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Enhanced Visible Light Response of TiO2 Codoped with Cr and Ta Photocatalysts by Electron Doping Fumiaki Amano, Masashi Nakata, Junie Jhon Magdadaro Vequizo, and Akira Yamakata ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00126 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 25, 2019

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ACS Applied Energy Materials

Enhanced Visible Light Response of TiO2 Codoped with Cr and Ta Photocatalysts by Electron Doping Fumiaki Amano a,b,*, Masashi Nakata a, Junie Jhon M. Vequizo c, Akira Yamakata c a

Department of Chemical and Environmental Engineering, The University of Kitakyushu, 1-

1 Hibikino, Wakamatsu, Kitakyushu, Fukuoka 808-0135, Japan. b

Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and

Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan. C

Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku,

Nagoya 468-8511, Japan. AUTHOR INFORMATION Corresponding Author *Fumiaki Amano, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The importance of electron concentration in rutile TiO2 photocatalysts motivated us to apply H2 reduction treatment to visible-light-responsive TiO2 photocatalysts sensitized by chromium ion doping. We found that H2 reduction treatment of rutile TiO2 particles codoped with Ta and Cr (TiO2:Ta/Cr) enhanced photocatalytic activity for O2 evolution by water oxidation under visible irradiation (> 2.2 eV). The enhanced visible light activity of H2-treated TiO2:Ta/Cr was attributed to the increase of electron concentration, which was confirmed by UV-vis diffuse reflectance and electron spin resonance (ESR) spectroscopy. The H2-treated TiO2:Ta/Cr photocatalyst was repeatedly used in aqueous media in spite of the presence of doped electrons. Photoluminescence and transient absorption spectroscopies revealed that electron doping with H2 treatment decreased the midgap states working as deep traps of photoexcited electrons, and increased the accumulation of the photoexcited electrons in the conduction band. The optimized H2 reduction temperature was decreased with an increase in the amount of higher valence Ta5+ used as a donor-type dopant. This study shows that the precise control of the bulk electronic structure of rutile TiO2 by a combination of codoping and H2 reduction treatment improves the visible-light-driven photocatalytic activity owing to the decrease of the trapping sites at deep energy levels and the recombination between deeply trapped electrons and valence band holes.

KEYWORDS Titanium dioxide, Reduced rutile, Visible light, Water oxidation, Photoexcited electron dynamics, Chromium, Tantalum


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INTRODUCTION Visible-light-driven photocatalysts for water splitting are expected to be developed to produce H2 using solar energy. TiO2 is the most studied photocatalyst because of its high stability, low toxicity, and relatively high activity for many photocatalytic reactions under UV irradiation. However, the wide bandgap of TiO2 (3.2 eV for anatase, 3.0 eV for rutile) is a disadvantage for sunlight utilization. Therefore, band engineering by impurity doping of anions such as nitrogen has been attempted to sensitize TiO2 to visible light contained in sunlight in large amounts.1,2 Metal cation doping of TiO2 has been reported to have low photocatalytic performance,3,4 while Cr-doped TiO2 exhibited a visible light response to a small extent.5-7 H2 reduction treatment is also a technique to enhance the visible light absorption by a color change from white to black through the introduction of disorder in the surface layers of the hydrogenated "black" TiO2 nanocrystals.8-12 Kudo et al. reported that rutile TiO2 codoped with chromium and antimony (TiO2:Cr/Sb), rhodium and antimony (TiO2:Rh/Sb), nickel and tantalum or niobium (TiO2:Ni/(Ta, Nb)) showed visible light response to photocatalytic O2 evolution by four-electron oxidation of water in the presence of silver ion (Ag+) as an electron acceptor.13-17 The occupied d orbitals of Rh3+ (4d6), Cr3+ (3d3), and Ni2+ (3d8) create electron donor levels and sub-bands in the band gap of TiO2.13-15 The lower valence cations are commonly used as an acceptor-type dopant of TiO2. For example, the incorporation of Cr3+ into the TiO2 lattice at low oxygen activity generates oxygen vacancies as expressed by Eq. 1 using Kröger–Vink notation.18 TiO2

Cr2O3

2CrTi + 3OO× + V•• O

(1)

where CrTi' is Cr3+ in the Ti4+ lattice site, OO× is O J in the oxygen lattice site, and VO•• is the oxygen vacancy with a double positive charge. However, when oxygen activity is high, electrons (e') react with molecular oxygen to form lattice oxygen according to the reaction of Eq. 2.


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2 V•• O + 2e + 1/2 O

OO×

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(2)

This reaction decreases the concentration of electrons in n-type oxides. However, the lower valence dopants (Rh3+, Cr3+, Ni2+) are frequently oxidized to undesirable higher valence cations (Rh4+, Cr6+, and Ni3+) at high temperatures in the presence of oxygen. Kudo et al. reported that the co-doping of Sb5+ and Ta5+ with a valence higher than that of Ti4+contributed to keep the charge balance by suppressing the formation of undesirable Rh4+ and Cr6+, and resulted in the success of the development of visible-light-responsive TiO2 and SrTiO3 photocatalysts.1315,17,19-22

However, the photocatalytic activity of TiO2-based photocatalysts doped with

transition metal cations is still not high enough to meet the demand for practical use under visible light irradiation. Thus, further understanding is necessary to develop visible-lightresponsive TiO2 photocatalysts for water splitting. Amano et al. have found that the photocatalytic activity of rutile TiO2 particles was decreased by high-temperature calcination, but the activity was recovered by H2 reduction treatment.23,24 The deactivation by calcination at high temperatures is attributed to the decrease of long-lived photoexcited electrons in TiO2. Amano et al. also found that H2 reduction treatment at 500°C–700°C is an effective method to enhance the photocatalytic activity of rutile TiO2 with large particle sizes for both H2 evolution and O2 evolution from water.24-27 The enhanced photocatalytic activity of H2-reduced rutile TiO2 is explained by the introduction of electrons rather than oxygen vacancies.24,28 The increased electron concentration in large TiO2 crystallites may enhance the charge separation and charge transport owing to the formation of a built-in electric field (upward band bending) at the water interface and the improvement of electrical conductivity.25,28,29 This indicates the importance of the control of electron concentration for rutile TiO2 photocatalysts by doping treatments. Considering that the transition metal-doped TiO2 photocatalysts were prepared using the solid-state reaction method, the sample may be deactivated by high-temperature calcination


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during the preparation.25 Based on this hypothesis, we performed H2 reduction treatment on metal-doped TiO2 to improve the photocatalytic activity under visible light excitation. Among the candidates (Cr3+, Ni2+, and Rh3+) for visible light sensitization, we selected Cr3+ as a doping cation as Ni2+ and Rh3+ may be reduced to metal by H2 reduction treatment. The toxicity of Cr3+ is low on the contrary to Cr6+. We selected Ta5+ rather than Sb5+ as a dopant for the charge compensation as Sb5+ may be reduced to Sb3+ under H2 treatment. The higher valence cations has been used as a donor-type dopant for the modification of the semiconducting properties of TiO2.18,30,31 The prepared TiO2 particles codoped with Ta and Cr (TiO2:Ta/Cr) were treated with H2 at different temperatures. The photocatalytic activity was investigated using O2 evolution by water oxidation in the presence of Ag+ as a sacrificial electron acceptor under visible light irradiation. The effects of dopant ratio and H2 reduction treatment were characterized by diffuse reflectance UV-vis spectroscopy and electron spin resonance (ESR) spectroscopy. The dynamics of the photoexcited electrons were evaluated using photoluminescence (PL) and infrared (IR) absorption measurements.

