Piao Jin ,Zichao Guan ,Yan Liang , Kai Tan , Xia Wang ,Guangling Song , Ronggui Du ,*

1 Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian Province,China.

2 Center for Marine Materials Corrosion and Protection, College of Materials, Xiamen University, Xiamen 361005, Fujian Province,China.

Abstract: Photocathodic protection by TiO2 semiconductor materials for metals has interested many corrosion researchers for years.However, a pure TiO2 semiconductor anode can only absorb ultraviolet light and cannot maintain the photocathodic protection in the dark.This has limited its practical applications to a great extent.Overcoming these limitations is significant as well as challenging.Therefore, the objective of this work is to prepare a modified TiO2 composite film with visible light absorption and charge storage capabilities for application in photocathodic protection.First, we fabricated an ordered TiO2 nanotube array film on a Ti substrate by electrochemical anodization.Then, we prepared NiO nanoparticles on the film via a hydrothermal reaction to obtain a p-n heterostructured NiO/TiO2 nanotube array composite film.The properties of the prepared films were investigated by scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV-Vis absorption spectroscopy, photoluminescence spectroscopy, and photoelectrochemical techniques.The results indicated that the electrochemically anodized TiO2 film had an anatase phase structure and consisted of vertically ordered nanotubes with an inner diameter of about 80 nm and length of 250 nm.After the NiO nanoparticles were deposited on the film, the TiO2 nanotube array structure remained intact.The main phase of TiO2 was still anatase, but the light absorption of the NiO/TiO2 composite film was extended into the visible region, which was in contrast to that of the simple TiO2 film.Moreover, the composite film showed lower photoluminescence intensities than the TiO2 film, implying that a higher charge carrier separation efficiency could be achieved by modification with NiO.Under white light illumination, the photocurrent density of the NiO/TiO2 composite film in a mixed solution of 0.5 mol·L-1 KOH and 1 mol·L-1 CH3OH reached 176 μA·cm-2, which was 2 times higher than that of the simple TiO2 nanotube film, indicating that the composite film had improved photoelectric conversion efficiency and photoelectrochemical properties.The potential of 403 stainless steel (403SS) in 0.5 mol·L-1 NaCl solution decreased by 380 and 440 mV relative to its corrosion potential when coupled to the TiO2 film and NiO/TiO2 composite film, respectively,under white light illumination.This indicated that the heterostructured NiO/TiO2 film as a photoanode could produce more effective photocathodic protection on the steel as compared with the pure TiO2 film.Even after 2.5 h of illumination, the composite film could continuously provide photocathodic protection to 403SS for about 15.5 h in the dark, suggesting that the NiO/TiO2 composite film had a charge storage capability that was significant for its practical applications.

Key Words: Anodic oxidation; Hydrothermal treatment; TiO2 nanotube; NiO; Photoelectrochemistry;Photocathodic protection; Stainless steel

1 Introduction

Titanium dioxide (TiO2), a wide-bandgap (3.0–3.2 eV)semiconductor, has many superior properties, such as low toxicity, low cost, good photosensitivity, and high stability.However, the inability of TiO2to utilize visible light limits its practical applications.In order to extend the optical absorption range of the TiO2semiconductor and improve its photoelectrochemical properties, researchers have modified TiO2by noble metal loading, ion doping and semiconductor coupling1–11,etc.Among these modifications, the coupling with semiconductors with suitable electronic band structures is considered to be one of the most effective methods to promote the separation of photoinduced electron-hole pairs and enhance the photoelectrochemical conversion efficiencies of TiO2composites6–15.NiO with a bandgap of about 3.55 eV is ap-type semiconductor with high hole mobility and cost effectiveness.Besides, its conduction band position is higher than that of TiO2,which is beneficial to the transfer of photoinduced electrons from NiO to TiO2.Ifp-type NiO is combined withn-type TiO2,ap-nheterojunction will form at their interface, which can enhance the photoelectrochemical activity of the NiO/TiO2composites14,16–19.For example, Kuetal.14used the NiO/TiO2composite as a photocatalyst to reduce Cr(VI) in the aqueous solution, and found that this composite showed high photocatalytic activity.Lietal.17synthesized the NiO/TiO2composite nanofibers to produce hydrogen by the photolysis, and found that the H2production rate of the composite reached to 337 μmol·h-1·g-1, which was 7 times higher than that of the pure TiO2sample.This was because NiO as a cocatalyst could suppress the recombination of photoinduced electron-hole pairs in the NiO/TiO2composite, lower the overpotential of H2production and accelerate the H2production.In the future, more cost-effective NiO/TiO2composite films will be pursued in the field of photoelectrochemistry.

