Effects of dopants on multiple functionalities of TiO 2 −coated TC4 alloys

Abstract

In this study, a TiO₂, Ag−doped or Cu−doped TiO₂ film was deposited on a TC4 alloy, and the effects of the dopants (i.e., Ag and Cu) on the photoelectrochemical response, biofilm suppression as well as corrosion resistance of the TiO₂−coated alloy were systematically investigated. It can be found that the Ag doping in TiO₂ can bring about the increased concentration of oxygen defects, thereby elevating the charge carrier density and enhancing the photoelectric and corrosion current responses, which can promote the photoelectrochemical response but concurrently deteriorate the corrosion resistance of the TC4 alloy substrate. Notably, a drastic decrease in the crystallite size of TiO₂ can be induced by Cu doping and an almost amorphous state is achieved, which is accompanied by the reduction in the charge carrier mobility as well as photoelectric densities. Nevertheless, both the Ag− and Cu−doped TiO₂ films exhibit pronounced corrosion resistance and biofilm suppression activity. Accordingly, the photocurrent response may be influenced by differences in charge-carrier density and mobility, corrosion resistance as well as crystallite size of the TiO₂ film.

Keywords

Ag/Cu dopant biofilm suppression photoelectrochemical response corrosion resistance

Introduction

TC4 titanium alloys exhibit significant application potential as base materials of solar interfacial evaporators owing to their high specific strength and outstanding corrosion resistance.^1–3 However, because the base materials of solar interfacial evaporators are generally prone to water−induced corrosion and contamination by organic matter and microorganisms, it is essential to enhance their resistance to contamination. Among these issues, biofilm accumulation on evaporator surfaces can reduce light absorption efficiency, obstruct pore channels, and ultimately decrease evaporation performance. In addition, biofilms can promote localized corrosion and structural degradation of the substrate. Therefore, resistance to surface biofilm formation is critical for maintaining long-term operational efficiency and durability.

Due to the merits of superior photoelectrochemical response, biofilm suppression performance as well as corrosion resistance, TiO₂ films have been extensively adopted in multiple applications such as development of self−cleaning glasses,^4,5 biofilm suppression performance,^6,7 and photovoltaics.^8,9 Their photocatalytic and biofilm suppression properties stem from the semiconducting property of TiO₂. TiO₂ generates superoxide radicals upon photoirradiation, which degrade and decompose organic substances into small inorganic molecules such as carbon dioxide and water.¹⁰ The biofilm suppression performance of TiO₂ is ascribed to the direct attack of these reactive species for destroying the bacterial cell walls/membranes and biological macromolecules, resulting in the death of bacteria and the loss of their proliferation.¹¹

The biofilm suppression activity of TiO₂ films can be enhanced by doping with metals, such as Ag or Cu, and the primary mechanism involves the release of metal ions that disrupt microbial cellular structures and metabolic functions.^12,13 Ag−, Cu−doped TiO₂ films are generally characterized by distinct optical transmittances and bandgap energies, which can directly affect the generation of photoelectrons. Moreover, the dopants within the film cause the occurrence of lattice distortions and thus the crystallinity of TiO₂ is reduced, leading to the diminished charge−carrier mobility, which may promote the increase in carrier concentration and decrease in mobility.¹⁴ The corrosion resistance of a semiconductive film is directly affected by its conductivity, i.e., its charge−carrier concentration and mobility.

In this study, pristine, Ag−, Cu−doped TiO₂ films were prepared through magnetron sputtering to investigate the effects of the Ag and Cu dopants on their multiple functions, including photoelectrochemical behavior related to photoelectrochemical response and biofilm suppression activities and corrosion resistance.

Materials and methods

TC4 plates were used as the substrate, and 99.95% pure titanium and 1 at.% Ag−doped or Cu−doped titanium plates were used as the targets for reactive magnetron sputtering. The TiO₂ films were prepared with background vacuum and deposition pressure of 3 × 10⁻³ and 0.14 Pa, respectively. A pulsed−target power supply and sputtering current of 1 A, along with a substrate bias voltage and oxygen flow rate of −60 V and 3 sccm, respectively, were employed. A deposition time of 3 h resulted in a film thickness of 250 nm.

