Highly Active Rutile TiO2 for Photocatalysis under Violet Light Irradiation at 405 nm

Abstract

Anatase TiO₂ is a widely investigated photocatalyst; however, it can only work under ultraviolet (UV) light with wavelengths less than 390 nm (band gap 3.2 eV). Rutile TiO₂ can absorb visible light at wavelengths less than 410 nm (band gap 3.0 eV); however, its photocatalytic activity is not high. Herein, we activated rutile TiO₂, which was prepared from Evonik TiO₂ P 25 through calcination at 800 °C using hydrogen reduction treatment at 700 °C. The photocatalytic activity of the hydrogen-treated TiO₂ was as high as P 25 under UV irradiation at 380 nm, which was significantly higher than P 25 under violet light irradiation at 405 nm for the oxidative decomposition of acetic acid in water. Electron spin resonance studies indicate that charge separation is enhanced in reduced TiO₂, and their oxygen reduction pathways differ between anatase and rutile. The formation of H₂O₂ was observed on rutile TiO₂; however, it was consumed during photocatalysis to accelerate acetic acid decomposition.

1. Introduction

Titania (TiO₂) is one of the most investigated photocatalytic materials for the environmental purification of air and water under ultraviolet (UV) light irradiation [1,2,3,4]. There are several studies on the photocatalytic activity of anatase TiO₂ for the oxidative decomposition of organic pollutants in air and water. In contrast, rutile TiO₂ has not been extensively studied due to the lower photocatalytic activity in the presence of O₂ as an oxidizing species [5,6,7]. However, rutile can be effectively used in photocatalytic applications because it can absorb violet light in the visible region owing to the band gap energy (E_g = 3.0 eV, onset wavelength λ_onset = 413 nm) being narrower than that of anatase (E_g = 3.2 eV, λ_onset = 388 nm) [3,8].

High photocatalytic activities are frequently reported for anatase TiO₂ with a high specific surface area [3,9]. Anatase TiO₂ irreversibly transforms into rutile, which is in a thermodynamically stable phase, through high-temperature calcination (Figure 1a). However, this heat treatment at high temperatures decreased the specific surface area due to the increase in the particle size and decreased photocatalytic activity. The surface OH groups also decreased under thermal treatment. The decrease in surface OH group density has been correlated with the lower photocatalytic activity of TiO₂ [5,10,11].

The low photocatalytic activity of rutile TiO₂ is also explained by the energy level of the conduction band minimum (CBM) of rutile, which is less than that of anatase (Figure 1b) [5]. Electrochemical studies indicate that the flat band potential, which is located just under the CBM for an n-type semiconductor, of rutile TiO₂(001) is more positive than that of anatase TiO₂(101) by 0.2 V [8]. This may affect the electron transfer to oxygen, which is the rate-determining process in photocatalysis [12,13,14]. The position of the CBM of rutile implies that a one-electron reduction in O₂ with conduction band electrons (e_cb⁻) to form superoxide radical anions (O₂^•−) may be difficult to achieve in rutile (Figure 1b).

In addition, several reports have concluded that the recombination of photogenerated carriers is likely to occur in rutile TiO₂ [15,16,17]. The electron mobility of rutile is less than that of anatase [18,19]. The difference in the depth of electron traps has also been elucidated by time-resolved visible and infrared absorption spectroscopies [15].

Nevertheless, it has been revealed that H₂ reduction treatment improves the photocatalytic and photoelectrochemical activities of rutile TiO₂ [20,21,22,23,24,25,26]. However, the application of H₂-treated rutile TiO₂ has been limited for photocatalytic oxidative decomposition of organic compounds since it is believed that rutile is not active for photocatalysis with oxygen reduction. In this study, we investigated the photocatalysis of H₂-treated rutile TiO₂ for the mineralization of acetic acid to carbon dioxide (CO₂). Acetic acid is one of the most studied model compounds for the photocatalytic mineralization of organic pollutants and intermediates during the oxidative decomposition of acetaldehyde. Herein, we firstly found that the H₂-treated rutile TiO₂ exhibited high photocatalytic activity for the oxidative decomposition reaction even under violet light irradiation (405 nm). We explored the mechanism for enhanced photocatalytic activity using electron spin resonance (ESR) spectroscopy and quantification of hydrogen peroxide (H₂O₂) during the photocatalytic reactions.

