One-pot synthesis and photocatalytic activity of Fe-doped TiO₂ films with anatase–rutile nanojunction prepared by plasma electrolytic oxidation

Abstract

A novel one-pot low temperature preparation of Fe-doped anatase–rutile TiO₂ (Fe-TiO₂) films is demonstrated using plasma electrolytic oxidation (PEO). Pale yellow TiO₂ films are obtained by PEO treatment of Ti metals in the electrolyte dispersing TiO₂ and Fe₂O₃ particles. The oxidized layer on Ti metal have a sponge-like structure with thickness and pore size of 10 and 0.1–1 μm, respectively. Investigation by X-ray diffraction, X-ray photoelectron spectroscopy, and UV–Vis absorption spectroscopy all indicate that dissolved Fe³⁺ ions in the strong acidic electrolyte are doped into the TiO₂ structure during PEO. The photocatalytic activity of Fe-TiO₂ samples was investigated by studying the photocatalytic decomposition of acetaldehyde. Fe-TiO₂ samples doped with optimum Fe content show visible light photocatalytic activity and further increased photocatalytic activity under UV illumination compared with that of pure TiO₂ films.

Introduction

Titanium dioxide (TiO₂) has aroused great interest because of its high photocatalytic activity, chemical stability, low-toxicity, and low cost [1, 2]. The large band gap of TiO₂ (anatase, 3.2 eV) requires UV excitation which accounts for only a small portion (3–5%) of the solar spectrum. To enhance photocatalytic activity under sunlight conditions, more efficient utilization of the solar spectrum is essential. The spectral response of TiO₂ can be extended to the visible region by doping with transition metals [3–5]. Fe is considered an appropriate candidate as the ionic radius of Fe³⁺ (0.78 Å) is similar to that of Ti⁴⁺ (0.74 Å) [6]. Fe³⁺ ions are easily incorporated into the crystal structure of TiO₂. Replacing Ti⁴⁺ with Fe³⁺ in the TiO₂ lattice forms a localized band near the bottom of conduction band, thus decreasing the band gap. Fe³⁺ doping has also been reported to improve charge separation of generated excited electrons and holes in TiO₂ [11]. Most authors have suggested that Fe³⁺ ions act as trapping sites for excited electrons and/or holes, resulting in the decrease of charge carrier recombination. Fe-doped TiO₂ can be excited by visible light and its photocatalytic activity under UV illumination is also enhanced [7–13]. To date, various synthetic routes to Fe-doped TiO₂ materials have been reported, most of which are sol–gel [7–10] and hydrothermal techniques [11–13]. These methods generally require long reaction times, high temperatures, and multi-step process.

Plasma electrolytic oxidation (PEO) of metals has attracted much industrial and academic attention because functional metal oxide films such as TiO₂, Al₂O₃, MgO, and BaTiO₃ can be easily obtained [14–18]. The PEO process is an effective method to prepare transition metal-doped TiO₂ photocatalysts. Cr-doped TiO₂ films prepared by PEO process show excellent photocatalytic activity for degradation of methylene blue and decomposition of water under visible light illumination [19]. Yao et al. [20] prepared Zn-doped TiO₂ films and investigated the photocatalytic activity in the reduction of potassium chromate under UV light illumination. We have developed a synthetic pathway to a TiO₂ photocatalyst by PEO of Ti metal in binary or trinary electrolyte solutions [21]. The one-pot method is low temperatures process and is adaptable to large-scales. Resulting TiO₂ films are a composite of anatase and rutile phases. Such phase coupling greatly improves charge separation of excited electrons and holes [22], and anatase–rutile coupled TiO₂ materials have higher photocatalytic activity compared with that of TiO₂ with anatase or rutile single phase. The anatase/rutile TiO₂ film formed (A/R) by PEO also shows high photocatalytic activity under UV illumination. Incorporation of anatase TiO₂ nanoparticles into the A/R film can be achieved by dispersing anatase TiO₂ nanoparticles within the electrolyte solution, and the resulting A/A/R TiO₂ material showed further increased photocatalytic activity [21].