EXPERIMENTAL SECTION Preparation of photocatalysts. TiO2 co-doped with Ta and Cr (TiO2:Ta/Cr) was prepared by the solid-state reaction method. Powders of rutile TiO2 (99.99%, about 2 µm), Ta2O5 (99.9%), and Cr2O3 (99.9%) purchased from Kojundo Chemical Laboratory (Sakado, Japan) were mixed using an alumina mortar and calcined in air at 1150°C for 10 h using an alumina crucible and a box furnace. The content of doping cations (Ta5+ and Cr3+) was adjusted to 2.5 mol% as a metals basis, (Ta + Cr) / (Ti + Ta + Cr) = 0.025. We changed the atomic ratio of Ta to Cr (Ta/Cr ratio) from 1.0 to 2.0 (Supporting Information, Table S1). The samples with different Ta/Cr ratios of x were denoted as TiO2:Ta/Cr(x). Thus obtained TiO2:Ta/Cr was heated under a stream of H2 at a flow rate of 50 mL/min at temperatures from 300°C to 700°C for 2 h using a quartz


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boat and a tube furnace. The temperature was naturally decreased to 300°C under the H2 flow and to room temperature in a N2 flow of 100 mL/min to purge H2. The H2-treated samples at y00°C are denoted as TiO2:Ta/Cr-Hy00(x). Non-doped TiO2 was prepared by calcination of TiO2 in air at 1150°C for 10 h. TiO2 codoped with Sb and Cr (TiO2:Sb/Cr(1.5), Sb = 1.5 mol%, Cr = 1.0 mol%) was prepared by a solid-state reaction method using Sb2O5 (99.9%) at 1150°C for 10 h.13,20 WO3 (99.99%) and CrO3 (99.9%) were purchased from Kojundo Chemical Laboratory.

Characterization. Brunauer–Emmett–Teller (BET) specific surface area was determined from N2 absorption isotherms measured at J L8M after pretreatment by evacuation at 200°C for 2 h (Bel Japan; BELSORP-mini). The morphology of the particles was observed using a scanning electron microscope (SEM, Hitachi; S-5200) and transition electron microscope (TEM, JEOL; JEM-3010). The crystal phase of TiO2 samples was confirmed by X-ray diffraction (XRD) pattern (Rigaku; RINT-2000/PC) using Cu KT radiation. The TiO2 sample was mixed with 30 wt% nickel oxide powder as an internal standard in an agate mortar. X-ray photoelectron spectroscopy (XPS) measurements were performed using Al KT radiation (Kratos; AXIS His). Chemical compositions were determined using an X-ray fluorescence (XRF) spectrometer (Rigaku; NEX CG). X-band electron spin resonance (ESR) spectra were recorded at J .,M (JEOL; JES-X320). The sample was diluted with

-Al2O3 and pre-evacuated at room

temperature before the ESR measurements. Diffuse reflectance spectra were recorded using barium sulfate as a standard material on a UV-vis spectrometer (ALS; SEC2000) with a fiber light source (Hamamatsu Photonics; L10290). The optical band gap was calculated from the Tauc plot assuming indirect allowed transition as expressed in Eq. 3.


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(F(R) !)0.5 = A ( ! – Eg)

(3)

where F(R) is the Kubelka-Munk function of the diffuse reflectance spectra, ! is the incident photon energy, and A is a constant.

Photocatalytic activity test. The photocatalytic activity was investigated using O2 evolution by water oxidation in the presence of silver cations as a sacrificial electron acceptor (4Ag+ + 2H2O U 4Ag0 + O2 + 4H+). The photocatalyst powder (50 mg) was dispersed in an aqueous solution of silver nitrate (50 mmol/L, 9.0 mL) in a glass tube with an outside diameter of 18 mm. The suspension was purged with argon for 10 min, and sealed with a rubber plug. Photoirradiation for the magnetically stirred suspension was performed at room temperature using blue light emitting diodes (LEDs, OptoSupply; OSUB5111A) attached with a cutoff filter (Sigma Koki; 44Y). The wavelength distribution of the emitted light shows a 470-nm peak wavelength and 25-nm bandwidth (Supporting Information, Figure S1). The irradiance of the blue light was measured to be about 20 mW/cm2 at the surface of the glass tube using an optical power meter and a sensor (Hioki; 3664 and 9742). The amount of evolved O2 in the gas phase was quantified every 20 min by a gas chromatograph (Shimadzu; GC-8A) with a Molecular Sieve 5A column and a thermal conductivity detector using an argon carrier. For the measurement of the action spectrum, photoirradiation was performed using monochromatic light emitted from a 300-W xenon lamp with bandpass filters (Asahi Spectra; MAX-303). The apparent (or external) quantum efficiency (AQY) for O2 evolution was calculated as the ratio of the number of electrons consumed for the four-electron process to the number of incident photons. The intensity of the incident photons was measured in the range of 9.4—26.6 mW for each wavelength (Table S2).


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PL and transient absorption spectroscopy. PL measurement was performed in vacuum using a continuous wave 375-nm laser diode (Toptica Photonics; iBeam smart) with a power of 6.5 mW and a CCD camera (Princeton Instruments; PIXIS:100F) attached to the grating spectrometer (Acton; SP2300). Microsecond transient absorption measurements were performed in reflection mode under 470-nm laser pulse irradiation originating from a Nd:YAG laser (Continuum; Surelite OPO, duration 6 ns, power 5 mJ, repetition rate 0.1–5.0 Hz). The probe light emitted from a 50-W halogen lamp was focused on the sample and detected using Si or InGaAs photodiodes. The output electric signal was amplified with an AC-coupled amplifier (Stanford Research Systems; SR560). FT-IR (Bruker Optics; Vertex-80) was used for the electron accumulation process. The sample was photoexcited by a continuous wave 470-nm LED light (Thorlab; M470L3, 10 mW/cm2), and the absorption change of the IR light was measured using the transmission mode.