Herein, in this study, a facile approach involving electrochemical anodization and hydrothermal treatment was developed to fabricate a heterostructured NiO/TiO2nanotube array film with charge storage capability in order to further improve its photocathodic protection property.

2 Experimental

2.1 Fabrication of NiO/TiO2 nanotube composite film

All the chemicals used in this work were of analytical grade.An electrochemical anodization combined with hydrothermal treatment method was used to fabricate a heterostructured NiO/TiO2nanotube array film, which would serve as a composite photoanode.An anatase TiO2nanotube array film was first prepared on a Ti substrate by anodization as reported in our earlier works7,10.Briefly, a Ti foil (purity 99.7%, dimensions 15 mm × 10 mm × 0.1 mm) was anodized in a 0.5% (mass fraction)HF solution for 30 min at a voltage of 20 V, and then the sample was calcined at 450 ℃ for 2 h to obtain an anatase TiO2film7,10.Then, NiO nanoparticles were preparedviahydrothermal treatment.Namely, the prepared TiO2film sample was immersed in a mixed solution of 6 mmol·L-1Ni(NO3)2·6H2O and 60 mmol·L-1CON2H4in a Teflon-lined autoclave and heated at 100 ℃ for 1 h.Finally, the film sample was calcined at 500 ℃ for 3 h to obtain a desired NiO/TiO2nanotube composite film.

2.2 Characterization of films

The morphologies of the prepared films were observed by scanning electron microscopy (SEM, Hitachi FE-SEM S4800).The chemical compositions were obtained by X-ray diffraction(XRD, Rigaku Ultima IV) with CuKαradiation (λ= 0.154 nm)and X-ray photoelectron spectroscopy (XPS, Qtac-100 LEISSXPS, AlKαradiation).The UV-Vis absorption and photoluminescence (PL) spectra of the films were obtained by a Varian Gray 5000 UV-Vis-NIR spectrophotometer and a fluorescence spectrometer (Hitachi F-7000, excitation wavelength 370 nm), respectively.

2.3 Photoelectrochemical measurements

The transient photocurrent responses of the TiO2and NiO/TiO2films were examined under open circuit conditions upon intermittent light illumination.The photocurrent density measurements were conducted by an electrochemical workstation (Ivium CompactState) in a photoelectrochemical cell with the prepared film (namely photoanode) as the working electrode, a Pt foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode.A mixed aqueous solution with 0.5 mol·L-1KOH and 1 mol·L-1CH3OH served as the electrolyte in the cell.The film photoanode was illuminated with a LHX 150 W Xe lamp.

Photocathodic protection measurements were performed by a home-built measurement system, as described in detail in our previous studies7,10.The prepared film served as a photoanode in a photoelectrochemical cell containing a mixed solution of 0.5 mol·L-1KOH and 1 mol·L-1CH3OH.403 stainless steel (403SS,exposed area of 1 cm2) served as the metal (namely working electrode) to be protected in a corrosion cell containing a 0.5 mol·L-1NaCl solution with a Pt foil counter electrode and an SCE reference electrode.The potentials of the 403SS uncoupled and coupled with the prepared film were measured to examine the photocathodic protection property of the film.The electrochemical impedance spectroscopy (EIS) tests on the 403SS at the corrosion potentials were also performed by using this system over the frequency range of 100 kHz to 10 mHz with an AC amplitude of 10 mV.

3 Results and discussion

3.1 Characterization of the prepared films

Fig.1 presents the SEM images of the prepared films,showing that the ordered nanotube array films were formed on their Ti substrate surfaces.The TiO2film consisted of vertically oriented nanotubes with about 80 nm inner diameter and 10 nm wall thickness (shown in Fig.1a), and the nanotube length was about 250 nm.As shown in Fig.1b, after the deposition of NiO,many nanoparticles were formed on the TiO2film surface.The nanotube array structure remained unchanged, but the nanotube surfaces became obviously rough.

Fig.1 SEM images of (a) the TiO2 nanotube film and (b) the NiO/TiO2 nanotube composite film.

