Preparation of FeS2/TiO2 nanocomposite films and study on the performance of photoelectrochemistry cathodic protection

FeS2/TiO2 nanotube array composite films with clean, high efficiency, low cost and low consumption were prepared by electrochemical anodization and hydrothermal methods. The modification of FeS2 nanoparticles on the surface of TiO2 nanotube array film not only broadens the light absorption range of TiO2, but also improves the utilization ratio of visible light and the separation rate of photogenerated electron–hole pairs, which greatly improves the photoelectrochemical cathodic protection performance of TiO2 for 304 stainless steel (304SS). Under visible light irradiation, the open circuit potential of 304SS coupled with the FeS2/TiO2 nanocomposite films decreased from − 170 to − 700 mV, and the electrode potential can still maintained at − 400 mV after the light was turned off. Compared with pure TiO2 nanotube array film, FeS2/TiO2 nanocomposite film has better photoelectrochemical cathodic protection effect on 304SS in 3.5 wt% NaCl corrosion medium.

www.nature.com/scientificreports/ of TiO 2 . The modification methods of TiO 2 mainly include: metal element doping (Ni 10 , Fe 11 , Zn 12 ), nonmetallic element doping (N 13 , S 14 ), and narrow band gap semiconductor sensitization (CdTe/TiO 2 15 , CdSe/TiO 2 16 , ZnSe/ TiO 2 17 , Bi 2 S 3 /TiO 2 18 ) and so on. Through modification, not only the band gap width of TiO 2 can be reduced, but also the separation efficiency and photoelectrochemical properties of electron hole pairs can be improved, so as to improve the photoelectrochemistry cathodic protection effect of 304SS 19 .
FeS 2 is widely distributed in nature, mainly in the form of hematite and pyrite. Hematite type has no photoelectric property, while it will change to pyrite type when the temperature exceeds 350 °C; pyrite FeS 2 is a kind of non-toxic, stable physical and chemical properties, low cost, easy to obtain semiconductor materials, and it has appropriate band gap width (0.95 eV indirect band gap, 1.03 eV direct band gap width), and high light absorption coefficient (6 × 10 -5 cm −1 ) 20 , which make FeS 2 is useful for the preparation of optical devices that can utilize visible light. Han 21 et al. prepared wormlike FeS 2 /TiO 2 nanotube composites, which were used for photocatalytic reduction of CO 2 to methanol under visible light, and the doping and recombination of FeS 2 nanoparticles on TiO 2 nanotubes not only broadened the light absorption range of TiO 2 , but also improved the photocatalytic reduction ability, which increased the methanol yield to 91.7 μmol/h/L. Rashid 22 and others prepared stable anatase TiO 2 composite pyrite FeS 2 composite nanocrystals to improve the photocatalytic decomposition of methyl blue, and the catalytic decomposition of organic compounds increased from 15 to 75 mg/L within 180 min. C Mutalik 23 et al. successfully prepared heterostructured TiO 2 -FeS 2 nanocomposites (NCs) by a facile solution approach to enhance light-induced antibacterial activity over a broader absorption range, and the result showed that TiO 2 -FeS 2 NCs had better antibacterial activity than that of only TiO 2 nanoparticles (NPs) or only FeS 2 NPs, which suggests that TiO 2 -FeS 2 NCs with superior light-induced antibacterial activity could be a promising antibacterial agent against bacterial infections. Li et al. 24 prepared FeS 2 nanoparticles by a simple hydrothermal method. The FeS 2 nanoparticles obtained by reducing FeCl 3 ·6H 2 O with C 5 H 10 NS 2 Na·3H 2 O were assembled on nickel foam as the anode and C nanoparticles as the cathode, and then placed together in a double-electrode alkaline electrolytic cell. Kuo 25 and others prepared a kind of clean hydrogen production based on TiO 2 and environmentally friendly light catalyst by wet chemical synthesis method. The catalyst is composed of FeS 2 -TiO 2 heterostructure nanocrystals, and its absorption range is wide, which can be extended from UV-Vis spectrum to near-infrared spectrum. A wider absorption wavelength range can improve the photocatalytic hydrogen production rate. Xin et al. 26 prepared photoanode materials by sensitizing TiO 2 nanotubes with FeS 2 of pyrite. Due to the heterogeneous structure formed at the interface between FeS 2 and TiO 2 , the recombination rate of photogenerated electron-hole pairs was reduced, and the light absorption capacity of TiO 2 increased from the ultraviolet region to the near-infrared region, thus improving the photochemical properties of composite nanomaterials.