RESULTS AND DISCUSSION Optical properties of the prepared photocatalysts. Figure 1 shows UV-vis-NIR diffuse reflectance spectra of non-doped TiO2 and TiO2:Ta/Cr samples with different Ta/Cr ratios. Non-doped TiO2, which is a white powder, showed intense photoabsorption with an onset at about 420 nm corresponding to the interband transitions of rutile TiO2 (band gap 3.0 eV). All TiO2:Ta/Cr samples showed new absorption bands in the visible light region in addition to the interband absorption. The TiO2:Ta/Cr(1.0) was a dark brown powder and showed a broad absorption with an onset at about 900 nm, which was attributed to a Cr6+ species of crystalline CrO3 (Figure S2). When TiO2:Ta/Cr(1.0) was treated with H2 at 500°C, the color changed to orange. In contrast, the samples with a Ta/Cr ratio greater than 1.5 were originally orange powders. This suggests that the doping of excess amounts of Ta5+ ions suppresses the formation of the Cr6+ species.13,19,20 The absorption spectra showed an onset at about 620 nm and a weak


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band maximized at 720 nm. The 720-nm band is assigned to the d-d transition (4A2g

4T ) 2g

of Cr3+ ions in octahedral symmetry.13 This indicates that the Cr3+ species is incorporated in the Ti4+ lattice site of TiO2. The absorption spectra of TiO2:Ta/Cr(2.0) were significantly different from the crystalline Cr2O3 green powder (Figure S2). The optical band gaps of the non-doped TiO2 and Ta5+-doped TiO2 were estimated to be 3.0 eV from their Tauc plots. The energy corresponds to the band gap of rutile TiO2. The energy gap of TiO2:Ta/Cr(2.0) was estimated to be 2.1 eV, which was similar to the reported values for Cr-doped TiO2 and SrTiO3.13,19,20 The narrow energy gap shows the formation of the in-gap levels owing to the doping of Cr3+ species in rutile TiO2. Figure 2 shows the diffuse reflectance spectra of TiO2:Ta/Cr(2.0) samples treated with H2 at different temperatures. The absorption edge at 620 nm was hardly changed by the H2 treatment. The optical energy gap of TiO2:Ta/Cr-H400(2.0) was estimated to be 2.2 eV from the Tauc plot. In contrast, the photoabsorption intensity at the NIR region was increased with an increase in the H2 treatment temperature. The d-d transition peak at 720 nm gradually disappeared in the broad absorption at the NIR region. The NIR absorption is assigned to the excitation of the shallowly trapped electrons to the conduction band (CB) and the intraband transition of free electrons in the CB. The spectra features indicate that the electron concentration of TiO2:Ta/Cr(2.0) increased with an increase in H2 treatment temperature. Thus, electron doping of TiO2:Ta/Cr was achieved by H2 reduction treatment.

Characterization of the photocatalysts. The BET specific surface area of TiO2:Ta/Cr(2.0) was slightly higher than that of non-doped TiO2 as shown in Table 1. Calcination at 1150°C decreased the specific surface area from 2 m2/g of the commercial rutile TiO2 to 1 m2/g owing to particle size growth. The particle size estimated from the specific surface area was consistent with that observed in SEM images (Figure S3). H2 reduction treatments did not significantly


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change the BET specific surface area, the size, and the morphology of the particles. In the case of hydrogenated "black" TiO2, surface disorder is introduced in the nanosized anatase particles by H2 treatment.8,10 However, such significant change of the surface structure was not confirmed in TEM observation (Figure S4). XRD pattern analysis showed that the weight fraction of rutile in the anatase-rutile mixture was 96.6% for the commercial rutile TiO2. The XRD pattern of non-doped TiO2 indicates that calcination at 1150°C changed the mixture to a single phase of rutile TiO2 (PDF 00-021-1276, Figure S5). We found that TiO2:Ta/Cr(2.0) contained very small peaks with 2# values of 22.9° and 28.3° assigned to orthorhombic Ta2O5 (PDF 00-025-0922). It is reported that Ta5+ species are enriched on the surface of Ta2O5-doped TiO2 under oxidizing conditions due to slower diffusion kinetics.32 In contrast, the absence of Cr2O3 suggests that Cr3+ was highly dispersed and incorporated into the TiO2 host crystalline structure. The XRD peaks of Ni-doped TiO2 are reportedly shifted to lower angles compared with those of non-doped TiO2 as the crystal radius of Ni2+ (0.83 Å) is larger than that of 6-coordinated Ti4+ (0.745 Å).14,33 However, the peaks of TiO2:Ta/Cr did not shift, probably because the crystal radius of Ta5+ (0.78 Å) and Cr3+ (0.755 Å) is similar to that of Ti4+.33 The XRD patterns of TiO2:Ta/Cr were not changed by H2 reduction treatment at 350°C–700°C. XRF measurement revealed that the chemical composition of TiO2:Ta/Cr was also not changed by H2 treatment at 400°C (Table S3). Figure 3 shows the ESR spectra of the TiO2:Ta/Cr(2.0). Five line signals with g values of 1.38, 1.67, 2.64, 4.94, and 5.62 appeared in the ESR spectrum in the range of 0 to 500 mT.3,34,35 The resonance peaks are assigned to the zero-field splitting of Cr3+(d3, S = 3/2) incorporated in TiO2 crystallites.36,37 The Cr3+ is substituted to the Ti4+ lattice site in distorted octahedral symmetry. There was no broad symmetrical line centered at about g = 1.98 (%-phase resonance) assigned to the Cr2O3 cluster.3,34,36 This indicates that the Cr3+ species is highly dispersed in the TiO2 host. The signal intensity of the substituted Cr3+ species was dependent on the content


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of Cr3+ in the TiO2:Ta/Cr samples. For the sample with Ta/Cr = 1.0, the substituted Cr3+ species was slightly small owing to Cr6+ species formation (Figure S6). This was consistent with the UV-vis-NIR diffuse reflectance spectroscopy result of TiO2:Ta/Cr(1.0). We did not observe a sharp line with a g value of 1.97 assigned to the surface Cr5+(d1) species (&-phase resonance).3,34-36 H2 reduction treatment at temperatures higher than 300°C decreased the intensity of the five lines of the Cr3+ species substituted in the TiO2 lattice (Figure 3). ESR signal due to the Ti3+ (d1) species was not formed after H2 reduction treatment in contrast to the case of non-doped TiO2.24,27,29 This indicates that the isolated Cr3+ species is changed to an ESR-silent species and Ti4+ is not reduced by H2 treatment. It is usually difficult to detect the ESR signal of Cr2+(d4) and Cr+(d5) even though the species in a distorted environment is ESR active. XPS showed that the oxidation state of the surface Cr3+ species was not changed after H2 reduction treatment and the majority of the oxidation states were Ti4+, Cr3+, and Ta5+ on the surface of H2-reduced TiO2:Ta/Cr (Figure S7). This suggests that the electron doped by H2 treatment is not localized on the Cr3+ species. The low amplitude of the ESR signal in H2-reduced TiO2:Ta/Cr may suggest that the zero-field splitting of Cr3+ species is interacted with the free electrons in the CB and shallow trap states, which was confirmed by the NIR absorption by H2 reduction treatment.

