Bi2Se3 Sensitized TiO2 Nanotube Films for Photogenerated Cathodic Protection of 304 Stainless Steel Under Visible Light.

Titanium dioxide (TiO2) nanotube arrays coupled with a narrow gap semiconductor-bismuth selenide (Bi2Se3)-exhibited remarkable enhancement in the photocathodic protection property for 304 stainless steel under visible light. Bi2Se3/TiO2 nanocomposites were successfully synthesized using a simple two-step method, including an electrochemical anodization method for preparing pure TiO2 and a chemical bath deposition method for synthesizing Bi2Se3 nanoflowers. The morphology and structure of the composite films were studied by scanning electron microscopy, energy dispersion spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction. In addition, the influence of the Bi2Se3 content on the photoelectrochemical and photocathodic protection properties of the composite films was also studied. The photocurrent density of the Bi2Se3/TiO2 nanocomposites was significantly higher than that of pure TiO2 under visible light. The sensitizer Bi2Se3 enhanced the efficient separation of the photogenerated electron-hole pairs and the photocathodic protection properties of TiO2. Under visible light illumination, Bi2Se3/TiO2 nanocomposites synthesized by the chemical bath deposition method with Bi3+ (0.5 mmol/L) exhibited the optimal photogenerated cathodic protection performance for 304 stainless steel.

Background

As important engineering materials, stainless steels have been widely applied to significant projects in numerous fields due to their excellent corrosion resistance. However, stainless steels can suffer serious corrosion when used in specific aggressive environments, such as acid environments, as well as under chloride-containing or high-temperature conditions [1-4]. Extensive research and applications of the traditional anti-corrosion method, including coatings [5, 6], use of a sacrificial anode [7] and impressed current cathodic protection [8, 9], have been developed during the past few decades. However, eco-friendly and long-lasting anticorrosion technology still remains a major objective. As a new anti-corrosion technology, photocathodic protection was first proposed by Yuan and Tsujikawa in 1995 [10] before receiving attention from corrosion researchers [11-14].

Titanium dioxide (TiO2) is an important photoelectric material with good photoelectric conversion and photocatalysis properties and is widely used in catalysts [15], solar cells [16] and gas sensors [17] due to its low cost, non-toxicity and stable chemical properties. TiO2 and TiO2-based composites are used for photogenerated cathodic protection: a promising technique for corrosion prevention that has undergone rapid development in recent years [18-23]. However, the bandgap (3.2 eV) of TiO2 restricts the photoresponse to only the ultraviolet region, which significantly depresses the utilization ratio of solar power. In addition, photo-induced charge carriers in bare TiO2 nanoparticles show a very short lifetime due to the rapid recombination of photo-excited electron-hole pairs, which reduces the photocathodic protection effect of pure TiO2 films. Thus, how to overcome the above deficiencies of TiO2 has become a widely studied topic. Many studies have been conducted on compounding TiO2 with non-metal elements (F, N and S) [12, 24, 25], metal atoms (Fe, Co, Cu and Ce) [26-29] and some narrow bandgap nano-semiconductors (Ag2O, ZnSe, WO3, CdS, Ag2S, CdSe and Bi2S3) [30-36] to improve the carrier separation and light utilization of TiO2.

Bismuth selenide (Bi2Se3) is a direct bandgap layered semiconductor and important member of the V2VI3 compound family. It has a high absorption coefficient in the visible and near-infrared light regions with a narrow bandgap (0.35 eV) [37]. As an important n-type chalcogenide, Bi2Se3 possesses many important characteristics, such as a high electrical conductivity [38], appreciable thermoelectric property [39], photosensitivity [40], electrochemical property [41] and photoconductivity [42]. Furthermore, Bi2Se3 is a popular topological insulator [43-45] and has the unique property of conductive surface states and insulated bulk states. High-quality Bi2Se3 nanostructures have been prepared using a high vacuum physical deposition method, chemical vapour deposition, atomic layer deposition, pulsed laser deposition and a vapour-liquid-solid technique at high temperature [44-49]. These synthetic methods for Bi2Se3 require a difficult fabrication, leading to a high production cost. In this paper, the above problems are overcome by employing a low-cost and simple chemical bath deposition method for Bi2Se3 nanoflower deposition on TiO2. The combination of a n-Bi2Se3/n-TiO2 heterojunction as an efficient photoanode was applied to the photogenerated cathodic protection of 304ss for the first time. The morphology, structure and optical absorption property of Bi2Se3/TiO2 nanocomposites were studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and UV-visible (UV-Vis) diffuse reflectance spectra, respectively.