However, there is no report on the application of FeS 2 particles modified TiO 2 composite nanotube arrays for photoelectrochemical cathodic protection of 304SS. In this study, FeS 2 /TiO 2 nanocomposite films were prepared by electrochemical anodization and hydrothermal methods to improve the optical absorption capacity of TiO 2 in visible light and the photoelectrochemical cathodic protection performance, and the protection mechanism for 304SS has also been studied.

Material and methods
Preparation of TiO 2 nanotube array films. Firstly, cut the industrial titanium plate (20 mm × 10 mm × 0.3 mm) with purity greater than 99.6%, and ultrasonic cleaning with acetone, absolute ethanol and ultrapure water for 10 min, separately, then make a mixture of 0.9 g ammonium fluoride, 5 mL ultra pure water, 12 mL hydrogen peroxide and 12 mL concentrated nitric acid to polish the titanium plate; secondly, platinum plate was used as counter electrode and titanium plate as anode, then put it in 80 mL ethylene glycol solution containing 0.44 g NH 4 F and 8 ml H 2 O and then it was anodized at a constant potential of 20 V for 1 h; thirdly, wash the obtained materials with deionized water and calcine it in a muffle furnace at 450 °C for 2 h to get TiO 2 nanotube array films, then take it out and place in a dust-free dryer for standby.
Preparation of FeS 2 /TiO 2 nanocomposite films. FeS 2 /TiO 2 nanocomposite films were prepared by hydrothermal deposition of FeS 2 nanoparticles on TiO 2 nanotubes. The detailed process is as follows: Firstly, FeCl 3 ·9H 2 O was dissolved with secondary water in a 100 mL beaker, and prepare 35 mL FeCl 3 solutions with concentrations of 5 mmol/L, 10 mmol/L and 20 mmol/L respectively, then dissolve thiourea ((NH 2 ) 2 CS) with secondary water in another beaker to make Fe:S = 1:2, and then stir the two solutions for 30 min respectively and then set aside; Secondly, add 35 mL thiourea solution into FeCl 3 solution and stir for 20 min after dropping to make FeCl 3 and thiourea fully mixed; thirdly, transfer the mixed solution to a 100 mL Teflon reactor containing TiO 2 nano film sheet, then placed it in the stainless steel jacket and put it into the blast drying oven, and set the heating temperature as 180 °C and heating time as 12 h; Finally, after the autoclave is cooled naturally, wash the obtained samples with deionized water and ethanol, and dried with dry cold air, then mark the obtained samples as 5-FST, 10-FST and 20-FST respectively for standby.
Characterization. The morphology of pure TiO 2 and FeS 2 /TiO 2 nanocomposite films were observed by scanning electron microscopy (SEM, Hitachi, S-8100); the chemical composition of FeS 2 /TiO 2 nanocomposites was measured by X-ray photoelectron spectroscopy (XPS, PHI, Quantum 2000); and the crystal structures of TiO 2 and FeS 2 /TiO 2 composites were detected by X-ray diffraction (XRD, Japan Science and technology company, 5°/min); and the optical absorption properties of TiO 2 and FeS 2 /TiO 2 composites were characterized by uv-drs (Hitachi).