Herein, we report a one-pot synthetic method for preparing Fe-doped A/A/R TiO₂ (Fe-TiO₂) films by PEO of Ti metal at room temperature. Anatase TiO₂ and α-Fe₂O₃ particles are both dispersed in the electrolyte solution composed of hydrogen peroxide, phosphoric acid, and sulfuric acid. The Fe₂O₃ particles are partly dissolved in the strongly acidic electrolyte solution, and Fe ions are incorporated into the TiO₂ film during PEO. Resulting films show visible light photocatalytic activity for the decomposition of acetaldehyde. Recently, Wu et al. [23] reported that TiO₂ films prepared with Fe³⁺ addition electrolyte of H₂SO₄ using micro-plasma oxidation showed higher photocatalytic activities than that prepared in the pure electrolyte. However, their and most other studies for Fe-doped TiO₂ photocatalyst have discussed the photocatalytic activity under UV or visible light illumination. We investigated the photocatalytic activity of Fe-TiO₂ under UV, visible, and UV–Vis light irradiation to reveal the effect of Fe³⁺ doping to TiO₂ photocatalyst in detail.

Experimental section

Preparation of Fe-doped A/A/R TiO₂ film by plasma electrolytic oxidation

A Ti plate (purity, 99.9%; working area, 15 cm²) was anodized galvanostatically at 3.0 A dm⁻² for 1 h by using a regulated DC power supply. An aqueous electrolyte was prepared from a solution of sulfuric acid (1.50 M), hydrogen peroxide (0.30 M), and phosphoric acid (0.03 M). Anatase TiO₂ (20 g L⁻¹; mean diameter, 30 nm; TAYCA, AMT-600) and α-Fe₂O₃ particles (0–7 g L⁻¹; mean diameter, 300 nm; Kanto Chemical) were added to the electrolyte. The initial electrolyte temperature was 5 °C, and then the temperature was gradually increased to ca. 30 °C during PEO process. PEO treatment was carried out in a double-walled glass cell with continuous stirring. A Ti sheet was used as the cathode. After oxidation, the samples were washed with distilled water and dried at room temperature. It was confirmed that the dispersed Fe₂O₃ particles were partly dissolved (ca. 2 wt%) in the electrolyte by inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurement.

Characterization

Field-emission scanning electron microscopy (SEM) and scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) were performed using Hitachi S-4800 type II (accelerating voltage, 10 kV). X-ray diffraction (XRD) was performed using Rigaku RINT 2500 using Cu Kα radiation. UV–Vis diffuse reflectance spectra were obtained on Hitachi U-4000 spectrophotometer. X-ray photoelectron spectroscopy (XPS) was performed using AXIS-NOVA, KRATOS. For XPS measurement, powder samples were obtained by scratching the surface of obtained Fe-TiO₂ films, and the powder adhered to double-faced conductive carbon tape on sample stage for XPS. The measured binding energies were referenced to the C 1s line at 284.6 eV. ICP-AES was performed using Shimadzu ICPS-7500.

Photocatalytic experiments

The photocatalytic activities of sample films were evaluated by measuring the change in concentration of acetaldehyde. A closed Pyrex glass (λ > 300 nm) was used as the photo-reactor vessel with a volume of 640 cm³. The sample film (30 × 30 mm²) was placed in the reaction vessel. Two hundred ppm of acetaldehyde was prepared in the vessel by injection of a commercial acetaldehyde gas (Takachiho Chemical Industrial Co. Ltd.; 594 ppm; balance gas, N₂). The photo-irradiations were conducted at room temperature after equilibrium between the gaseous and adsorbed acetaldehyde had been reached (as ascertained by monitoring the concentration chromatographically about every 15 min). A 500-W Xe lamp (Wacom Electric, HX-500) was used as a light source. Three types of light beam, (i) UV light shorter than 400 nm, I _320–380 = 2.0 mW cm⁻²; (ii) visible light longer than 420 nm, I _400–485 = 2.0 mW cm⁻²; (iii) UV/visible light, I _320–380 = 2.0 mW cm⁻², were irradiated to the samples by using cut filters (AGC Techno glass, UV-D36A; Toshiba, L-42). The decrease in acetaldehyde concentration during photoirradiation was measured using a gas chromatograph (Shimadzu, GC-2014, FID detector).