Fig.2 displays the XRD patterns of the two nanotube films.Two diffraction peaks located at 25.3° and 48.1° were ascribed to the (101) and (200) crystal faces of anatase TiO2(JCPDS No.71-1166), respectively, indicating that the TiO2prepared by the anodization and 450℃ calcination was in the anatase phase (Fig.2a).As shown in Fig.2b, after the deposition of NiO nanoparticles, the diffraction peaks of 37.4°, 43.4°, and 63.0°were attributed to the (111), (200) and (220) crystal faces of NiO(JCPDS No.75-0197), suggesting that NiO was prepared on the film.Besides the main peaks corresponding to anatase TiO2, the two small peaks at 27.4° and 36.0° were indexed to the (110) and(101) plans of rutile TiO2(JCPDS No.72-1148), suggesting that small amounts of the anatase TiO2in composite film was converted into rutile TiO2after the 500 ℃ calcination.

Fig.2 XRD patterns of (a) the TiO2 nanotube film and (b) the NiO/TiO2 nanotube composite film.

The high-resolution XPS spectra of the NiO/TiO2composite film for Ti 2p, Ni 2p, and O 1sare displayed in Fig.3.As shown in Fig.3a, the two peaks at 458.8 and 464.5 eV corresponded to the Ti 2p3/2and Ti 2p1/2, respectively, demonstrating that Ti in the film was in a Ti4+state9,10.In Fig.3b, all the peaks belonged to Ni2+, among which the peaks at 855.9 and 873.4 eV corresponded to Ni 2p3/2and Ni 2p1/2, respectively.Satellite peaks due to shake-up processes appeared at 861.9 and 880.9 eV for the composite film20,21.In Fig.3c, the peaks at 530.0 eV22and 530.8 eV19resulted from the lattice oxygen of TiO2and NiO, respectively, and the peak at 531.5 eV was attributed to the adsorption oxygen23.The above XRD and XPS results indicated that a NiO/TiO2composite film had been successfully synthesized.

Fig.3 XPS spectra of the NiO/TiO2 composite film: (a) Ti 2p, (b) Ni 2p and (c) O 1s.

Fig.4a exhibits the UV-Vis absorption spectra of the prepared films.The absorption edge of the NiO/TiO2composite film showed an obvious red shift towards the visible region relative to the simple TiO2film, and its photoabsorption was stronger in the 400–550 nm range, which was probably due to the conduction band of the Ti 3dorbital overlapping with that of the Ni 3dorbital and narrowing the band gap of the NiO/TiO2composite film24,25.Fig.4b displays the PL spectra of the prepared films.Peaks in the PL spectra were mainly derived from the electron-hole recombination26,27.Obviously, the NiO/TiO2film showed a weaker peak intensity than the TiO2film, indicating that the deposition of NiO on the TiO2nanotubes could effectively inhibit the electron-hole recombination in the NiO/TiO2composite film, which is of significance to improve the photoelectrochemical properties of the composite film.

Fig.4 (a) UV-Vis absorption spectra and (b) PL spectra of the different nanotube films.

3.2 Photoelectrochemical properties

The time evolution of the photocurrent densities of the TiO2and NiO/TiO2films upon white light illumination on/off cycles was recorded at open circuit conditions for evaluating their photoelectrochemical responses.As presented in Fig.5, the reproducible photocurrent responses clearly showed the good photostability of the two films.Particularly, the photocurrent densities of the TiO2film and NiO/TiO2composite film upon the illumination were about 89 and 176 μA·cm-2, respectively,illustrating that the photoelectric conversion efficiency of the NiO/TiO2film was much higher than that of the TiO2film.Namely, the NiO/TiO2composite photoanode showed better photoelectrochemical property, which is of importance for improving its photocathodic protection effect.

Fig.5 Time evolution of the photocurrent densities of the different nanotube films: (a) TiO2 and (b) NiO/TiO2.

The photocathodic protection properties of the prepared films were investigated when they were coupled with the 403SS specimen.Fig.6 shows the potential variation of the 403SS coupled with the TiO2or NiO/TiO2film photoanode.The 403SS corrosion potential (Ecorr) was about 100 mVvsSCE before coupling.After coupling with TiO2film or NiO/TiO2composite film, the 403SS potential decreased owing to the galvanic action.When the light was turned on, the 403SS potentials were further decreased, and were 380 and 440 mV lower than the corrosion potential for coupling with the TiO2and NiO/TiO2films (Fig.6a,b), respectively, suggesting that the 403SS was under photocathodic protection, and the NiO/TiO2film had a more effective cathodic protection effect.After the light was turned off, the 403SS potential was immediately shifted back to the level before illumination for coupling with the TiO2film (Fig.6a), indicating that the photocathodic protection disappeared because of the recombination of photoinduced electrons and holes in the film.It is noteworthy that the potential of the 403SS coupled with the NiO/TiO2composite film increased only slightly from -340 to -320 mVvs.SCE after the first illumination was stopped, and the potential was lower than the level before illumination, indicating that the composite film could maintain the photocathodic protection resulting from its charge storage capability.After about 2.0 h, the illumination was provided again, the potential of the 403SS decreased as before,showing good reproducibility for the photocathodic protection effect of the composite film.After about 2.5 h illumination, the light was turned off again, the potential of the 403SS coupled with the NiO/TiO2film rose slowly back to the level before the first illumination in 15.5 h (Fig.6b).It indicated that the composite film had a charge storage capacity and provided some photocathodic protection in the dark for about 15.5 h.