Methods

All of the chemicals used in this study were of analytical grade and used as received without further purification. All of the aqueous solutions were prepared using deionized water.

Preparation of TiO2 Film

Ti foils (20 mm × 10 mm × 0.3 mm; > 99.9% purity) were polished using a mixture containing NH4F (2.25 g), H2O (12.5 mL), H2O2 (30 wt%, 30 mL) and HNO3 (68 wt%, 30 mL), and then, Ti pieces were cleaned with deionized water and ethanol. TiO2 film was prepared on Ti foil by the anodic oxidation method reported in the literature [50]. The Pt plate was chosen as the cathode, and the Ti foil was the anode at 20 V for 1 h in an ethylene glycol solution containing NH4F (0.22 g), H2O (4 mL) and ethylene glycol (40 mL) at ambient temperature. After that, the samples were rinsed with deionized water and ethanol. Finally, the specimens were annealed at 450 °C for 2 h and cooled in ambient air to obtain TiO2 film.

Synthesis of Bi2Se3 on the TiO2 Film

The Bi2Se3 was prepared by the chemical bath deposition method. In the experimental procedure, 8 mmol of nitrilotriacetic acid (H3NTA) and 0.4 mmol of Bi(NO3)3·5H2O were added to deionized water (400 mL) to form the bismuth chelate, with a Bi3+ concentration of 1.0 mmol/L in the mixed solution. Two millimoles of ascorbic acid as the reducing reagent was added to the above solution, and then, ammonium hydroxide was cautiously added, dropwise, until the pH of the mixture was adjusted to approximately 8.6~8.9 and mixed solution appeared colourless and transparent. Finally, Na2SeSO3 (20 mL, 30 mmol/L) was injected into the above solution. In all of the above experiments, the aqueous solutions were thoroughly stirred with a magnetic stirrer to obtain a homogeneous solution. Then, a TiO2 substrate was immersed in the final solution (40 mL) in a beaker (100 mL). The beaker covered with cling film was then transferred into an oven heated to a temperature of 80 °C for 200 min to obtain the Bi2Se3 nanoflower on the TiO2 substrate. Finally, the sample was removed from the beaker and washed several times with deionized water and ethanol and then allowed to dry in ambient air. In this way, Bi2Se3-sensitized TiO2 films were obtained and labelled with Bi2Se3/TiO2-1.0. For simplicity, different quantities of Bi2Se3 on TiO2 substrates are designated as Bi2Se3/TiO2-γ in this paper, where γ denotes the concentration of Bi3+ in the H3NTA and Bi (NO3)3·5H2O solution. With the quantities of the other reagents held constant, Bi2Se3/TiO2-0.5 and Bi2Se3/TiO2-0.25 were obtained for Bi3+concentrations of 0.5 mmol/L and 0.25 mmol/L, respectively. The influence of different quantities of Bi2Se3 on the photoelectrochemical and photocathodic protection properties of the composite films was investigated in this paper.

Morphology and Composition Analysis

Scanning electron microscopy (SEM, Hitachi S-4800, Japan) was used to investigate the morphologies of the prepared films. Energy-dispersive X-ray spectroscopy (EDS, Oxford Energy 350 X-ray energy spectrum analyser) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) were employed to determine the chemical composition of the Bi2Se3/TiO2 nanocomposites. UV-Vis DRS (Japan Hitachi UH4150) was used to determine the light absorbance of the samples. The crystalline phase composition of the samples was characterised by an X-ray diffractometer (XRD, Germany Bruker AXSD8) using Cu Kα radiation (γ = 1.54056 Å) from 10° to 80°.

Photoelectrochemical Measurements

As shown in Fig. 1, a coupling system comprising photolysis and electrolytic cells was used for the photoelectrochemical measurements, and a proton exchange membrane was used to link the two cells together. The photolysis cell contained a 0.1 mol/L Na2S and 0.2 mol/L NaOH mixed solution, which played the role of a sacrificial agent for promoting the separation of electrons and holes [33, 51], while a 0.5 mol/L NaCl solution was used as the electrolyte for the electrolytic cell. In the electrolytic cell, a three-electrode system was adopted with a Pt foil as the counter electrode (CE), saturated calomel electrode as the reference electrode (RE) and 304ss as the working electrode (WE). Bi2Se3/TiO2 nanocomposite samples placed in the photolysis cell were connected to a 304ss electrode immersed in the electrolytic cell by a copper wire. The light source in the visible light range was irradiated by a high-pressure xenon lamp (PLS-SXE 300 C, Beijing Perfectlight Company, China). The changes in the open-circuit potential (OCP) and photocurrent curves were measured using a Gamry potentiostat/galvanostat/ZRA system (GAMRY 3000, Gamry Instruments, USA) before and during light irradiation.