Results and discussion

Figure 1a shows photographic images of a Ti plate before and after PEO. A colorless A/A/R TiO₂ film was uniformly formed on Ti plate in the absence of Fe₂O₃ in the electrolyte (Fig. 1a; right, A/A/R TiO₂). A pale yellow film was obtained by anodizing a Ti plate in electrolyte containing Fe₂O₃ (Fig. 1a; right, Fe-TiO₂). SEM images of a typical sample are shown in Fig. 1b, c. The oxidized layer had a sponge-like structure with thickness and pore size of 10 and 0.1–1 μm, respectively. Nano-sized particles were apparent on the Fe-TiO₂ surface, and anatase nanoparticles were expected to be deposited during PEO [21].

Fig. 1

a Photographic images of a Ti plate before (left) and after (right) PEO. b Plane and c cross-sectional scanning electron microscopy images of a Fe-TiO₂ film formed from PEO. The electrolyte dispersion contained 3 g of Fe₂O₃

The crystalline structure was investigated from XRD patterns (Fig. 2). Diffractions from Ti and anatase and rutile TiO₂ were observed in the XRD spectra of all samples (JCPDS no. anatase, 21-1272; rutile, 21-1276; Ti, 44-1294). This was in contrast to diffractions of Fe₂O₃. Fe contents of prepared Fe-TiO₂ films were determined by ICP-AES measurement (Fig. 3). Fe content increased with dispersed Fe₂O₃ particle content (x g L⁻¹). Fe ions dispersed in the electrolyte were expected to be incorporated into the TiO₂ film crystal lattice. However, for Fe-TiO₂ samples, large particles (diameter, 100–300 nm) which are different from TiO₂ nanoparticles (diameter, 30 nm) were slightly seen on the surface of formed TiO₂ films as indicated by arrows on Fig. 4b, c. Figure 5 shows a SEM image and SEM/EDX elemental mappings of the Fe-TiO₂ film (x, 7 g L⁻¹). The signals attributed with Ti and O were evenly seen on the images. On the other hand, regions where large particles exist were particularly rich in elemental Fe. Thus, the large particles were probably Fe₂O₃ particles deposited on the surface of TiO₂ during PEO process. It is assumed that XRD signals attributed with Fe₂O₃ was not able to detect because of the negligible amount of the deposited Fe₂O₃ particles. The changes of cell volume and lattice parameters (a, b, and c) of anatase TiO₂ were not seen on the obtained Fe-TiO₂ samples [23]. Anatase TiO₂ nanoparticles dispersed in the electrolyte will be deposited into Fe-TiO₂ films during PEO process [21]. XRD signals attributed to anatase crystal doped with Fe³⁺ ions are hidden due to those of the deposited anatase nanoparticles. Thus, slight changes of the diffraction angle should not emerge on Fe-TiO₂ samples. Interestingly, the anatase:rutile diffraction intensity ratio changed by varying the Fe₂O₃ content in the electrolyte. This crystalline transformation was associated with local exothermic heat caused by the spark discharge process. Anatase:rutile ratio changes have also been reported for the PEO preparation of TiO₂ films from Ti plate in electrolytes containing Ni ions [24].

Fig. 2

XRD patterns of Fe-TiO₂ films obtained from PEO. Electrolyte dispersions contained various amounts of Fe₂O₃ (x g L⁻¹) as stated

Fig. 3

Fe contents of Fe-TiO₂ samples prepared by the PEO process. Electrolyte dispersions contained various amounts of Fe₂O₃ (x g L⁻¹) as stated

Fig. 4

SEM images of A/A/R TiO₂ film (a) and Fe-TiO₂ films obtained in electrolytes containing 3 g L⁻¹ (b) and 7 g L⁻¹ (c) Fe₂O₃ particles

Fig. 5

SEM/EDX micrographs of Fe-TiO₂ (x, 7 g L⁻¹), with elemental analysis maps of Ti (top right), O (bottom left), and Fe (bottom right). Scale bar 200 nm