The film thickness was measured using a profilometer (Alpha−Step D−100, KLA, USA), with a stylus force of 0.03 mg and scanning step of 2 mm. The surface morphologies of the TiO₂ films were characterised through atomic force microscopy (AFM, CSPM 5500, Guangzhou, China) in contact mode at a scanning frequency of 1.8 Hz; the needle insertion voltage was between −15 and −20 V. The roughness and three−dimensional morphology of the films were analysed with the Imager analysis software. The crystal structures of the TiO₂ films were analysed through X−ray diffraction (XRD, D8 ADVANCE, Bruker, Germany); the scanning mode was continuous PHD fast, the scanning step size was 0.117 s, and the phases of the films were characterised using the High Score Plus software. Further, the valence states of the surface elements were determined through X−ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, USA), conducted at a test passing energy of 20 eV and step size of 0.1 eV. The electrical parameters of the TiO₂ films were evaluated using a Hall−effect tester (Swin Hall 8000, Taiwan, China), applying an input current of 10⁻⁶ A, a voltage of 10 V, and a magnetic field strength of 4000 Gs. The carrier concentration and electron mobility of the film were output based on the ‘four−point probe’ method. Photocurrent studies were conducted in a 0.5 mol/L Na₂SO₄ solution with an electrochemical workstation (Vertex C) under simulated sunlight (100 mW/cm²) obtained using a CEL−HXUV xenon lamp (Zhongjiao Jinyuan Company). The test sample was subjected to light switching every 10 s to evaluate its photocurrent density. To avoid randomness, multiple samples of the same group were soaked in the test solution in biological contamination experiments. The samples were inoculated in the prepared sulfate−reducing bacteria (SRB) suspension for 7 days; thereafter, the biofilms were fixed with a 5% glutaraldehyde solution and finally subjected to gradient dehydration with ethanol solutions, followed by dry storage for SRB observation by scanning electron microscopy (SEM, Apreo 2, USA), which was performed at a working voltage of 1–3 kV to achieve good imaging contrast. For quantitative analysis, three randomly selected regions from each sample were observed by SEM to ensure the statistically representative data. The biofilm areas were calculated using ImageJ software, and the coverage percentage was defined as (biofilm area / field area) × 100%. The reported coverage percentage was the average data of three independent measurements with standard deviation less than 5% for each region. The doping contents of Ag and Cu were only 1 at.%, which cannot be characterized by CFU counts or inhibition rates due to the limited ion overflow rate. The culture medium consisted of the following components: K₂HPO₄ (0.5 g/L), NH₄Cl (1.0 g/L), Na₂SO₄ (0.5 g/L), C₃H₅O₃Na (3.0 ml/L), yeast extract (1.0 g/L), CaCl₂ (0.1 g/L), MgSO₄ (2.0 g/L), C₆H₈O₆ (0.1 g/L),and Fe(NH₄)₂·(SO₄)₂·6H₂O (0.2 g/L). The corrosion resistance was measured using a three−electrode system in 3.5 w.t.% NaCl solution in order to simulate the real marine environment, using the test sample, platinum foil, and saturated calomel electrode as the working, counter, and reference electrodes. Before measurement, the test samples were soaked in the solution for until the open−circuit potential stabilized, the scan rate of the polarization test was 1.6 mV/s and the duration of the open circuit potential measurement was 30 min, the polarisation curve of the sample was recorded on the basis of the open−circuit potential. The test range was between −200 and +400 mV of the open−circuit potential. The resulting experimental data were fitted with the Tafel extrapolation.

Results and discussion

Morphology and structure

Figures 1(a)−(c) and Table 1 show the AFM of the films. The TiO₂ film exhibits a dense, fine morphology with a roughness of 2.93 nm. In contrast, the maximum roughness of 5.88 nm was observed in the TiO₂(Ag) film, which is attributed to the formation of large clusters of grains induced by Ag doping. Moreover, the TiO₂(Cu) film exhibits a larger roughness of 3.93 nm, because Cu doping deteriorates the uniformity of the grains.

Figure 1.

AFM morphologies of the (a) TiO₂, (b) TiO₂(Ag), and (c) TiO₂(Cu) films, and (d) XRD patterns (PDF #: anatase, 01−073−1764; rutile, 00−034−0180).