Fig. 1

Schematic sketches of the experimental setup used for photoelectrochemical analysis

Results and Discussion

Characterization of Pure TiO2 and Bi2Se3/TiO2

Figure 2a shows typical top view and cross-sectional topographies for TiO2 films prepared under the anodization method. The TiO2 nanotube arrays show a nanoporous structure composed of well-ordered and high-density nanotubes with an average inner diameter and length of approximately 55 nm and 680 nm, respectively. As shown in Fig. 2b–d, the TiO2 nanotube surfaces were successfully modified by Bi2Se3 via the chemical bath deposition method for different concentrations of Bi3+. For Bi2Se3/TiO2-0.25, the Bi2Se3 nanoflakes were sporadically distributed and aggregated unevenly across the TiO2 nanotubes (Fig. 2b). When the concentration of Bi3+ was 0.5 mmol/L, Bi2Se3 was composed almost entirely of flower-like patterns of pliable ultrathin nanoflakes with a diameter of approximately 800 nm, without blocking the nozzle of the TiO2 nanotubes or damaging them (Fig. 2c). Bi2Se3 nanoflowers that were observed to be evenly distributed on the surface of the TiO2 showed an internal cross-linked structure for the ultrathin nanoflakes, which effectively prevented lamella aggregation and maintained a long-standing lifetime of the architectures, as shown in Fig. 2c. After the concentration of Bi3+ was increased to 1.0 mmol/L, the amount and diameter of the Bi2Se3 nanoflowers significantly increased, and the agglomeration of nanoflowers blocked the nanotubes, as shown in Fig. 2d. The corresponding EDS spectrum of the Bi2Se3/TiO2-0.5 films shown in Fig. 2e revealed that the characteristic peaks for Ti, O, Bi and Se were marked with atomic percentages of Bi and Se of 0.9% and 1.3%, respectively. It is well known that the measurement error of EDS test is increased with the decrease of content of test element. So, it is acceptable that the atomic ratio of Bi and Se is close to 2:3.

Fig. 2

SEM images for a pure TiO2, b Bi2Se3/TiO2-0.25, c Bi2Se3/TiO2-0.5 and d Bi2Se3/TiO2-1.0; e EDS spectrum for Bi2Se3/TiO2-0.5 films

Figure 3a shows the XRD spectra for pure TiO2 (curve a) and Bi2Se3/TiO2-1.0 nanocomposites (curve b). Aside from the Ti substrate peaks, the diffraction peaks at 25.38°, 38.03°, 48.01°, 54.05°, 55.17°, 62.71° and 70.44° were well matched with the lattice planes (101), (004), (200), (105), (211), (204) and (220) of anatase TiO2, respectively (JCPDS 21-1272). Except for the TiO2 peaks, the distinctive diffraction peaks at 29.35° and 74.90° were indexed to the lattice planes (015) and (0216) of the rhombohedral crystal Bi2Se3 (JCPDS 33-0214). However, there is no obvious peak of Bi2Se3/TiO2-1.0 due to low content Bi2Se3 deposited on TiO2 and the XRD spectra conformed to the SEM and EDS results. X-ray photoelectron spectroscopy (XPS) was used to further determine the chemical compositions and states of the Bi2Se3/TiO2 nanocomposites. As shown in Fig. 3b, XPS revealed the existence of Bi, Se, Ti and O components in addition to C contaminants due to adventitious hydrocarbon contamination. Figure 3c shows the peak positions for Ti 2p at 458.7 and 464.5 eV, indicating that the titanium oxides mainly consisted of TiO2 [52]. As illustrated in Fig. 3d, the O 1s semaphores matched with two Gaussian peaks: the maximum at the lower binding energy (530.0 eV) was attributed to the lattice oxygen (OL) in Bi2Se3/TiO2 nanocomposites and the second at the higher binding energy (531.5 eV) was derived from the adsorbed oxygen (OA), including weak bonding oxygen or hydroxyl groups. The existence of OA was due to the generation of oxygen vacancies on the surface of the nanocomposites, which might improve the photoelectric conversion properties of Bi2Se3/TiO2 nanocomposites in photocathodic protection [53]. Figure 3e shows that the 4f7/2 asymmetric peak for Bi resolved into two peaks (157.5 and 159.4 eV), with the Bi 4f5/2 spectrum similarly divided into two bands at 162.8 and 164.7 eV, respectively. The positions of the lower peaks (157.5 eV and 162.8 eV) were in good agreement with those in Bi2Se3, with the higher peaks corresponding to bismuth oxide at 159.4 eV and 164.7 eV [54, 55]. It can be concluded that a handful of bismuth metal was oxidized during the synthetic process with Bi2Se3 modifying pure TiO2. As shown in Fig. 3f, the two peaks were assigned to the 3d3/2 and 3d5/2 core levels of Se at 55.1 and 54.2 eV, respectively, indicating that Se existed in the form of Se2- [56].