Photocatalytic activity. Non-doped TiO2 and TiO2:Ta exhibited photocatalytic activity for O2 evolution from AgNO3 solution under UV irradiation.28 However, the TiO2:Ta/Cr(2.0) and TiO2:/Cr samples showed negligible activity under irradiation of not only visible light but also UV light (Figure S8). This would be because the doped chromium species work as recombination centers. The photocatalytic activity of non-doped TiO2 under UV irradiation


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was enhanced by electron doping with H2 treatment as reported previously,24,28,29 but it did not work under visible light irradiation unlike the "black" TiO2 prepared by H2 treatment.8 We found that H2-treated TiO2:Ta/Cr induced photocatalytic O2 evolution by water oxidation under 470-nm blue LED irradiation (Figure 4A). H2 treatment at temperatures higher than 300°C enhanced the photocatalytic activity of TiO2:Ta/Cr(2.0). The rate of visible-light-driven O2 evolution was maximized by H2 treatment at 400°C. However, temperatures higher than 600°C significantly diminished the photocatalytic activity. The optimized H2 treatment temperature was dependent on the Ta/Cr ratio in the 2.5 mol% cation-doped TiO2: the photocatalytic O2 evolution rate was maximized at 500°C in the case of TiO2:Ta/Cr(1.5) (Figure 4B). The optimized H2 treatment temperature decreased with an increase of the amount of higher valence Ta5+. Action spectrum analysis revealed that the photocatalytic O2 evolution over H2-treated TiO2:Ta/Cr was promoted under 550-nm irradiation, which corresponds with the energy gap of 2.2 eV estimated from the UV-vis spectrum (Figure 5, Table 2). This indicates that the visible light response is attributed to the mid-gap state corresponding to the Cr3+ species. The negligible photocatalytic activity at wavelengths longer than 600 nm indicates that the excitation of the d-d transition of Cr3+ and the photoabsorption of free electrons in CB are not effective to induce water oxidation. The apparent quantum efficiency was about 0.5% under the visible light region (> 2.2 eV), and less than that under UV irradiation. Therefore, the visible-light-driven process induced by Cr doping was moderate compared with the band gap photoexcitation process of rutile TiO2. The rate of O2 evolution over TiO2:Ta/Cr-H400(2.0) was 2.3 Z

? under 470-nm blue

LED irradiation. Photocatalytic activity was much higher than that of visible-light-driven photocatalysts for water oxidation such as WO3 and TiO2:Sb/Cr(1.5) (Table 2 and Figure S9). The low photocatalytic activity of WO3 is explained by the fact that the band gap (2.6 eV) was


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not suitable for blue light absorption.38 The TiO2:Sb/Cr showed visible light activity as previously reported,13,20 but the O2 evolution rate was low compared with that of H2-treated TiO2:Ta/Cr(2.0). It should be noted that H2 treatment was not effective for TiO2:Sb/Cr contrary to the case of TiO2:Ta/Cr. We confirmed the stability of the H2-treated sample during the photocatalytic reaction (Figure S10). The O2 evolution rate slightly decreased with photoirradiation time. The pH of 6.1 in the initial solution was found to have decreased to about 3.0 after 200 min (Table S4). The decrease in pH was consistent with the value calculated from the amount of evolved O2 assuming the associated formation of H+ (2H2O + 4h+

O2 + 4H+). It has been reported that

acidic environments retard O2 evolution due to the suppression of Ag+ adsorption on the TiO2 surface.39 We confirmed that the photocatalytic activity recovered to the original value after changing the solution, even in the presence of Ag particles deposited on the photocatalyst surface. The reduced state of TiO2:Ta/Cr-H400 was stable in aqueous solution and under the continuous generation of the positive hole under irradiation.

Photoexcited electron dynamics. Photoluminescence (PL) spectra provide information about the trapping states of photogenerated charge carriers. The emission band of rutile TiO2 was reported to be observed at 840 nm (1.5 eV).40 The NIR emission can be assigned to a radiative deactivation of photoexcited electrons deeply trapped in crystalline defects such as oxygen vacancies.40 Figure 6 shows the PL spectrum of TiO2:Ta/Cr(2.0) by 375-nm photoexcitation. The doping of Ta and Cr drastically enhanced the NIR emission assigned to the deeply trapped electrons, suggesting that it led to the formation of crystalline defects working as a recombination center in the TiO2 lattice. In contrast, we found that the NIR emission at 840 nm was significantly decreased by H2 reduction treatment. This suggests that the density of the mid-gap states and the recombination of the deeply trapped electrons was reduced by the


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increase in electron concentration. There was a small visible emission at about 590 nm (2.1 eV) in addition to the large NIR emission at 840 nm. The radiative deactivation at visible regions (500–650 nm) can be assigned to the recombination of shallowly trapped electrons.40 The recombination dynamics of the shallowly trapped electrons were not affected by the electron doping of H2 treatment. The carrier dynamics after 470-nm pulse photoexcitation were investigated using visibleNIR transient absorption spectroscopy (Figure 7). The difference in the absorbance before and after the pulse, [

" shows the number of charge carriers generated in doped TiO2

samples by visible light excitation. The time-resolved visible–NIR absorption spectra of TiO2:Ta/Cr(2.0) show that there are two broad absorptions at around 11800 cmJ (850 nm) and 17000 cmJ (590 nm). The literature has assigned the broad peak at 11800 cmJ to the absorption by the electrons trapped at deep levels and the broad peak at 17000 cmJ to the absorption relating to both photoexcited electrons and holes in the case of rutile TiO2.41,42 The absorption at 11800 cmJ (850 nm) indicates that the photoexcited electrons in TiO2:Ta/Cr are trapped at defects located at 1.5 eV below the CB. In contrast, H2-treated TiO2:Ta/Cr exhibited weak transient absorption in the NIR region. The decrease of [

at 11800 cmJ

indicates that the number of deeply trapped electrons is significantly decreased by H2 reduction treatment at 450°C. We found that the deep level defects trapping photoexcited electrons were decreased with an increase in the electron density. It should be noted that measurement of the transient IR absorption was difficult for the TiO2:Ta/Cr samples after 470-nm pulse photoexcitation owing to the low signal-to-noise ratio and contribution of the breached absorbance. To confirm the photoexcited electron dynamics, we measured the [

change at

900 cmJ under 470-nm continuous photoexcitation (Figure 8). The IR absorption was assigned to the electrons in the CB and shallow trap states. The [

was increased with


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irradiation time, indicating the accumulation of photoexcited electrons and the slow electron recombination process. The saturated value of the [

shows the maximum electron

density under steady-state photoirradiation, where there is a balance between the generation rate by photoexcitation and the disappearance rate by recombination. The number of accumulated electrons in TiO2:Ta/Cr-H400(2.0) was slightly higher than that in TiO2:Ta/Cr (2.0), indicating that the lifetime of the photoexcited electrons in the CB was enlarged by H2 reduction treatment. The recombination rate is usually accelerated with the increase in the electron concentration in n-type semiconductors. In contrast, herein, we found that the recombination of photogenerated carriers is decelerated by the increase in the electron concentration for H2-reduced TiO2:Ta/Cr. However, the effect of H2 treatment on the CB electron dynamics was not significant to explain the difference in the visible-light-responsive photocatalytic activity.