Photoelectrochemistry cathodic protection performance measurement. The photoelectrochemical properties are evaluated by measuring the photocurrent density time curve (I-t), and the test device is  Fig. 1a. The reference electrode and counter electrode of the electrochemical workstation CHI660E are short circuited, and a small resistance ammeter is formed between the working electrode and the grounding end. The electrolyte in the electrolytic cell is 0.1 mol/L Na 2 SO 4 , and the light source is the incident light filtered out of ultraviolet rays. As shown in Fig. 1b, the device for testing the performance of photoelectrochemical cathodic protection (including OCP, Tafel and EIS) is a dual cell system consisting of a corrosion cell containing 3.5 wt% NaCl solution and a photocell containing 0.1 mol/L Na 2 S solution, and the two cells are connected by Nafion membrane. The electrochemical work station (P4000 + ,USA) of three electrode system was used for electrochemical measurement: platinum plate as counter electrode, saturated calomel electrode as reference electrode, and copper wire as working electrode between 304SS electrode and optical electrode. The light source is a 300 W high pressure xenon lamp with visible light wavelength greater than 400 nm.

Results and discussion
Analysis of open circuit potential time curve. Figure 2 shows the open circuit potential time curves of 304SS electrode, 304SS electrode coupled with pure TiO 2 nanotube array film, and 304SS electrode coupled with three kinds of FeS 2 /TiO 2 nanocomposite films as photoanode under intermittent visible light and in 3.5 wt% NaCl electrolyte, and the OCP reflects the corrosion state of stainless steel electrode. It can be seen from the figure that the self corrosion potential of 304SS in 3.5 wt% NaCl corrosion medium is − 170 mV. As shown in curve (a), the electrode potential of 304SS coupling with pure TiO 2 sample decreased from − 210 to − 340 mV under www.nature.com/scientificreports/ visible light, and the electrode potential after reaching stability was more negative than that of 304SS (− 170 mV), because TiO 2 photo anode provided power supply for 304SS electrode under light condition, which reduced the electrode potential, thus realizing the effect of photoelectrochemical cathodic protection; when the light stopped, the OCP returned to the coupling electrode potential before the light, which indicated that pure TiO 2 had no photoelectrochemical cathodic protection effect on 304SS electrode in the dark. Curves (b-d) show the OCP of 304SS coupling samples 5-FST, 10-FST and 20-FST as photoanodes, and the electrode potential of 304SS with three kinds of FeS 2 /TiO 2 nanocomposite films decreased to − 455 mV, − 708 mV, − 361 mV under visible light, respectively, which indicates that the deposition of FeS 2 nanoparticles on the surface of TiO 2 nanofilms can improve the photoelectrochemical properties of TiO 2 . With the increase of reactant concentration in hydrothermal reaction, the photoinduced potential drop first increases and then decreases, and the photochemical cathodic protection ability also increases first and then decreases. Combined with the SEM surface morphology in Fig. 4, it is speculated that the reason may be that the excessive FeS 2 nanoparticles not only reduce the active sites for light energy absorption of TiO 2 films, but also increase the recombination sites of photoinduced electron hole pairs. When the light stopped, the electrode potential of 304SS coupled with three samples increased to − 320 mV, − 390 mV and − 310 mV, respectively, however, these three potentials are lower than that of 304SS bare electrode, which indicates that FeS 2 /TiO 2 nanocomposite film still has good photoelectrochemistry cathodic protection effect under dark conditions, and the reason may be FeS 2 /TiO 2 nanocomposite materials having certain energy storage function.