Figure 6 shows UV–Vis diffuse reflectance spectra for the obtained samples. All samples showed an intense broad absorption band below 400 nm which was attributed to the TiO₂ band-gap transition. The absorption bands in the visible and near infrared (NIR) region of the A/A/R TiO₂ film (0 g L⁻¹) were attributed with d−d transition of Ti³⁺ species which were slightly formed in the TiO₂ lattice by the plasma electrolytic oxidation process [21, 25]. For Fe-TiO₂ samples, an absorption bands emerged in visible, extending to the NIR region (400−750 nm), and its intensity increased with dispersed Fe₂O₃ content. Visible absorption (400–500 nm) may have been attributed to the excitation of Fe³⁺ 3d electrons to the TiO₂ conduction band at 415 nm or charge transfer transition between Fe ions (such as Fe³⁺ + Fe³⁺ → Fe⁴⁺ + Fe²⁺) at 500 nm [7]. The increases of absorption intensity in the Vis–NIR region (500–750 nm) have been usually observed on Fe-doped TiO₂ samples prepared by other procedures such as metal plasma ion implantation [26], sol–gel [27], hydrothermal [28], ultrasonic-hydrothermal [29], and incipient wetness impregnation methods [30]. The absorption bands are generally associated with one type of transitions, i.e., the promotion of an electron from a localized orbital on one atom to a higher-energy localized orbital on the same atom [31]. In this study, the absorption was associated with d−d electron transition of doped iron atoms and slightly deposited Fe₂O₃ particles.

Fig. 6

UV–Vis diffuse reflectance spectra of Fe-TiO₂ films. Electrolyte dispersions contained various amounts of Fe₂O₃ (x g L⁻¹) as stated

XPS spectra of Fe-TiO₂ samples are shown in Fig. 7. Ti 2p_1/2 and Ti 2p_3/2 spin-orbital splittings were located at 464.4 and 458.7 eV, respectively, and were in agreement with literature values of Ti⁴⁺ in pure TiO₂ (Fig. 7a) [32]. These energies were not affected by the low doping content. Peaks attributed to Ti³⁺ were not observed in XPS spectra because of the very low contents in the TiO₂ crystal lattice. Peaks at 711.4–712.7 and 725.2–726.6 eV were assigned to Fe 2p_3/2 and Fe 2p_1/2, respectively (Fig. 7b). These signals showed a positive shift compared to those for crystalline α-Fe₂O₃ (711.0 and 724.4 eV for 2p_3/2 and 2p_1/2, respectively [33, 34]). Such positive shifts have been previously shown for Fe³⁺-doped TiO₂ materials [7, 13]. Slight shifts in Fe 2p binding energy probably indicated some Fe³⁺ replaced Ti⁴⁺ within the TiO₂ lattice, and subsequent electron transfer from Fe³⁺ to Ti⁴⁺. XRD, UV–Vis absorption, and XPS spectral data all indicated that dissolved Fe³⁺ was doped into the TiO₂ structure during PEO.

Fig. 7

Ti 2p (a) and Fe 2p (b) XPS spectra of Fe-TiO₂ samples obtained by the PEO process. Electrolyte dispersions contained various amounts of Fe₂O₃ (x g L⁻¹) as stated

The photocatalytic activity of Fe-TiO₂ samples was investigated by studying the photocatalytic decomposition of acetaldehyde under UV, visible, and UV–vis illumination. Photocatalytic degradation kinetics of organic compounds usually follows the Langmuir–Hinshelwood mechanism [35, 36], simplified to the apparent first-order equation:

$$ \ln \left( {{\frac{{C_{0} }}{C}}} \right) = kKt = k_{app} t $$

where C ₀ and C are the concentration of acetaldehyde initially and after t min, respectively, k is the reaction rate constant, and K is the adsorption coefficient of the reactant. The apparent first-order rate constant, k _app is given by the gradient of the graph of ln(C ₀/C) versus t.