2. Results

2.1. Preparation and Characterization

We used Evonik/Degussa Aeroxide^® P 25, which is a commercial TiO₂ with high photocatalytic activity [27], with varying compositions of anatase (~85%) and rutile (~15%) [28]. P 25 powder was calcined in air at 800 °C for 2 h. Thus-obtained P800 was further treated with H₂ flow at 700 °C for 2 h. The H₂-treated TiO₂ is denoted as P800-H700.

Figure 2 shows the scanning electron microscope (SEM) images of the TiO₂ samples. The particle size increased from ~30 to ~100 nm after calcination. In contrast, the hydrogen treatment did not change the particle size of P800. These phenomena were supported by the BET-specific surface areas (SSA_BET), which were 55, 9.6, and 9.8 m² g⁻¹ for P 25, P800, and P800-H700, respectively.

Figure 3 shows powder X-ray diffraction (XRD) patterns of TiO₂ with nickel oxide, which was added to 30 wt% as an internal standard. P 25 was composed of anatase (87%) and rutile (13%) [24,29]. Calcination at 800 °C converted anatase to pure rutile (100%). P800-H700 also consisted of a single phase of rutile TiO₂.

2.2. Optical Properties of TiO₂ Particles

Figure 4a shows the diffuse reflectance UV-visible spectra using barium sulfate as a reference. The optical absorption onset (λ_onset) of P 25 was exhibited at ~410 nm, which is consistent with 3.02 eV of rutile TiO₂. However, the absorption edge of P 25 was not steep because rutile was not the major phase in P 25. P800 and P800-H700 exhibited steeper absorption edges than P 25. The λ_onset of P800 and P800-H700 was ~415 nm (2.99 eV), which is consistent with the E_g of rutile TiO₂ [8,30,31].

As shown in Figure 4b, P800 was white, whereas P800-H700 was pale blue. This blue color is related to visible light absorption. The 1—reflectance values at wavelengths longer than 420 nm were not zero for all samples. However, the values of P 25 and P800 originate from scattering. In contrast, the extinction of P800-H700 is attributed to electron transitions from the shallow traps in Ti³⁺-enriched TiO₂ [32,33,34]. The blue TiO₂ implies the formation of partially reduced TiO₂ with Ti³⁺ species.

2.3. Photocatalytic Activity Test

Figure 5 shows the time course of photocatalytic CO₂ evolution through the decomposition of acetic acid in water. CO₂ evolution was effectively induced by the highly active P 25 and P800-H700 under UV illumination at 380 nm. In contrast, CO₂ evolution over P800 was sluggish because of the phase transformation to rutile. The decreased activity may be explained by the decrease in the specific surface area. However, P800-H700 with low SSA_BET (9.8 m² g⁻¹) also exhibited high activity, which is comparable to that of highly active P 25 (SSA_BET = 55 m² g⁻¹). This implies that the intrinsic activity per unit surface area was significantly high for H₂-treated rutile TiO₂.

Figure 5b shows the results of the photocatalytic reaction under violet light at 405 nm. In this visible light region, the photocatalytic activity of P 25 was very low because the main component of anatase could not absorb violet light. The CO₂ evolution of P 25 was as low as P800 under 405-nm irradiation. In contrast, P800-H700 exhibited significantly higher photocatalytic activity than anatase-rich P 25 and conventional rutile TiO₂. Rutile can absorb violet light; however, its photocatalytic activity is generally less than that of anatase. However, H₂-treated rutile exhibited high photocatalytic activity even in the violet light region. The apparent quantum efficiencies of CO₂ evolution were 8.4% at 380 nm and 4.9% at 405 nm when four electrons were required for one molecule of CO₂ formation (CH₃COOH + 2H₂O → 2CO₂ + 8H⁺ + 8e⁻).

2.4. ESR Study

To determine the reason for the enhanced activity, we measured the ESR spectra of H₂-reduced rutile (Figure 6). The signal assigned to the Ti³⁺ species at g = 2.0–1.8 was not observed for P800. In contrast, P800-H700 exhibited a broad signal of Ti³⁺ at g = 2.0–1.8. The signal at g⊥ = 1.97 is assigned to Ti³⁺ in rutile TiO₂ [35,36]. The signal broadening in H₂-reduced TiO₂ suggests a magnetic dipole–dipole interaction due to the high density of Ti³⁺ species [34].