Analysis of photocurrent density time curve. Figure 3 shows the time curve of photocurrent density measured under intermittent visible light and 0.1 mol/L Na 2 SO 4 solution by connecting TiO 2 nanotube array film and FeS 2 /TiO 2 composite film prepared at different concentrations as photoanode. The photocurrent density time curve can directly show the photoelectric conversion efficiency of photoelectric materials. The higher the photocurrent density, the higher the photoelectric conversion efficiency, and the stronger the metal protection. As shown in the figure, when the visible light irradiates the surface of the materials, the photocurrent provided by these materials as photoanodes to the cathode increases instantaneously and then stabilizes at a constant value, indicating that they all have photoelectric response performance. It can be seen from curve (a) that a very small photocurrent density (13 μA/cm 2 ) is generated by the TiO 2 photoanode, which is due to the large band gap of pure TiO 2 and the small light absorption in the visible region. The curve (b-d) show that the current densities provided by the three kinds of FeS 2 /TiO 2 composite films are 25 μA/cm 2 , 43 μA/cm 2 and 18 μA/cm 2 , respectively, which are higher than those of pure TiO 2 nano film as photoanode. The results show that the modification of TiO 2 nano film by FeS 2 nanoparticles greatly improves the photoelectric conversion efficiency, which is due to FeS 2 semiconductor with narrow band gap is easy to absorb visible light, and the photo excited electrons migrate from the FeS 2 conduction band to the TiO 2 conduction band, while the photogenerated holes gather in FeS 2 , which realizes the separation of photogenerated electron-hole pairs, thus improving the light utilization and conversion efficiency of TiO 2

27
. Similar to pure TiO 2 nanotube array film, many experiments on opening and closing light show that FeS 2 /TiO 2 nanocomposite films have very high stability and can not be corroded by light.
Morphology characterization. The surface morphology of the samples is shown in Fig. 4, among them,   Fig. 4a shows the SEM of pure TiO 2 , Fig. 4b-d shows the SEM of 5-FST, 10-FST and 20-FST, respectively. It can be seen from Fig. 4a that the tubular structure is formed by electrochemical oxidation, and these nanotubes  Figure 4b is the SEM image of 5-FST sample, and it can be seen that nanoparticles with a diameter of 10-20 nm are attached to the edge of the tube mouth, and there is no deformation or collapse of TiO 2 nanotubes after hydrothermal reaction, which indicates that the thermodynamic properties of TiO 2 crystal are very stable. Figure 4c is the morphology of 10-FST sample, and it can be clearly observed that with the increase of FeS 2 nanoparticles, the coverage area of FeS 2 on the TiO 2 nanotube film expands, and gradually agglomerates on the surface of TiO 2 then fills the nanotubes, but the nozzle of TiO 2 is still open. The morphology of 20-FST sample is shown in Fig. 4d, and the large FeS 2 particles almost seal the mouth of TiO 2 film. In conclusion, with the increase of the concentration of hydrothermal deposition reactants, the deposition amount of FeS 2 nanoparticles increases, while the open part of TiO 2 nanotubes decreases, as well as the contact area of photoelectrochemical reaction decreases, which indicates that the concentration of reactants will affect the morphology of FeS 2 /TiO 2 nanocomposite films. Figure 3e is the energy spectrometer point distribution diagram of FeS 2 /TiO 2 nanocomposite films. There is not only a large amount of Ti, O elements, but also a small amount of Fe and S elements were detected, and the ratio of Fe to s is about 1:2. For the analysis of the specific composition and the compound state of the substance, we will further determine it by means of X-ray photoelectron spectroscopy.