Mechanism of the enhanced photocatalytic activity. We propose the energy levels of H2reduced TiO2:Ta/Cr photocatalysts (Figure 9) based on the following findings. 1) Diffuse reflectance spectra showed that the band gap of rutile TiO2 was 3.0 eV and the energy gap of the J transition of Cr 3d was 1.7 eV. 2) In PL spectra, the NIR emission due to the recombination between deeply trapped electrons and photogenerated holes indicates that the defects are located at 1.5 eV above the valence band (VB). 3) The transient absorption spectra indicate that the deep traps for photoexcited electrons are located at 1.5 eV below the CB. 4) The H2 reduction treatment significantly decreased the density of deep traps for photoexcited electrons and slightly increased the accumulation of long-lived photoexcited electrons in the CB and shallow traps as shown in Figures 6, 7, and 8.


15


Page 16 of 34

The d orbitals of Cr are split into t2g and eg groups in octahedral crystal field. There was no photocatalytic activity under the excitation of the d-d transition of Cr3+, (t2g)3

(t2g)2(eg)1, at

730 nm (1.7 eV). The O2 evolution was observed under irradiation of photons with energies higher than 2.2 eV. This suggests that the possible excitation mechanisms are those from Cr t2g orbitals to the CB, and from the VB to Cr eg orbitals. Considering the potential energies, the photoexcited electrons in the CB of TiO2 can reduce protons to H2 (2H+ + 2eJ

H2, E° = 0 V

vs. SHE). However, photocatalytic H2 evolution was not observed for H2-reduced TiO2:Ta/Cr in the presence of ethanol and platinum under visible light irradiation. This suggests that the photoexcited electrons in CB would be recombined with photogenerated holes before the promotion of H+ reduction over platinized TiO2:Ta/Cr even after H2 treatment. The accumulation of photoexcited electrons in the CB was not drastically affected by H2 reduction treatment in contrast to the significant decrease of the NIR emission assigned to deep trap states. This indicates that the contribution of the photoexcitation from Cr t2g orbitals to the CB of TiO2 is small for the visible-light-driven O2 evolution. As previously proposed,43 the electrons in the Cr eg orbital can reduce Ag+ to Ag metal (Ag+ + eJ

Ag, E° = 0.799 V vs. SHE). Therefore,

the photoexcitation from the VB of TiO2 to the eg orbital of the doped Cr species is proposed to be the dominant photocatalytic mechanism of H2-reduced TiO2:Ta/Cr under visible light irradiation. The four-electron oxidation of water to O2 (E° = 1.23 V vs. SHE) is promoted by the photogenerated holes in the VB of TiO2. Kudo et al. have proposed that the visible light response of TiO2:Sb/Cr and SrTiO3:Sb/Cr photocatalysts is induced by electron excitation from the donor levels consisting of Cr3+ t2g orbitals to CB.13 SrTiO3:Sb/Cr evolved H2 from aqueous methanol solution, but the activity for O2 evolution from silver nitrate (AgNO3) solution was very low under visible irradiation. It is reported that the low activity of SrTiO3 host for O2 evolution was drastically improved by the loading of IrOX cocatalyst.21 This suggests that the ability of SrTiO3 surface is poor for four-


16

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electron oxidation of water to evolve O2. In contrast, TiO2:Sb/Cr was active only for O2 evolution under visible light irradiation. The reason of the negligible activity of TiO2:Sb/Cr for H2 evolution is explained by the CB energy of rutile TiO2 less than that of SrTiO3.13,14 However, H2-reduced rutile TiO2 exhibited high activity for H2 evolution under UV irradiation. Therefore, the negligible activity of TiO2:Sb/Cr and H2-reduced TiO2:Ta/Cr for H2 evolution might be accounted from the fast recombination of the photoexcited electrons in the CB with holes in the Cr t2g orbitals. It should be noted that the photoexcited electrons in the Cr eg orbital cannot reduce protons to H2. In the case of non-doped rutile TiO2 photocatalysts, the enhanced photocatalytic activity by H2 reduction treatment is explained by the increase in electron density in the CB and shallow energy levels, which provide efficient charge separation by built-in potential and fast transport of photogenerated electrons.25,28,29 In this study, we found that the H2 reduction treatment decreased the midgap states working as deep traps of photoexcited electrons, and the radiative recombination between the deeply trapped electrons and the VB holes. As the H2 reduction treatment increases the concentration of electrons, the traps at deep levels have already filled with the electrons before photoirradiation.44 Therefore, the fall of photoexcited electrons in Cr eg orbitals into the deep traps is delayed by electron doping. Yamakata et al. reported that the photoexcited electrons in rutile TiO2 are trapped at deep levels within a few picoseconds.41,42 The suppression of the fast trapping of photoexcited electrons increased the charge transfers of photogenerated carriers over the photocatalyst surface. The electron doping mechanism is reasonable to explain the enhanced photocatalytic activity under visible light irradiation.

CONCLUSION We investigated the effect of H2 reduction treatment on TiO2:Ta/Cr photocatalysts for water oxidation under visible light irradiation. H2 reduction treatment was found to improve the


17


Page 18 of 34

visible light response of Cr3+-doped TiO2 photocatalysts with an increase of the electron concentration. The optimized H2 treatment temperature was decreased with an increase of the content of higher valence Ta5+ as a donor dopant. Electron doping by H2 reduction treatment was confirmed by NIR absorption in diffuse reflectance spectroscopy, and the ESR active Cr3+ species was decreased with an increase of the electron concentration. PL and transient absorption spectroscopy revealed that the H2 reduction treatment decreased the radiative recombination of deeply trapped electrons with VB holes. This is because the trap sites at deep energy levels have been previously occupied by the electron doping before visible light photoexcitation. Therefore, more photoexcited electrons can reduce Ag+ escaping from the recombination. Since the dynamics of the photoexcited electrons in CB and shallow traps were not drastically changed by H2 treatment, the photoexcitation from the VB to the midgap states of the Cr eg orbital is suggested to be the possible pathway of the visible light response of H2reduced TiO2:Ta/Cr. This study demonstrates that the electron concentration plays an important role in the enhanced photocatalytic activity of metal-doped TiO2 photocatalysts owing to the decrease of the trapping sites at deep energy levels. Electron doping by H2 reduction treatment is one of the available methods to develop visible-light-driven TiO2 photocatalysts for water splitting to produce solar H2.