When the sample was exposed to UV light, the signal changed slightly. The different spectra before and after UV irradiation indicate an increase in Ti³⁺ and the formation of photogenerated holes trapped on the lattice oxygen atom (O_L^•−) with g values in the range of 2.02–2.00. The increment in the signal intensities was higher for P800-H700 than for P800, suggesting more efficient charge separation in H₂-reduced rutile TiO₂ under UV irradiation. The longer lifetime of the trapped electrons indicates the high activity of P800-H700.

We further investigated the oxygen reduction reaction over anatase and rutile TiO₂ using ESR spectroscopy (Figure 7). After evacuation, P 25 exhibits a signal at g = 1.98, which is assigned to the Ti³⁺ in anatase TiO₂ [35,36]. A signal intensity less than that of P800-H700 indicates the low density of Tⁱ³⁺ in P 25. In the presence of molecular oxygen (1.0 Torr), there are several signals of paramagnetic O₂ exhibited in the gas phase (Figure S1 in the Supplementary Materials) [37]. Under UV irradiation, P 25 exhibited signals at g = 2.04–1.98. The line shape was not resolved and was superimposed on several radicals. The superoxide anion (O₂^•−) on anatase can be identified as g₁ = 2.020–2.029, g₂ = 2.009, and g₃ = 2.003 [38,39,40,41,42,43]. When photogenerated e_cb⁻ is used for O₂ reduction, the valence band hole is trapped on the surface (O_L^•−). The g-values of O_L^•− in anatase were g₁ = 2.016, g₂ = 2.012, and g₃ = 2.002 [35,38,39]. Therefore, we assigned the unresolved signals to O₂^•− and O_L^•− to anatase. However, the O₂^•− signal was not formed for P800-H700, even under UV irradiation. The weak and broad signal at g = 2.015 can be assigned to O_L^•− on rutile. The absence of the O₂^•− signal suggests that a one-electron reduction pathway (O₂ + e_cb⁻ → O₂^•−) does not occur in Ti³⁺-rich rutile (P800-H700). A similar result was reported for the Bi₂WO₆ photocatalyst [44].

2.5. Effect of H₂O₂ Addition on the Photocatalytic Activity

The fact that one-electron oxygen reduction was not involved in the photocatalysis of rutile TiO₂ motivated us to confirm the presence of a two-electron pathway (O₂ + 2H⁺ + 2e_cb⁻ → H₂O₂). Table 1 summarizes the formation of H₂O₂ during the photocatalytic reaction of acetic acid decomposition. H₂O₂ was analyzed by the colorimetric method using the oxidation of iodide to triiodide ions (Equations (1) and (2)) [45,46,47].

H₂O₂ + 2H⁺ + 2I⁻ → 2H₂O + I₂(1)

I₂ + I⁻ → I₃⁻(2)

We confirmed that H₂O₂ was formed in the photocatalytic reaction over 20 min; however, the formed amount was significantly low for P800-H700 (Table 1). Because H₂O₂ is easily decomposed, we investigated the degradation behavior of H₂O₂ under photocatalytic conditions. Table 2 summarizes the effect of the presence of 9 µmol of H₂O₂ in the feed. Without TiO₂ photocatalysts, H₂O₂ did not decompose even under light irradiation. In contrast, the added H₂O₂ did not remain after 20 min of photocatalysis over Ti³⁺-rich rutile TiO₂. This fast degradation can explain the low amount of H₂O₂ formed during the photocatalytic reaction. Notably, CO₂ evolution was significantly enhanced by the H₂O₂ addition for P800-H700. This suggests that H₂O₂ promotes the decomposition of acetic acid over the Ti³⁺-rich rutile TiO₂.

3. Discussion

The flat band potential of rutile TiO₂ is reported to be +0.05 or +0.13 V vs. the standard hydrogen electrode (SHE) [30,31]. The flat band potential of anatase is more negative than that of rutile TiO₂ by 0.20–0.26 eV [8,48]. Thus, the CBM of anatase and rutile is roughly assumed to be located at approximately −0.2 and 0 V vs. SHE, respectively (Figure 1b). To reduce oxygen, the CBM should be more negative than the standard electrode potentials of the oxygen reduction reactions, as expressed in Equations (3)–(5) [49].