Fig. 3

a XRD patterns for pure TiO2 and Bi2Se3/TiO2-1.0 nanocomposites; b the total survey spectrum, c Ti 2p, d O 1s, e Bi 4f and f Se 3d XPS spectra for Bi2Se3/TiO2-1.0 nanocomposites

Figure 4 shows the light absorption abilities of pure TiO2 and Bi2Se3/TiO2-1.0 nanocomposites. The characteristic absorption edge for pure TiO2 was approximately 380 nm within the UV region due to the bandgap energy of anatase TiO2 (3.2 eV) (curve a). Pronounced adsorption was observed for Bi2Se3/TiO2 nanocomposites in the visible light region (350–800 nm) (curve b), with visible light absorption abilities higher than those of pure TiO2 due to the incorporation of the Bi2Se3 nanoflower. This phenomenon can be ascribed to the fact that Bi2Se3 is excited under visible light due to its narrow bandgap (0.35 eV), with electrons and holes produced in its conduction band (CB) and valence band (VB). Therefore, the addition of Bi2Se3 effectively increases the visible light absorption capability of Bi2Se3/TiO2 nanocomposites.

Fig. 4

UV-visible absorption spectra for TiO2 (a) and Bi2Se3/TiO2-1.0 (b)

Photocathodic Protection Performance of Pure TiO2 and Bi2Se3/TiO2

As shown in Fig. 5, the OCP curves for 304ss coupled with pure TiO2 and Bi2Se3/TiO2 nanocomposite photoanodes were measured under intermittent visible light, with the OCP response to illumination prompted and shifted to a negative potential for all of the coupled electrodes. At the initial phase of light on, the OCP for all of the coupled electrodes showed a negative shift over a short time, which was due to the transfer of the excited photoelectrons from the pure TiO2 and Bi2Se3/TiO2 nanocomposite to the 304ss electrode [1, 57]. Subsequently, the relatively stable OCP values can be attributed to the balancing rate between the creation and recombination of photogenerated electrons [32]. After switching off the irradiation, the OCP values for the Bi2Se3/TiO2 nanocomposites returned to their original values at a slower speed compared to pure TiO2. This phenomenon might be attributed to the electron pool effect of Bi2Se3/TiO2 nanocomposites, which can store photoinduced electrons under light irradiation and slowly release these electrons without light irradiation. Under visible light irradiation, the OCP value for 304ss was approximately − 450 mV when coupled with TiO2 (curve a), and the OCP values for 304ss coupled with Bi2Se3/TiO2-0.25 (curve b), Bi2Se3/TiO2-0.5 (curve d) and Bi2Se3/TiO2-1.0 (curve c) reached − 905 mV, − 996 mV and − 958 mV, respectively. These results indicated that 304ss was cathodically polarized once coupled with Bi2Se3/TiO2 nanocomposites and that a good cathodic protection for 304ss might be provided by the Bi2Se3/TiO2 photoanodes. As shown in Fig. 5 d, the 304ss coupled to Bi2Se3/TiO2-0.5 possessed most negative potential indicated that the best photocathodic protection performance for 304ss. This result might be because the active sites and light harvesting increased with the increasing Bi2Se3 content. However, an excessive amount of Bi2Se3 particles served as the recombination sites for electrons and holes, which hindered the charge transfer from the Bi2Se3/TiO2 nanocomposites to 304ss.