Crystal structure analysis of FeS 2 /TiO 2 nanocomposite films. Figure 5 shows the XRD spectra of pure TiO 2 nanotube array film sample, FeS 2 /TiO 2 nanocomposite film sample and pure FeS 2 powder prepared by hydrothermal method. As shown in the XRD curve of pure TiO 2 , there are two groups of characteristic diffraction peaks in the sample. A group of characteristic diffraction peaks with 2θ at 25.3°, 37.8°, 48.0°, 53.9° and 55.0° respectively belong to anatase TiO 2 (JCPDF-21-1272), and these five diffraction peaks correspond to the 101, 004, 200, 105, 211 crystal face of TiO 2 , among them, 101 crystal face diffraction peak is the strongest, which indicates that 101 crystal face has the strongest activity; the 2 θ of the other group of characteristic diffraction peaks is 38.4°, 40.2°, 53.0°, 62.9°, 70.7° and 77.4° respectively, which is the characteristic diffraction peak of Ti metal lattice (JCPDF-44-1294), and the diffraction peak intensity of Ti is much stronger than that of TiO 2 , which verifies that the electrochemical oxidation of TiO 2 is a nano material; while there is no diffraction peak of rutile TiO 2 on the curve of this sample, which indicates that the crystal structure of calcined samples is anatase. In the XRD curves of FeS 2 /TiO 2 nanocomposite film, in addition to the characteristic diffraction peaks of pure TiO 2 , there are also characteristic absorption peaks at 28.52° 33.05° 37.07° 40.75° 56.23° etc., which indicates that pyrite FeS 2 nanoparticles were successfully deposited on anatase TiO 2 nanofilms to form FeS 2 /TiO 2 nanocomposites, which was consistent with the results of SEM. From the XRD spectra of FeS 2 powder, it can be seen that the characteristic diffraction peak appears with 2θ at 28.  25 , and there are no other impurity diffraction peaks in the curve, which indicates that the preparation method used in this study can prepare pyrite FeS 2 nanoparticles with high purity and crystallinity.

XPS analysis of FeS 2 /TiO 2 nanocomposite films.
In order to confirm the composition of the nanocomposite films more clearly, we used X-ray photoelectron spectroscopy to detect the valence orbital structure and valence of elements. Figure 6 shows the XPS spectra of FeS 2 /TiO 2 nanocomposite film prepared under hydrothermal reaction time of 12 h, ferric chloride concentration of 0.15 mmol/L and reaction temperature of   www.nature.com/scientificreports/ full spectrum, while the peak of C1s is the impurity peak when the instrument is calibrated. Figure 6b is a high resolution narrow spectrum of Fe2p, and it can be seen that the binding energies at 707.1 eV and 719.4 eV correspond to the orbits of Fe2p 3/2 and Fe2p 1/2 , and the distance between the two peaks is 12 eV, which is similar to that of FeS 2 . Figure 6c is a high-resolution narrow spectrum of S2p orbit, and 163.8 eV corresponds to the characteristic peak of binding energy of S2p orbit. According to the literature, the S element in the sample is − 1 valence, so FeS 2 exists in the complex 28 . Figure 6d is a high resolution narrow-band XPS image of Ti2p, and the binding energies of the two characteristic absorption peaks are 458.4 eV and 464.2 eV, respectively, which are corresponding to the orbital absorption peaks of Ti2p 3/2 and Ti2p 1/2 , indicating that the valence state of Ti is + 4, and the Ti element in the sample mainly exists in the form of TiO 2 . Figure 6e is a high resolution narrow spectrum of O1s, and it can be seen from the figure that the characteristic peak of binding energy at 529.9 eV corresponds to the metal oxide, which is consistent with the narrow spectrum scanning of Ti2p, and due to the O1s peak is very sharp, there is no other oxide in the sample. In conclusion, the composition of the nanocomposite is only FeS 2 and TiO 2 , which is consistent with the results of EDS and XRD.