O₂ + e⁻ = O₂^•−, E° = −0.33 V vs. SHE(3)

O₂ + H⁺ + e⁻ = HO₂^•, E° = −0.046 V vs. SHE(4)

O₂ + 2H⁺ + 2e⁻ = H₂O₂, E° = +0.695 V vs. SHE(5)

As shown in Figure 1, the CBM of anatase is close to or more negative than the thermodynamic theoretical potential for one-electron reduction to form oxygen radicals (O₂^•− and HO₂^•). In contrast, the CBM of rutile TiO₂ cannot promote one-electron reduction, as demonstrated by ESR. Nevertheless, two-electron reduction of oxygen is thermodynamically possible for rutile. In addition, the formation of H₂O₂ during the photocatalytic reaction was confirmed (Table 1). H₂O₂ formation has also been reported in Pt-WO₃ and Bi₂WO₆ photocatalysts with deep CBM [46,47,50].

The low amount of H₂O₂ during photocatalysis can be explained by the rapid degradation by the e_cb⁻ in the rutile photocatalyst, as expressed in Equation (6) [46,49].

H₂O₂ + e⁻ = ^•OH + OH⁻, E° = +1.14 V vs. SHE(6)

The in situ-generated H₂O₂ can act as an electron scavenger to retard the recombination of photogenerated carriers [50]. The reaction of e_cb⁻ with H₂O₂ is easier than with O₂. Moreover, the hydroxyl radical (^•OH) formed is a very strong oxidant (E° = +2.38 V vs. SHE), thus promoting acetic acid degradation [5,10,51]. This hypothesis agrees with the enhanced CO₂ formation by adding H₂O₂ (Table 2).

Figure 8 illustrates the proposed reaction scheme for a highly active rutile TiO₂ photocatalyst for the oxidative decomposition of acetic acid. It can be observed that charge separation is enhanced in the Ti³⁺-rich rutile under UV and violet light irradiation (λ < ~415 nm). The photogenerated hole can oxidize acetic acid to the CH₃COO^• radical, which will be ^•CH₃ radical vis decarboxylation [52]. The two-electron reduction of oxygen by e_cb⁻ is the plausible pathway in thermodynamics. The formed H₂O₂ retards the recombination and further accelerates the decomposition of acetic acid via radical pathways.

4. Materials and Methods

Evonik/Degussa Aeroxide TiO₂ P 25 was obtained from Nippon Aerosil Co. (Yokkaichi, Japan). Calcination in air was performed in an alumina crucible in an electric furnace at 800 °C for 2 h. The treatment under H₂ flow (50 mL min⁻¹) was performed in a quartz boat using a quartz tube-type reactor at 700 °C for 2 h. The temperature was naturally cooled to 300 °C under H₂ flow and to room temperature under N₂ flow (100 mL min⁻¹).

SEM images were recorded using a Hitachi S-5200 field emission scanning electron microscope (Tokyo, Japan). The BET-specific surface areas were determined at −196 °C using a MicrotracBEL BELSORP-mini instrument (Osaka, Japan). The XRD patterns were recorded using a Rigaku RINT-2000 diffractometer (Akishima, Japan). UV-visible spectra were obtained using an ALS SEC2000-UV/vis spectrometer (Tokyo, Japan) with a Hamamatsu Photonics L10290 fiber light source (Hamamatsu, Japan).

The photocatalytic activity was examined by measuring CO₂ evolution from an aqueous solution of 1 vol% acetic acid. The reactor was a glass test tube with an outer diameter of 18 mm and the volume of the liquid was 9.0 mL. A suspension of 50 mg TiO₂ particles was sonicated for 1 min and bubbled with oxygen for 5 min. After stirring for 30 min in the dark, photoirradiation was performed using a LED at 25 °C. The evolved CO₂ was detected using a TCD-GC with a Porpak-Q column and helium carrier. The concentration of H₂O₂ was quantified from the UV-visible spectra of I₃⁻ formed from H₂O₂ (2 mL), 0.2 M KI (1 mL), and 0.2 M H₂SO₄ (1 mL) at 25 °C. The absorbance of I₃⁻ was measured at 351 nm wavelength.

The ESR spectra were recorded at −253 °C using liquid helium on a JEOL JES-RE1X (Akishima, Japan). The ESR sample tube was irradiated using an ultra-high-pressure mercury lamp with a 365 nm band path filter. A JEOL JES-X320 instrument (Akishima, Japan) was used for the oxygen reduction reaction at −150 °C. UV irradiation was performed using a xenon lamp through a 365 nm band path filter. The samples were pre-evacuated at room temperature for 60 min on a vacuum line before the ESR measurements, and oxygen gas (1.0 Torr) was added to the ESR sample tube at 25 °C.