Fig. 5

OCP for 304ss coupled with pure TiO2 and Bi2Se3/TiO2 nanocomposites in a 0.5 mol NaCl solution

As shown in Fig. 6, the photocurrent density vs. time curves for TiO2 and Bi2Se3/TiO2 nanocomposites showed a rapid and reproducible photoresponse under intermittent visible light illumination, which reflected the photoelectric conversion performance of the materials. The photogenerated current was relatively small under visible light due to weak visible light absorption (curve a). However, the photogenerated current increased remarkably under visible light illumination following sensitization of TiO2 by the Bi2Se3 nanoflower (curves b to d). The data implied that the Bi2Se3/TiO2 nanocomposites were capable of utilizing visible light and that the heterojunction between TiO2 and Bi2Se3 promoted the separation of photogenerated electrons and holes [58]. Furthermore, the photoelectrons produced in the conduction band of the Bi2Se3 nanoflower can be easily transferred to the more positive conduction band of the TiO2 nanotubes under visible light illumination. After three irradiation interval, the photocurrent maintained a relatively steady value and no photocurrent degradation was detected, illustrating the good photochemical stability of the Bi2Se3/TiO2 nanocomposite films. For different concentrations of Bi3+, the Bi2Se3/TiO2 nanocomposites showed different intensities for the photocurrent response. In particular, the transient photocurrent density for Bi2Se3/TiO2-0.5 (415 μA/cm2) was higher than that for Bi2Se3/TiO2-0.25 (85 μA/cm2) and Bi2Se3/TiO2-1.0 (160 μA/cm2), indicating that Bi2Se3/TiO2-0.5 possessed an ideal separation efficiency for the photogenerated electron-hole pairs. The active sites and light harvesting were decreased because of the deficiency of Bi2Se3 nanoflowers on the Bi2Se3/TiO2 nanocomposite films, while recombination sites for electrons and holes increased in the presence of an excessive amount of Bi2Se3 nanoflowers. Under visible light illumination, the largest photoinduced current density of the Bi2Se3/TiO2-0.5 photoanode was consistent with the largest photoinduced potential drops illustrated in Fig. 5, further validating the optimal photocathodic protection performance of Bi2Se3/TiO2-0.5 for 304ss.

Fig. 6

Photocurrent density vs. time curves for pure TiO2 and Bi2Se3/TiO2 nanocomposites in 0.1 mol/L Na2S and 0.2 mol/L NaOH mixed solution

Figure 7 shows the photoelectric conversion and transportation processes for the Bi2Se3/TiO2 nanocomposites. Under visible light, Bi2Se3 nanoflowers can readily absorb photons as they contain adsorbed oxygen (OA) and have a narrow bandgap width (0.35 eV). When the photons are absorbed by the Bi2Se3 nanoflowers, photoexcited electrons will be generated by excitation from the valence band (VB) of Bi2Se3 to the conduction band (CB) of Bi2Se3. The photoexcited electrons in the CB of Bi2Se3 are shifted to the CB of TiO2, while the photogenerated holes in the VB of TiO2 are transferred to the VB of Bi2Se3, and then are captured by S2− in the electrolyte to turn into S on the surface of photoanode film. When the photoexcited electrons exit the photoanode and transfer to 304ss, they will react with the oxygen gas and water to convert OH−. Furthermore, Na+ is transported from electrolytic cell to photolysis cell by proton exchange membrane, so that the coupling system is electrically neutral as a whole. As a consequence, the photogenerated charges are effectively separated and the recombination probability for photogenerated electron-hole pairs is reduced. Once 304ss receives photoexcited electrons from the Bi2Se3/TiO2 nanocomposite through the wire, the potential of 304ss shifts negatively. Under visible light illumination, the Bi2Se3/TiO2 nanocomposites can reduce the corrosion rate of 304ss. Therefore, the efficient separation of photo-excited electron-hole pairs in Bi2Se3/TiO2 nanocomposites will accelerate the redox reaction and generate effective photocathodic protection for 304ss.

Fig. 7

Schematic representation of the electron transfer processes in Bi2Se3/TiO2

Conclusions

In this paper, TiO2 nanotube arrays were prepared by the anodization method and Bi2Se3 nanoflowers were grown on TiO2 nanotubes by chemical bath deposition. The Bi2Se3/TiO2 nanocomposites showed a homogeneous distribution and ordered characteristics. Electrochemical tests for the nanocomposites and pure TiO2 coupled with 304ss showed that the photogenerated cathodic protection performance of the Bi2Se3/TiO2 nanocomposites was superior compared to that for pure TiO2. The OCP value for 304ss coupled with Bi2Se3/TiO2-0.5 showed a negative shift to − 996 mV under visible light illumination due to the active sites and light harvesting of TiO2 sensitized by Bi2Se3. By comparing the results of the electrochemical tests for three Bi2Se3/TiO2 nanocomposites, the nanocomposite prepared using 0.5 mmol/L Bi3+ in the electrolyte exhibited optimal performance.