over four cycles without loss of activity.

4. Conclusions

The actual study showed the significant influence of hot-water treatment on the structural, optical, and functional properties of sputtered TiO2 thin films. Increasing the treatment temperature from 50 to 95 °C improved crystallinity, surface hydroxylation, and hydrophilicity, as confirmed by XRD, FT-IR, and contact angle analyses. The optimized film treated at 95 °C for 72 h exhibited superior photocatalytic activity and bacterial inactivation, achieving a bleaching time of 245 s and complete E. coli inactivation within 90 min. This improvement is attributed to the formation of the anatase phase, with a narrowing of the optical band gap thus enhancing reactive oxygen species (ROS) generation. The bacterial inactivation followed a two-phase mechanism involving an initial membrane oxidation phase and subsequent rapid cell destruction. Furthermore, the treated films displayed excellent stability and reusability over multiple cycles. Notably, the hot-water treatment used in this work operates at low temperature, making it particularly suitable for TiO2 coating of thermally fragile substrates such as plastics and textile without making degradation or deformation. Overall, this work highlights the efficiency of low-temperature thermal treatment in improving the multifunctional performance of sputtered TiO2, providing great potential for self-cleaning and antibacterial surface applications on fragile and flexible substrates.

Author Contributions

Conceptualization, M.B.; methodology, M.B.; formal analysis, D.M.; investigation, M.B.; resources, D.M. and F.N.; data curation, D.M.; writing—original draft, O.B.; writing—review and editing, F.N. and O.B.; supervision, F.N. and O.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Algerian Ministry of Higher Education and Scientific Research and was partly supported by the German Academic Exchange Service (DAAD) (50015559).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

During the preparation of this manuscript/study, the authors used ChatGPT (OpenAI, GPT-5) for the purposes of improving the grammar and clarity of the manuscript; All scientific content and conclusions are the authors’ own. The authors have reviewed and edited the output and take full responsibility for the content of this publication.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns of sputtered TiO2 thin films: (1) untreated sample S0 and treated by hot water for 72 h °C at (2) S1: 50 °C, (3) S2: 70 °C, and (4) S3: 95 °C.
Figure 1. XRD patterns of sputtered TiO2 thin films: (1) untreated sample S0 and treated by hot water for 72 h °C at (2) S1: 50 °C, (3) S2: 70 °C, and (4) S3: 95 °C.
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Figure 2. UV–Vis transmittance spectrum of sputtered TiO2 films: untreated and treated with hot water for 72 h with various degrees in temperature treatment. Inset: indirect band gap estimation of as-deposited films.
Figure 2. UV–Vis transmittance spectrum of sputtered TiO2 films: untreated and treated with hot water for 72 h with various degrees in temperature treatment. Inset: indirect band gap estimation of as-deposited films.
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Figure 3. FTIR spectra of sputtered TiO2 thin films: Untreated and treated with hot water for 72 h with various degrees in temperature treatment.
Figure 3. FTIR spectra of sputtered TiO2 thin films: Untreated and treated with hot water for 72 h with various degrees in temperature treatment.
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Figure 4. (a) Photographs of water droplets on (1) untreated sample S0: 77.4°, and treated samples (2) S1: 65.3°, (3) S2: 44.5°, and (4) S3: 19.7° for 72 h. (b) Contact angle as a function of treatment temperature.
Figure 4. (a) Photographs of water droplets on (1) untreated sample S0: 77.4°, and treated samples (2) S1: 65.3°, (3) S2: 44.5°, and (4) S3: 19.7° for 72 h. (b) Contact angle as a function of treatment temperature.
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Figure 5. (a) A series of images recorded at 60 s irradiation intervals for untreated and treated samples. (b) A plot of variation in the RGB (red) with irradiation time, a Blak-Ray® XX-15 lamp, (λmax = 352 nm, ~2 mW·cm−2).
Figure 5. (a) A series of images recorded at 60 s irradiation intervals for untreated and treated samples. (b) A plot of variation in the RGB (red) with irradiation time, a Blak-Ray® XX-15 lamp, (λmax = 352 nm, ~2 mW·cm−2).
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Figure 6. E. coli inactivation kinetics by various sputtered TiO2 samples irradiated under Osram Lumilux lamp for different times: (1) untreated S0, (2) S1, (3) S2, and (4) S3.
Figure 6. E. coli inactivation kinetics by various sputtered TiO2 samples irradiated under Osram Lumilux lamp for different times: (1) untreated S0, (2) S1, (3) S2, and (4) S3.
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Figure 7. (a) Time to bleach (ttb) and bacterial inactivation at the surface of the sample S3 under different light intensities. (b) Recycling of the sample S3 over five cycles.
Figure 7. (a) Time to bleach (ttb) and bacterial inactivation at the surface of the sample S3 under different light intensities. (b) Recycling of the sample S3 over five cycles.
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Table 1. Crystallite size and mesh parameters of sputtered TiO2.
Table 1. Crystallite size and mesh parameters of sputtered TiO2.
Immersion Temperature
(°C)		Lattice Parameters (Å)Crystallite Size (nm) [ I 004 ] [ I ( 101 ) ]
a = bc
03.72919.83802.430.35
503.69899.83528.190.37
703.69859.834210.20.41
953.69749.831725.60.48
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Boukazzoula, M.; Maghnia, D.; Neumann, F.; Baghriche, O. Low-Temperature Hot-Water Treatment as a Green Strategy to Enhance the Self-Cleaning and Antibacterial Performance of Sputtered TiO2 Thin Films. Photochem 2026, 6, 4. https://doi.org/10.3390/photochem6010004

AMA Style

Boukazzoula M, Maghnia D, Neumann F, Baghriche O. Low-Temperature Hot-Water Treatment as a Green Strategy to Enhance the Self-Cleaning and Antibacterial Performance of Sputtered TiO2 Thin Films. Photochem. 2026; 6(1):4. https://doi.org/10.3390/photochem6010004

Chicago/Turabian Style

Boukazzoula, Manel, Djamila Maghnia, Frank Neumann, and Oualid Baghriche. 2026. "Low-Temperature Hot-Water Treatment as a Green Strategy to Enhance the Self-Cleaning and Antibacterial Performance of Sputtered TiO2 Thin Films" Photochem 6, no. 1: 4. https://doi.org/10.3390/photochem6010004

APA Style

Boukazzoula, M., Maghnia, D., Neumann, F., & Baghriche, O. (2026). Low-Temperature Hot-Water Treatment as a Green Strategy to Enhance the Self-Cleaning and Antibacterial Performance of Sputtered TiO2 Thin Films. Photochem, 6(1), 4. https://doi.org/10.3390/photochem6010004

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