Optical absorption properties of FeS 2 /TiO 2 nanocomposite films. Figure 7 shows the UV-Vis diffuse reflectance spectra of pure TiO 2 nanotube film, pure FeS 2 powder tablet and FeS 2 /TiO 2 nanocomposite film. It can be seen from the curve of pure TiO 2 sample that the light absorption edge of pure TiO 2 nano film is 387 nm, and the light absorption intensity in visible light region is very weak, corresponding to the energy band width of TiO 2 is 3.2 eV. From the curve of pure FeS 2 sample we can see that the light absorption edge of pyrite FeS 2 is 780 nm, and the radiation in visible spectrum has strong light response, and the corresponding band gap is 1.5 eV, indicating that pyrite FeS 2 is an excellent semiconductor material with narrow band gap. According to the curve of FeS 2 /TiO 2 nanocomposite films, the light absorption edge of FeS 2 /TiO 2 nanocomposite is 496 nm, and the corresponding band gap is 2.5 eV, which indicates that the deposition of FeS 2 nanoparticles on the TiO 2 nano film can expand the light absorption range to the visible light region, and reduce the band gap of TiO 2 . In summary, nanosized pyrite FeS 2 has a very significant effect on improving the optical absorption properties of TiO 2 nanotube films.
Tafel polarization curve analysis of FeS 2 /TiO 2 nanocomposite films. After the metal contacts with the medium in the environment, there are multiple electrode reactions on the metal surface. The anodic reaction makes the metal dissolve and enter the solution in the form of ions, which is called anode process; and the cathodic reaction is that the medium on the metal surface absorbs the electrons from the anode, and the metal does not dissolve. The existing state of metal depends on the addition of total cathodic reaction and total anode reaction, and the anode process causes electrochemical corrosion of metal, while the principle of cathodic protection is the polarization of metal under external conditions, therefore, the total potential of metal changes from corrosion potential to negative, the total cathodic current increases and the anode current decreases.     Figure 9a shows the Nyquist diagram of 304SS in 3.5 wt% NaCl solution without coupling and coupling FeS 2 /TiO 2 nano film photoanode, and each curve has only one capacitance arc. Obviously, under visible light irradiation, the capacitance arc of 304SS coupling FeS 2 /TiO 2 nano film photoanode is much smaller than that of 304SS bare electrode, and the difference between them is two orders of magnitude. The EIS spectrum can be simulated by the equivalent circuit shown in Fig. 9b. In the equivalent circuit model, RS, RCT and CPE rep-   Mechanism analysis of photoelectrochemical cathodic protection. The electron transfer process of FeS 2 /TiO 2 nanocomposite film coupled with 304SS electrode under visible light is shown in Fig. 10. It takes less photon energy to excite FeS 2 , and the electron in the valence band of FeS 2 can be excited by visible light and then transited to the conduction band. Because the conduction band potential of FeS 2 is lower than that of TiO 2 , the conduction band electrons in FeS 2 can be transferred to the conduction band of TiO 2 , while the holes in FeS 2 remain in the valence band, which greatly improves the separation efficiency of photogenerated electron-hole pairs. Under the action of external circuit, photogenerated electrons are transferred to 304SS and gathered on the surface of it, which makes 304SS cathode polarized to achieve the purpose of cathodic protection. Therefore, FeS 2 /TiO 2 nanocomposite film has excellent photoelectrochemical cathodic protection effect on 304SS under visible light irradiation.

Conclusion
(1) FeS 2 /TiO 2 nanocomposite films were successfully prepared by anodic oxidation and hydrothermal methods, and when the concentration of reactants was designed to be 10 mmol/L, FeS 2 /TiO 2 nanocomposite films had the optimal surface morphology of absorbing sunlight. (2) After the modification of FeS 2 nanoparticles, the light absorption range of TiO 2 extends from UV to visible, which not only improves the separation efficiency of photogenerated electron-hole pairs, but also increases the photoelectric conversion ability. (3) Compared with pure TiO 2 , the modification of FeS 2 nanoparticles on the surface of TiO 2 nanotube array film greatly improves the photoelectrochemistry cathodic protection performance of 304SS. (4) Under visible light irradiation, the OCP of 304SS coupled with the composite can be reduced to about − 700 mV, and the electrode potential of the stainless steel can still be maintained at − 400 mV after the light is turned off, that is, FeS 2 /TiO 2 Nanocomposite filmscan still play the role of electrochemical cathodic protection in the dark.