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Article

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

by
Fumiaki Amano
1,*,
Akira Yamamoto
2 and
Jun Kumagai
3
1
Department of Applied Chemistry for Environment, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
2
Department of Interdisciplinary Environment, Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-Nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan
3
Institute of Materials and Systems for Sustainability, Division of Materials Research, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(10), 1079; https://doi.org/10.3390/catal12101079
Submission received: 5 September 2022 / Revised: 16 September 2022 / Accepted: 17 September 2022 / Published: 20 September 2022
(This article belongs to the Special Issue UV/Vis/NIR Photocatalysis and Optical Properties)
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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.

Graphical Abstract

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 (Eg = 3.0 eV, onset wavelength λonset = 413 nm) being narrower than that of anatase (Eg = 3.2 eV, λonset = 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 (ecb−) 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 (SSABET), 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 (λonset) 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 λonset of P800 and P800-H700 was ~415 nm (2.99 eV), which is consistent with the Eg 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 SSABET (9.8 m2 g−1) also exhibited high activity, which is comparable to that of highly active P 25 (SSABET = 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 (OL•−) 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 ecb− is used for O2 reduction, the valence band hole is trapped on the surface (OL•−). The g-values of OL•− 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 OL•− 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 OL•− on rutile. The absence of the O2•− signal suggests that a one-electron reduction pathway (O2 + ecb− → 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+ + 2ecb− → 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
I2 + I− → I3−
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
O2 + H+ + e− = HO2•, E° = −0.046 V vs. SHE
O2 + 2H+ + 2e− = H2O2, E° = +0.695 V vs. SHE
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 ecb− in the rutile photocatalyst, as expressed in Equation (6) [46,49].
H2O2 + e− = •OH + OH−, E° = +1.14 V vs. SHE
The in situ-generated H2O2 can act as an electron scavenger to retard the recombination of photogenerated carriers [50]. The reaction of ecb− 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 ecb− 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 OL•−; 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 Eg (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.

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.

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.

Funding

This research was funded by Japan Society for the Promotion of Science (JSPS KAKENHI) (grant number JP20H02525).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01079 g001 550
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 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.
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Catalysts 12 01079 g002 550
Figure 2. FE-SEM images of TiO2 particles (P 25, P800, and P800-H700).
Figure 2. FE-SEM images of TiO2 particles (P 25, P800, and P800-H700).
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Catalysts 12 01079 g003 550
Figure 3. XRD patterns of TiO2 powders mixed with NiO as an internal standard: (●) Rutile TiO2, (▲) anatase TiO2, and (■) NiO.
Figure 3. XRD patterns of TiO2 powders mixed with NiO as an internal standard: (●) Rutile TiO2, (▲) anatase TiO2, and (■) NiO.
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Catalysts 12 01079 g004 550
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 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.
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Catalysts 12 01079 g005 550
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 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.
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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 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.
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Catalysts 12 01079 g007 550
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 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.
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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.
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.
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Table
Table 1. Photocatalytic reaction in 1 vol% CH3COOH aqueous solution after violet light irradiation for 20 min.
Table 1. Photocatalytic reaction in 1 vol% CH3COOH aqueous solution after violet light irradiation for 20 min.
PhotocatalystFormed H2O2/µmolEvolved CO2/µmol
P 250.171.19
P8000.030.88
P800-H7000.093.36
blank–0.00
Table
Table 2. Photocatalytic reaction in 1 vol% CH3COOH aqueous solution with 9 µmol H2O2 after violet light irradiation for 20 min.
Table 2. Photocatalytic reaction in 1 vol% CH3COOH aqueous solution with 9 µmol H2O2 after violet light irradiation for 20 min.
PhotocatalystResidual H2O2/µmolEvolved CO2/µmol
P 252.930.89
P8001.482.39
P800-H7000.038.60
blank9.390.00
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Amano, F.; Yamamoto, A.; Kumagai, J. Highly Active Rutile TiO2 for Photocatalysis under Violet Light Irradiation at 405 nm. Catalysts 2022, 12, 1079. https://doi.org/10.3390/catal12101079

AMA Style

Amano F, Yamamoto A, Kumagai J. Highly Active Rutile TiO2 for Photocatalysis under Violet Light Irradiation at 405 nm. Catalysts. 2022; 12(10):1079. https://doi.org/10.3390/catal12101079

Chicago/Turabian Style

Amano, Fumiaki, Akira Yamamoto, and Jun Kumagai. 2022. "Highly Active Rutile TiO2 for Photocatalysis under Violet Light Irradiation at 405 nm" Catalysts 12, no. 10: 1079. https://doi.org/10.3390/catal12101079

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