Table 1.

Performance and parameters the pristine and Ag−/Cu−doped TiO₂ films.

Film	Ra nm	Crystal size nm	Photo current density μA·cm⁻²	Corrosion potentials mV	Corrosion current density ×10⁻⁹ A·cm⁻²	Defect oxygen%	Carrier concentration×10¹⁷ cm⁻³	Carrier mobility cm²·V⁻¹·s⁻¹	Coverage area of SRB%
TC4				−268.9 ± 4.2	9.48 ± 0.14				100
TiO₂	2.93	11.9	11.2	−176.7 ± 3.0	2.09 ± 0.18	8.1	0.56	5.6	100
TiO₂(Ag)	5.88	10.1	15.5	−191.5 ± 3.8	2.31 ± 0.15	11.5	1.68	4.9	18.84
TiO₂(Cu)	3.93	6.75	6.8	−191.4 ± 4.1	1.03 ± 0.07	10.9	1.57	1.4	16.11

Figure 1(d) presents the XRD patterns of the films, which reveal that all samples are composed of mixed anatase and rutile phases. The crystallite size calculated from the rutile (110) peak is approximately 11.9 nm for the TiO₂ film, according to the equation of D = kλ/βcosθ.¹⁵ For the TiO₂(Ag) film, the intensity of the rutile (110) peak is enhanced, this is because Ag may be incorporated into the TiO₂ film either through partial lattice incorporation or as segregated metallic/oxide species, resulting in the formation of unsaturated covalent bonds. Additionally, the crystallite size is slightly reduced to 10.1 nm after Ag doping, indicating the lattice distortion caused by the difference in the ionic radii between Ag⁺ (115 pm) and Ti⁴⁺ (74.5 pm),¹⁶ consistent with the AFM in Figure 1(b). The diffraction peaks of the TiO₂(Cu) film are weak, and the crystal size of TiO₂ is reduced to 6.75 nm, indicating that the crystallisation is suppressed by the Cu dopant. Unlike in the case of Ag⁺ doping, the ionic radius of Cu²⁺ (73 pm) is comparable to that of Ti⁴⁺, leading to relatively minimal lattice distortion and a smaller change in the valence bonding.¹⁷

Figures 2(a)−(d) show the XPS survey spectra and Ag3d, Cu2p, Ti2p, O1 s XPS spectra of the TiO₂ films. The survey spectra in Figure 2(a) and the Ag3d and Cu2p spectra of the doped samples in Figure 2(b) confirm the presence of the Cu and Ag dopants in the corresponding doped TiO₂ films. The Ag3d spectrum of the Ag−doped film is characterized by Ag3d_3/2 and Ag3d_5/2 peaks.¹⁸ Similarly, the Cu2p spectrum of the Cu−doped film exhibits Cu2p_1/2 and Cu2p_3/2 peaks.¹⁹ The intensity difference of the Ag and Cu peaks is mainly derived from the different ionisation cross−sections resulting from the different radii of the Ag3d and Cu2p orbitals.²⁰ The Ti2p spectra of the films show pronounced Ti2p_3/2 and Ti2p_1/2 peaks, accompanied by satellite peaks.²¹ The shock−induced satellite peaks of Ti2p are a typical feature of the XPS spectra of TiO₂ films.²² The O1 s spectra of the films could be deconvoluted into peaks representing lattice (O_α), defect (O_β), and adsorbed oxygen (O_γ) atoms.²³ Notably, O_β is extensively recognized as a critical contributor to photocatalytic activity because of its role in promoting the charge−carrier separation or delay the electron−hole recombination.²⁴ The proportion of O_β within the TiO₂ film was 8.1%, which is increased to 11.5 and 10.9% in the TiO₂(Ag) and TiO₂(Cu) films, respectively. This increase is attributed to the substitution of lattice Ti⁴⁺ with the Ag⁺ and Cu²⁺ species, resulting in the generation of charge imbalance and oxygen−related defects.

Figure 2.

X−ray photoelectron spectroscopy of the TiO₂ films. (a) survey spectra, high−resolution spectra of (b) Ag and Cu, (c) Ti, and (d) O.