Figure 8 shows k _app values of Fe-TiO₂ samples obtained from varying amounts of Fe₂O₃ (x) dispersed in the electrolyte. The A/A/R TiO₂ (x, 0 g L⁻¹) film showed photocatalytic activity under UV illumination. On the other hand, Fe-TiO₂ samples (1 < x < 6) clearly showed visible light photocatalytic activity. Figure 9 shows the changes in ln(C ₀/C) values of the Fe-TiO₂ (x, 3 g L⁻¹) and the A/A/R TiO₂ (x, 0 g L⁻¹) under visible light irradiation. The negligible photocatalytic activity of the A/A/R TiO₂ film was attributed with the self-doped Ti³⁺ species [37]. On the other hand, the visible light photocatalytic activity of the Fe-TiO₂ film was about ten times as high as that of the A/A/R TiO₂ film. Fe³⁺ 3d electrons can be excited by the visible light and transit to the conduction band of TiO₂. The photocatalytic activities were investigated in gas phase containing air, N₂, and acetaldehyde. H₂O and O₂ molecules from air were present in the reaction vessel. Additionally, H₂O molecules are generated as products of photocatalytic oxidation of acetaldehyde [38]. The photo-generated holes and electrons are reacted with H₂O and O₂ on the surface of Fe-TiO₂, resulting in the formation of highly activated radicals, such as O₂ ⁻ and HO. Acetaldehyde molecules adsorbed on the surface of the photocatalyst are oxidatively decomposed by the formed radicals. In addition, the slightly deposited Fe₂O₃ particles onto the surfaces of TiO₂ were involved in the increase of visible light photocatalytic activity [39]. The photocatalytic decomposition of acetaldehyde occurs on also pure TiO₂ by generating holes and excited electrons with UV light illumination. For Fe-TiO₂ samples under excitation, k _app initially increased with dispersed Fe₂O₃ content, and then plateaued at 3 g L⁻¹ of Fe₂O₃. Further Fe₂O₃ increase caused a decrease in photocatalytic activity. A small amount of doped Fe³⁺ can act as trapping sites for holes and excited electrons, respectively, generated in the valence band and the conduction band of TiO₂ under UV illumination and reduce the recombination probability [11]. Thus, photocatalytic activities of Fe-TiO₂ (1 < x < 3) were enhanced under UV light excitation compared with that of pure TiO₂. This enhancement was probably associated with the increased anatase:rutile ratio. Anatase TiO₂ photocatalysts show higher photocatalytic activity compared with that of rutile TiO₂ [40]. All the above photocatalytic reaction pathways are involved in the photocatalytic activity of Fe-TiO₂ under visible and UV light illumination (Fig. 8). In addition, the interaction between Fe-TiO₂ and acetaldehyde which was changed due to the formation of surface Fe–OH sites perhaps influenced the photocatalytic activity. However, doped Fe³⁺ may also act as recombination centers for photo-generated holes and electrons when their concentration is higher than the optimal level [41–43]. Thus, photocatalytic activity under both visible and UV illumination decreased due to less photo-generated carriers when the doped Fe content exceeded a certain concentration (4 < x < 7).

Fig. 8

First-order rate constants (k _app) for the photocatalytic degradation of acetaldehyde by Fe-TiO₂ films under UV–Vis (filled triangle), UV (filled square), and visible (filled circle) light excitation, as a function of electrolyte Fe₂O₃ content (x g L⁻¹)

Fig. 9

Kinetics of the photocatalytic degradation of acetaldehyde by Fe-TiO₂ (filled triangle) and A/A/R TiO₂ (filled square) under visible light irradiation, as a function of irradiation time

Conclusions

In conclusion, porous Fe-doped TiO₂ films containing both anatase and rutile crystal phases were obtained by a one-pot PEO process. The Fe-TiO₂ samples possessed an absorption band in the visible region and showed visible light photocatalytic activity for the decomposition of acetaldehyde. In addition, the photocatalytic activities of Fe-TiO₂ samples were enhanced under also UV illumination due to the decrease of recombination probability of photo-generated holes and electrons by formation of trapping sites attributed to incorporation of Fe³⁺ ions into TiO₂ crystal lattice. We envisage that metal-doped TiO₂ materials of various compositions can be obtained from PEO.