Fig.6 Variation of the potential of 403SS coupled to the different films under intermittent white light illumination.

The photocathodic protection properties of the prepared films were further investigated by the EIS analyses.EIS spectra of the 403SS specimen in the 0.5 mol·L-1NaCl solution in the corrosion cell under different conditions are presented in Fig.7,and they could be modeled by using the equivalent circuit given in the inset28,29.In the equivalent circuit,RsandRctare the solution resistance and the charge transfer resistance,respectively.CPE is the constant phase element representing the electric double layer capacitance of the 403SS electrode28–30.TheRctvalues of the uncoupled and coupled 403SS specimens could be used for examining photocathodic protection effects of the different films.For the uncoupled 403SS, itsRctvalue was 451.2 kΩ·cm2by fitting the EIS data.For coupling with the TiO2and NiO/TiO2films, theRctvalues of the 403SS were 95.4 and 47.6 kΩ·cm2, respectively, which were much lower than that of the uncoupled 403SS.These results indicated that large numbers of photoinduced electrons were transferred from the illuminated TiO2or NiO/TiO2film to the coupled 403SS, which promoted the cathodic reaction on the 403SS surface and inhibited the steel corrosion.Therefore, the two films could offer photocathodic protection to the 403SS, but the heterostructured NiO/TiO2composite film showed a better photocathodic protection effect because of the lowerRctvalue corresponding to the 403SS coupled with the composite film.

Fig.7 Impedance spectra of 403SS in the 0.5 mol·L-1 NaCl solution.

3.3 Mechanism

The enhanced photocathodic protection mechanism of the NiO/TiO2composite film and its charge storage capacity is illustrated schematically in Fig.8.As described in the schematic,the combination ofn-TiO2andp-NiO in the composite film formedp-nheterojunctions, which created an internal electric field at their interface, and resulted in the diffusion of electrons to the positive side and the diffusion of holes to the negative side31,32.Under white light illumination, the semiconductors absorbed light and generated the photoinduced electron-hole pairs.The electrons of the semiconductors were excited from the valence bands to the conduction bands.Subsequently, in the NiO/TiO2heterojunction electric field, the photoinduced electrons in the NiO conduction band migrated to the TiO2conduction band, and meanwhile the photoinduced holes were transferred from the TiO2valence band to the NiO valence band.This effectively inhibited the recombination of photoinduced charge carriers.Therefore, the photoelectric conversion efficiency and photocathodic protection performance of the NiO/TiO2composite film were enhanced.Besides the electric field effects, NiO also served as holes recombination centers.When the light illumination was turned on, the photoinduced holes might react with NiO according to the equation as follows33:

Fig.8 Schematic diagram of the separation and migration of electron-hole pairs in the heterostructured NiO/TiO2 composite film.

NiO + OH-+ h+↔ NiOOH

As a result, the holes were stored in the NiO so that more photoinduced electrons were accumulated in the TiO2and/or transferred to the 403SS, which contributed to the metal charge transfer from Ni2+to Ni3+.After the illumination was turned off,the holes were released and mainly consumed by hole scavengers in the electrolyte, and the accumulated electrons were released slowly from the TiO2to the 403SS to maintain the cathodic protection33.This was the possible reason that the NiO/TiO2composite film could provide a certain photocathodic protection effect in the dark.Further study is needed in this respect.

4 Conclusions

The NiO/TiO2nanotube composite film prepared by anodization combined with hydrothermal treatment showed the enhanced photoelectrochemical properties compared with the simple TiO2nanotube film.The absorption edge of the composite film was red-shifted to the visible region, and its photocurrent density was 2 times that of the TiO2film.Under white light illumination, the composite film could offer better photocathodic protection to the 403SS in a 0.5 mol·L-1NaCl solution, which made the 403SS potential decrease by 440 mV relative to the corrosion potential.Especially, after the illumination was stopped, the NiO/TiO2composite film was able to continuously provide the photocathodic protection in the dark for about 15.5 h due to its charge storage capability.