Functionality

Due to the limited specific surface area of the films, a quantitative comparison of their degradation efficiencies could not be reliably conducted. Nevertheless, earlier studies have demonstrated photocurrent reflect photoinduced charge separation behavior and serve as an indirect indicator of potential photocatalytic response.^25,26 Therefore, the photocurrent density of the films was evaluated (Figure 3(a)). As shown in Table 1, the TiO₂(Ag) film exhibits the highest photocurrent density (15.5 μA·cm⁻²), followed by the TiO₂ film (11.2 μA·cm⁻²) and the TiO₂(Cu) film (6.8 μA·cm⁻²). This result indicates that the Ag dopant may promote the photoelectrochemical response of TiO₂, but the Cu dopant was associated with a lower photocurrent response. Figure 3(b) shows the polarisation curves of the films. The corrosion potentials of the TC4 substrate and TiO₂, TiO₂(Ag), and TiO₂(Cu) films are −268.9, −176.7, −191.5, and −191.4 mV, respectively. Additionally, the corrosion current densities are 9.48 × 10⁻⁷, 2.09 × 10^–7, 2.31 × 10^–7, and 1.03 × 10^–7 A·cm⁻², respectively (Table 1). The results reveal that the deposition of TiO₂ films on the TC4 alloy can enhance the corrosion resistance of the substrate.

Figure 3.

(a) Photocurrent densities, (b) polarisation curves of the TiO₂ films.

As mentioned in the introduction, the application of this study is the solar interfacial evaporator. The critical biofilm suppression requirement is to suppression of biofilm formation on the sample surface, in order to prevent reducing the light absorption efficiency and the intensifying corrosion, rather than inhibiting bacteria in the surrounding environment. Hence, the biofilm suppression performance of the films was evaluated, as shown in the Figures 4(a)−(d). The entire surface of the TC4 alloy is extensively colonized by the biofilms, demonstrating its inferior biofilm suppression properties²⁷ (Figure 4(a)). The TiO₂ film exhibits significant biofouling, and flocculent biological remains can be observed on its surface (Figure 4(b)). This is because the TiO₂ film cannot produce reactive oxygen species on the surface in the absence of light, thereby rendering it ineffective against biological contamination. Conversely, the TiO₂(Ag) and TiO₂(Cu) films exhibit significantly reduced amounts of SRB residues, with the coverage areas of SRB were 18.84 and 16.11%, respectively (Figures 4(c)−(d) and Table 1). The improved performance can be attributed to the presence of Ag⁺/Cu²⁺ species, which can disrupt the functional proteins of the SRB cell walls, damage their genetic material integrity and ultimately cause metabolic dysfunction and bacterial death,²⁸ as shown below:

H^{+} + Ag / Cu / {Cu}^{+} \to {Ag}^{+} / {Cu}^{2 +} + H

(1)

\begin{aligned} {Ag}^{+} / {Cu}^{+} / {Cu}^{2 +} + PR - COOH; \\ - SH; - {NH}_{2} \to PR - COOAg / Cu; \\ - SHAg / Cu; - NHAg / Cu + H^{+} \end{aligned}

(2)

Figure 4.

Biofilm−resistant morphologies of the (a) TC4 alloy, and (b) TiO₂, (c) TiO₂(Ag), and (d) TiO₂(Cu) films.