18

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. BET specific surface area (SSA) of the prepared samples, and their particle sizes calculated from SSA assuming spherical particles. SSA / m2 gJ

Particle size / µm b

Commercial TiO2 a

2.3

0.61

Non-doped TiO2

0.78

1.8

TiO2:Ta/Cr(2.0)

0.97

1.5

TiO2:Ta/Cr-H400(2.0)

0.83

1.7

Sample

a

The precursor for non-doped TiO2 and TiO2:Ta/Cr(2.0).

b The

diameter of the spherical particles with the density of rutile TiO2 (4.25 g/cm3).

Table 2. The initial rate of O2 evolution over photocatalysts under 470-nm light irradiation. Energy gap / eV b r(O2) / Z

Sample Non-doped TiO2

3.0

0

WO3 a

2.6

0.38

TiO2:Ta/Cr (2.0)

2.1

0

TiO2:Ta/Cr-H400(2.0)

2.2

2.40

TiO2:Sb/Cr(1.5)

2.1

0.24

a WO

hJ

c

3 (99.99%, Kojundo Chemical Laboratory), BET specific surface area 6.0 m

2/g, Mixture

of monoclinic phase and triclinic phase. b Optical

energy gap estimated from diffuse reflectance spectrum using Tauc plot.

c The

initial rate of O2 evolution from an aqueous solution of 50 mmol/L AgNO3 over 50 mg of photocatalyst powder under 470-nm blue light (20 mW/cm2).


19


A (a)

(b)

(c)

(d)

100

B

100 - %R

80 60 (b)

40 20

(a)

0 200

0.5

400 600 800 Wavelength / nm

(d) (c) 1000

C

5

(F(R)*hv)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

4 (d) Ta/Cr(2.0) 2.1 eV

3

(a) non-doped TiO2 3.0 eV

2 1 0

1

2

3 hv / eV

4

5

Figure 1. (A) Photographs and (B) UV-vis-NIR diffuse reflectance spectra of (a) non-doped TiO2, (b) TiO2:Ta/Cr(1.0), (c) TiO2:Ta/Cr(1.5), and (d) TiO2:Ta/Cr(2.0). (C) Tauc plots of (a) and (d) for the estimation of the optical energy gap.


20

Page 21 of 34

A Ta/Cr = 2.0

H200

H400

H600

100

B

100-%R

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

H600

60

H500 H450 H400 H300

40 20 0 200


H200 Ta/Cr TiO2 1000

Figure 2. (A) Photographs and (B) UV-vis-NIR diffuse reflectance spectra of TiO2:Ta/Cr(2.0) and the samples treated with H2 at different temperatures.


21


4.94

H200

H200

Ta/Cr

Intensity / -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

H300 H400 H500 H600

5.62 126 130 134 138 142

2.64 1.38 1.67

100

200

300

400

500

Magnetic field / mT Figure 3. ESR spectra recorded at J .,M of TiO2:Ta/Cr(2.0) and the samples treated with H2 at different temperatures from 200°C to 600°C.


22

Page 23 of 34

5

O2 evolution / mol

A

H400 H450 H500

4 3 2

H300

1

H200 H600 Ta/Cr=2.0

0 -1

0

Rate of O 2 evolution / mol h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

40 60 80 100 Irradiation time / min

120

3

B

Ta/Cr = 2.0

2

1

0 100

Ta/Cr = 1.5

200 300 400 500 600 700 o H2 treatment temperature / C

Figure 4. (A) Photocatalytic O2 evolution over TiO2:Ta/Cr(2.0) and the H2-treated samples from an aqueous solution of 50 mmol/L AgNO3 under 470-nm blue light (20 mW/cm2), (B) effect of H2 treatment temperature on the initial rate of photocatalytic O2 evolution for TiO2:Ta/Cr(1.5) and TiO2:Ta/Cr(2.0) samples.


23


4

100

60 2 40 1

100 - %R

80

3 AQY (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

20

0

0 300


700

Figure 5. Apparent quantum efficiency (AQY) action spectra of TiO2:Ta/Cr-H400(2.0) for photocatalytic O2 evolution and the UV-vis diffuse reflectance spectrum.


24

Page 25 of 34

3

6x10

(b) TiO 2:Ta/Cr

PL intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

4x10

3

2x10

(a) TiO 2 0 400

500

(c) H400

600 700 800 900 1000 Wavelength / nm

Figure 6. PL spectra of (a) rutile TiO2, (b) TiO2:Ta/Cr(2.0), and (c) TiO2:Ta/Cr-H400(2.0) measured in a vacuum. The samples were excited by a 375-nm continuous wave diode laser.


25


-3

5x10

2.4

Energy / eV 2.1 1.8

4x10

Absorbance

1.5

(a) TiO 2:Ta/Cr

-3

-3

3x10

5 s 10 s 20 s 50 s 100 s 1000 s 10000

-3

2x10

-3

1x10

0 20000 2.4 -3

1x10

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

15000 2.1

1.8

1.5

(b) TiO 2:Ta/Cr-H400

5 s 10 s 20 s 50 s 100 s 1000 s

-4

5x10

0 20000

15000 Wavenumber / cm -1

10000

Figure 7. Transient visible-NIR absorption spectra of (a) TiO2:Ta/Cr(2.0) and (b) TiO2:Ta/CrH400(2.0). The samples were excited by 470 nm laser pulses in a vacuum.


26

Page 27 of 34

-2

1x10

(b) TiO 2:Ta/Cr-H400

-3

8x10

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-3

6x10

(a) TiO 2:Ta/Cr

-3

4x10

-3

2x10

0 0

200 400 600 800 1000 1200 Irradiation time / s

Figure 8. Accumulation curves of photogenerated electrons at 900 cmJ in (a) TiO2:Ta/Cr(2.0) and (b) TiO2:Ta/Cr-H400(2.0). The samples were excited by 470-nm continuous wave LED in vacuum.

e 900 1 cm

e

e

e

UV Vis

trap

h+

850 nm h+

(a) TiO2:Ta/Cr

e CB

e eg e

850 nm

UV Vis

h+

e

Ag+/Ag e

O2/H2O

h+ t2g h+

h+

VB

(b) H2-treated TiO2:Ta/Cr

Figure 9. Proposed energy diagram of (a) TiO2:Ta/Cr(2.0) and (b) TiO2:Ta/Cr-H400 estimated from the results of UV-vis-NIR diffuse reflectance, PL, and transient absorption spectroscopies.