5. Conclusions

We investigated TiO₂ photocatalysts for the oxidative decomposition of acetic acid to CO₂ under UV and violet light irradiation. Rutile TiO₂ samples were prepared from Evonik TiO₂ P 25 through calcination at 800 °C, followed by H₂ treatment at 700 °C. H₂-treated TiO₂ exhibited high photocatalytic activity, which is comparable to highly active anatase TiO₂, despite its rutile structure and low specific surface area. The high photocatalytic activity under UV irradiation was due to the increase in Ti³⁺ density and the improvement in charge separation, as confirmed by ESR. In the presence of O₂, anatase TiO₂ promoted the formation of O₂^•− and O_L^•−; however, rutile TiO₂ did not exhibit an O₂^•− signal. This is because the conduction band edge of rutile is not suitable for the one-electron reduction of oxygen. We observed that the two-electron reduction of O₂ to H₂O₂ was dominant in rutile TiO₂ photocatalysis, and the in situ-formed H₂O₂ accelerated the decomposition of acetic acid. We also found that Ti³⁺-rich rutile TiO₂ can work even under violet light irradiation (wavelength = 405 nm) owing to its narrower E_g (3.0 eV), and its intrinsic activity per unit surface area is significantly high. The apparent quantum efficiency of H₂-treated TiO₂ for CO₂ evolution was 4.9% at 405 nm, where conventional TiO₂ photocatalysts do not work efficiently.

Author Contributions

Investigation and visualization, F.A., A.Y. and J.K.; writing the original draft, F.A. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. (a) Effect of calcination temperature on the properties of TiO2. (b) Schematic energy diagram of anatase and rutile TiO2 and the standard electrode potentials for the oxygen reduction reaction. The conduction band (CB) minimum positions were assumed from the reported flat band potentials, and the valence band (VB) maximum positions were derived from their band gap energies.

Figure 2. FE-SEM images of TiO2 particles (P 25, P800, and P800-H700).

Figure 3. XRD patterns of TiO2 powders mixed with NiO as an internal standard: (●) Rutile TiO2, (▲) anatase TiO2, and (■) NiO.

Figure 4. (a) Diffuse reflectance UV-visible spectra of TiO2 samples and emission spectra of 380 and 405 nm light-emitting diodes (LEDs) used for photocatalytic reaction. (b) Photograph of TiO2 powders: P800 and P800-H700.

Figure 5. Photocatalytic CO2 evolution through oxidative decomposition of acetic acid under (a) 380-nm UV irradiation (9 mW cm−2) and (b) 405-nm violet light irradiation (18 mW cm−2). Photocatalyst: 50 mg, liquid phase: 1 vol% CH3COOH aq. (9.0 mL), gas phase: Oxygen (15 mL), and temperature 25 °C.

Figure 6. (a) ESR spectra of P800 and P800-H700 with helium (18 Torr) at −253 °C. (b) Difference spectra between before and after UV irradiation at 365 nm.

Figure 7. ESR spectra at −150 °C of (a) P 25 and (b) P800-H700. The sample was measured after (i) evacuation at 25 °C for 1 h, (ii) introduction of 1.0 Torr of oxygen, and (iii) UV irradiation with oxygen. The difference spectra between before and after photoirradiation are also shown.

Figure 8. Schematic of the (a) proposed reaction mechanism and (b) energy diagram for oxidative decomposition of acetic acid by Ti3+-rich rutile TiO2 photocatalysis.

Table 1

Photocatalytic reaction in 1 vol% CH₃COOH aqueous solution after violet light irradiation for 20 min.

Photocatalyst	Formed H2O2/µmol	Evolved CO2/µmol
P 25	0.17	1.19
P800	0.03	0.88
P800-H700	0.09	3.36
blank	–	0.00

Table 2

Photocatalytic reaction in 1 vol% CH₃COOH aqueous solution with 9 µmol H₂O₂ after violet light irradiation for 20 min.

Photocatalyst	Residual H2O2/µmol	Evolved CO2/µmol
P 25	2.93	0.89
P800	1.48	2.39
P800-H700	0.03	8.60
blank	9.39	0.00

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal12101079/s1. Figure S1: ESR spectrum of pure O₂ recorded at −150 °C.

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