The corrosion current and photocurrent densities of the films are related to their crystallinity and O_β proportion, as well as the concentration and mobility of the free charge carriers. As shown in Figure 5, a significantly increased O_β proportion can be detected in the TiO₂(Ag) and TiO₂(Cu) films (11.5 and 10.9%, respectively), which is attributed to the different bonding interactions of Ag⁺, Cu²⁺, and Ti⁴⁺ with oxygen, leading to an increase in the concentration of free charge carriers within the doped films. Specifically, the free charge−carrier concentration is enhanced from the order of 10¹⁶ in the TiO₂ film to 10¹⁷ in the doped films, as shown in Figure 5(a) and Table 1. The O_β proportion and free carrier concentration can be measured as 8.1% and 0.56 × 10¹⁷ cm⁻³ in the TiO₂ film, 10.9% and 1.57 × 10¹⁷ cm⁻³ in the Cu−doped film, and 11.5% and 1.68 × 10¹⁷ cm⁻³ in the Ag−doped film, respectively. Cu²⁺ induces slightly greater changes than Ag⁺ in covalently bonding with O^2–, and the O_β proportion and free carrier concentration of the TiO₂(Cu) film are slightly lower than those of the Ag−doped film. Further, the TiO₂(Ag) and TiO₂(Cu) films exhibit smaller crystallite sizes (6.75–10.1 nm) than the TiO₂ film (11.9 nm). Hence, the mobility of the free charge carriers is decreased from 5.6 cm²·V⁻¹·s⁻¹ in the TiO₂ film to 4.9 and 1.4 cm²·V⁻¹·s⁻¹ in the TiO₂(Ag) and TiO₂(Cu) films, respectively. In particular, the TiO₂(Cu) film with an almost amorphous structure (Figure 1(d)) exhibits a low carrier mobility²⁹ (Figure 5(b) and Table 1). In contrast, the TiO₂(Ag) film is characterized by a substantially higher carrier concentration and relatively high mobility, and the photocurrent density of 15.5 μA·cm⁻².³⁰ The relatively large crystallite size and excellent semiconductive properties of TiO₂(Ag) can provide interfacial boundaries that facilitate the formation of galvanic cells and the participation of charge carriers during the electrochemical corrosion process. Consequently, the TiO₂(Ag) film exhibits a higher corrosion current density (2.31 × 10⁻⁷ A/cm²) than the TiO₂ film (Figure 5(c) and Table 1), indicating that the enhanced electrochemical activity is closely associated with its higher carrier density and mobility. In contrast, the extremely low carrier mobility within the TiO₂(Cu) film restricts the charge motion, and thereby the generation of photoelectrons is suppressed, resulting in a relatively low photocurrent density (6.8 μA·cm⁻²). Besides, the almost amorphous structure of the TiO₂(Cu) film confines the corrosion process at the interface, leading to a lower corrosion current density (1.03 × 10⁻⁷ A/cm²) than the TiO₂ film.

Figure 5.

(a) Carrier concentration and proportion of defect oxygen, (b) carrier mobility and crystal size, (c) corrosion current and photocurrent densities.

The TiO₂(Ag) film exhibits a better photoelectrochemical response than the TiO₂ film. This behaviour is ascribed to the combined effect of high concentration of free charge carriers, a high mobility and relatively large crystallite sizes. In contrast, the TiO₂(Cu) film displays lower photoelectrochemical response than the TiO₂ film, which is derived from the low mobility of the free charge carriers and its almost amorphous structure. The deposition of TiO₂ films on the TC4 alloy can enhance the corrosion resistance of the substrate. It should be noted that the improved biofilm suppression may be related to the presence and possible release of Ag- or Cu-containing species.

Conclusions

In this study, the TC4 alloy was imparted with photoelectrochemical response, corrosion resistance, and biofilm suppression functions through the deposition of TiO₂, TiO₂(Ag), and TiO₂(Cu) films. It can be found that compared with the TiO₂ and TiO₂(Ag) films, the Ag−doped TiO₂ film exhibits superior photoelectrochemical response. The intrinsic and doped TiO₂ films on the TC4 alloy significantly enhanced the corrosion resistance of the substrate. Particularly, both the Ag and Cu dopants can remarkably improve the biofilm suppression activity of TiO₂ films. This outstanding comprehensive performance can play a significant role in the application of the TiO₂−coated TC4 alloy as a solar interfacial evaporator for water evaporation, protecting against corrosion, bacterial growth, and organic matter adsorption.

Footnotes

Acknowledgements

This work is supported by the National Natural Science Foundation of China in Nos. of 52271056, the education department of Liaoning in Nos. of LJ212410146060, LJ232510146002, the research fund of national key laboratory of marine corrosion and protection of luoyang ship material research institute under the contract No. KJS2403.

ORCID iD

Yanwen Zhou

Author contribution(s)

Zhiwei Su: Methodology; Writing – original draft; Writing – review & editing.

Caibo Yan: Data curation; Investigation; Software.

Tingdong Ren: Data curation; Investigation; Software.

Yanwen Zhou: Conceptualization; Writing – review & editing.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China, education department of Liaoning, (grant number 5227105, LJ212410146060, LJ232510146002).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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