27


Page 28 of 34

Supporting Information Available. The Supporting Information is available free of charge on the ACS Publications website. Amount of doping cations for TiO2:Ta/Cr (Tables S1), Incident photon intensity for the AQY measurement (Table S2), Chemical composition determined by XPS (Table S3), Change of pH after photocatalytic reaction (Table S4), Emission spectrum of LEDs for photocatalytic reaction (Figure S1), UV-vis-NIR diffuse reflection spectra (Figure S2), SEM images (Figure S3), TEM images (Figure S4), XRD pattern (Figure S5), ESR spectra (Figure S6), XPS (Figure S7), Time course of photocatalytic O2 evolution (Figures S8JS10).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the General Sekiyu Research Scholarship Foundation and the JST, Precursory Research for Embryonic Science and Technology (PRESTO), grant number JPMJPR18T1. We thank Dr. A. Yamamoto and Prof. T. Tanaka from Kyoto University for the ESR measurement.


28

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REFERENCES (1)

Asahi, R.; Morikawa, T.; Irie, H.; Ohwaki, T. Nitrogen-Doped Titanium Dioxide as

Visible-Light-Sensitive Photocatalyst: Designs, Developments, and Prospects. Chem. Rev. 2014, 114, 9824-9852. (2)

Miyoshi, A.; Vequizo, J. J. M.; Nishioka, S.; Kato, Y.; Yamamoto, M.; Yamashita, S.;

Yokoi, T.; Iwase, A.; Nozawa, S.; Yamakata, A.; Yoshida, T.; Kimoto, K.; Kudo, A.; Maeda, K. Nitrogen/Fluorine-Codoped Rutile Titania as a Stable Oxygen-Evolution Photocatalyst for Solar-Driven Z-scheme Water Splitting. Sustain. Energy Fuels 2018, 2, 2025-2035. (3)

Dvoranová, D.; Brezová, V.; Mazúr, M.; Malati, M. A. Investigations of Metal-Doped

Titanium Dioxide Photocatalysts. Appl. Catal. B 2002, 37, 91-105. (4)

Serpone,

N.;

Lawless,

D.;

Disdier,

J.;

Herrmann,

J.-M.

Spectroscopic,

Photoconductivity, and Photocatalytic Studies of TiO2 Colloids: Naked and with the Lattice Doped with Cr3+, Fe3+, and V5+ Cations. Langmuir 1994, 10, 643-652. (5)

Tian, B.; Li, C.; Zhang, J. One-Step Preparation, Characterization and Visible-Light

Photocatalytic Activity of Cr-doped TiO2 with Anatase and Rutile Bicrystalline Phases. Chem. Eng. J. 2012, 191, 402-409. (6)

Inturi, S. N. R.; Boningari, T.; Suidan, M.; Smirniotis, P. G. Visible-Light-Induced

Photodegradation of Gas Phase Acetonitrile using Aerosol-Made Transition Metal (V, Cr, Fe, Co, Mn, Mo, Ni, Cu, Y, Ce, and Zr) Doped TiO2. Appl. Catal. B 2014, 144, 333-342. (7)

Fan, X.; Chen, X.; Zhu, S.; Li, Z.; Yu, T.; Ye, J.; Zou, Z. The Structural, Physical and

Photocatalytic Properties of the Mesoporous Cr-doped TiO2. J. Mol. Catal. A 2008, 284, 155160. (8)

Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis

with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746-750.


29


(9)

Page 30 of 34

Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang,

J. Z.; Li, Y. Hydrogen-treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 3026-3033. (10)

Naldoni, A.; Allieta, M.; Santangelo, S.; Marelli, M.; Fabbri, F.; Cappelli, S.; Bianchi,

C. L.; Psaro, R.; Dal Santo, V. Effect of Nature and Location of Defects on Bandgap Narrowing in Black TiO2 Nanoparticles. J. Am. Chem. Soc. 2012, 134, 7600-7603. (11)

Jiang, X.; Zhang, Y.; Jiang, J.; Rong, Y.; Wang, Y.; Wu, Y.; Pan, C. Characterization

of Oxygen Vacancy Associates within Hydrogenated TiO2: A Positron Annihilation Study. J. Phys. Chem. C 2012, 116, 22619-22624. (12)

Yang, C.; Wang, Z.; Lin, T.; Yin, H.; Lü, X.; Wan, D.; Xu, T.; Zheng, C.; Lin, J.;

Huang, F.; Xie, X.; Jiang, M. Core-Shell Nanostructured “Black” Rutile Titania as Excellent Catalyst for Hydrogen Production Enhanced by Sulfur Doping. J. Am. Chem. Soc. 2013, 135, 17831-17838. (13)

Kato, H.; Kudo, A. Visible-Light-Response and Photocatalytic Activities of TiO2 and

SrTiO3 Photocatalysts Codoped with Antimony and Chromium. J. Phys. Chem. B 2002, 106, 5029-5034. (14)

Niishiro, R.; Kato, H.; Kudo, A. Nickel and Either Tantalum or Niobium-Codoped TiO2

and SrTiO3 Photocatalysts with Visible-Light Response for H2 or O2 Evolution from Aqueous Solutions. Phys. Chem. Chem. Phys. 2005, 7, 2241-2245. (15)

Niishiro, R.; Konta, R.; Kato, H.; Chun, W. J.; Asakura, K.; Kudo, A. Photocatalytic

O2 Evolution of Rhodium and Antimony-Codoped Rutile-Type TiO2 under Visible Light Irradiation. J. Phys. Chem. C 2007, 111, 17420-17426. (16)

Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting.

Chem. Soc. Rev. 2009, 38, 253-278.


30

Page 31 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17)

Miyoshi, A.; Nishioka, S.; Maeda, K. Water Splitting on Rutile TiO2-Based

Photocatalysts. Chem. Eur. J. 2018, 24, 18204-18219. (18)

Nowotny, M. K.; Sheppard, L. R.; Bak, T.; Nowotny, J. Defect Chemistry of Titanium

Dioxide. Application of Defect Engineering in Processing of TiO2-based Photocatalysts. J. Phys. Chem. C 2008, 112, 5275-5300. (19)

Ishii, T.; Kato, H.; Kudo, A. H2 Evolution from an Aqueous Methanol Solution on

SrTiO3 Photocatalysts Codoped with Chromium and Tantalum Ions under Visible Light Irradiation. J. Photochem. Photobiol. A 2004, 163, 181-186. (20)

Ikeda, T.; Nomoto, T.; Eda, K.; Mizutani, Y.; Kato, H.; Kudo, A.; Onishi, H.

Photoinduced Dynamics of TiO2 Doped with Cr and Sb. J. Phys. Chem. C 2008, 112, 11671173. (21)

Niishiro, R.; Tanaka, S.; Kudo, A. Hydrothermal-Synthesized SrTiO3 Photocatalyst

Codoped with Rhodium and Antimony with Visible-Light Response for Sacrificial H2 and O2 Evolution and Application to Overall Water Splitting. Appl. Catal. B 2014, 150-151, 187-196. (22)

Asai, R.; Nemoto, H.; Jia, Q.; Saito, K.; Iwase, A.; Kudo, A. A Visible Light

Responsive Rhodium and Antimony-Codoped SrTiO3 Powdered Photocatalyst Loaded with an IrO2 Cocatalyst for Solar Water Splitting. Chem. Commun. 2014, 50, 2543-2546. (23)

Amano, F.; Nakata, M.; Asami, K.; Yamakata, A. Photocatalytic Activity of Titania

Particles Calcined at High Temperature: Investigating Deactivation. Chem. Phys. Lett. 2013, 579, 111-113. (24)

Amano, F.; Nakata, M.; Yamamoto, A.; Tanaka, T. Effect of Ti3+ Ions and Conduction

Band Electrons on Photocatalytic and Photoelectrochemical Activity of Rutile Titania for Water Oxidation. J. Phys. Chem. C 2016, 120, 6467-6474.


31


(25)

Page 32 of 34

Amano, F.; Nakata, M. High-Temperature Calcination and Hydrogen Reduction of

Rutile TiO2: A Method to Improve the Photocatalytic Activity for Water Oxidation. Appl. Catal. B 2014, 158-159, 202-208. (26)

Amano, F.; Nakata, M.; Ishinaga, E. Photocatalytic Activity of Rutile Titania for

Hydrogen Evolution. Chem. Lett. 2014, 43, 509-511. (27)

Amano, F.; Nakata, M.; Yamamoto, A.; Tanaka, T. Rutile Titanium Dioxide Prepared

by Hydrogen Reduction of Degussa P25 for Highly Efficient Photocatalytic Hydrogen Evolution. Catal. Sci. Technol. 2016, 6, 5693-5699. (28)

Amano, F.; Tosaki, R.; Sato, K.; Higuchi, Y. Effects of Donor Doping and Acceptor

Doping on Rutile TiO2 Particles for Photocatalytic O2 Evolution by Water Oxidation. J. Solid State Chem. 2018, 258, 79-85. (29)

Amano, F. In Titanium Dioxide; Janus, M., Ed.; IntechOpen: 2017, DOI:

10.5772/intechopen.68603. (30)

Akubuiro, E. C.; Verykios, X. E. Effects of Altervalent Cation Doping on Electrical

Conductivity of Platinized Titania. J. Phys. Chem. Solids 1989, 50, 17-26. (31)

Karakitsou, K. E.; Verykios, X. E. Effects of Altervalent Cation Doping of TiO2 on Its

Performance as a Photocatalyst for Water Cleavage. J. Phys. Chem. 1993, 97, 1184-1189. (32)

Sheppard, L. R.; Hager, S.; Holik, J.; Liu, R.; Macartney, S.; Wuhrer, R. Tantalum

Segregation in Ta-doped TiO2 and the Related Impact on Charge Separation During Illumination. J. Phys. Chem. C 2015, 119, 392-400. (33)

Shannon, R. D. Revised Effective Ionic Radii and Systematic Studies of Interatomic

Distances in Halides and Chalcogenides. Acta Cryst. A 1976, 32, 751-767. (34)

Köhler, K.; Schläpfer, C. W.; Von Zelewsky, A.; Nickl, J.; Engweiler, J.; Baiker, A.

Chromia Supported on Titania: I. An EPR Study of the Chemical and Structural Changes Occurring During Catalyst Genesis. J. Catal. 1993, 143, 201-214.


32

Page 33 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(35)

Egerton, T. A.; Harris, E.; Lawson, E. J.; Mile, B.; Rowlands, C. C. An EPR Study of

Diffusion of Chromium into Rutile. Phys. Chem. Chem. Phys. 2000, 2, 3275-3281. (36)

O'Reilly, D. E.; Maciver, D. S. Electron Paramagnetic Resonance Absorption of

Chromia-Alumina Catalysts. J. Phys. Chem. 1962, 66, 276-281. (37)

Evans, J. C.; Relf, C. P.; Rowlands, C. C.; Egerton, T. A.; Pearman, A. J. An Electron

Paramagnetic Resonance (EPR) Study of the Effect of Calcination Temperature on Cr(3+)Impregnated Polycrystalline Rutile TiO2. J. Mater. Sci. Lett. 1984, 3, 695-698. (38)

Amano, F.; Ishinaga, E.; Yamakata, A. Effect of Particle Size on the Photocatalytic

Activity of WO3 Particles for Water Oxidation. J. Phys. Chem. C 2013, 117, 22584-22590. (39)

Ohtani, B.; Okugawa, Y.; Nishimoto, S. I.; Kagiya, T. Photocatalytic Activity of TiO2

Powders Suspended in Aqueous Silver Nitrate Solution. Correlation with pH-Dependent Surface Structures. J. Phys. Chem. 1987, 91, 3550-3555. (40)

Vequizo, J. J. M.; Kamimura, S.; Ohno, T.; Yamakata, A. Oxygen Induced

Enhancement of NIR Emission in Brookite TiO2 Powders: Comparison with Rutile and Anatase TiO2 Powders. Phys. Chem. Chem. Phys. 2018, 20, 3241-3248. (41)

Yamakata, A.; Vequizo, J. J. M.; Matsunaga, H. Distinctive Behavior of

Photogenerated Electrons and Holes in Anatase and Rutile TiO2 Powders. J. Phys. Chem. C 2015, 119, 24538-24545. (42)

Vequizo, J. J. M.; Matsunaga, H.; Ishiku, T.; Kamimura, S.; Ohno, T.; Yamakata, A.

Trapping-Induced Enhancement of Photocatalytic Activity on Brookite TiO2 Powders: Comparison with Anatase and Rutile TiO2 Powders. ACS Catal. 2017, 7, 2644-2651. (43)

Tanigawa, S.; Irie, H. Visible-Light-Sensitive Two-Step Overall Water-Splitting Based

on Band Structure Control of Titanium Dioxide. Appl. Catal. B 2016, 180, 1-5. (44)

Nishioka, S.; Hyodo, J.; Vequizo, J. J. M.; Yamashita, S.; Kumagai, H.; Kimoto, K.;

Yamakata, A.; Yamazaki, Y.; Maeda, K. Homogeneous Electron Doping into


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Enhanced Visible Light Response of TiO2 ... - ACS Publications

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