IceâWater Quenching Induced Ti3+ Self-doped TiO

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Ice-water Quenching Induced Ti Self-Doped TiO With Surface Lattice Distortion and the Increased Photocatalytic Activity Baoshun Liu, kai Cheng, Shengchao Nie, Xiujian Zhao, Huogen Yu, Jiaguo Yu, Akira Fujishima, and Kazuya Nakata J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06274 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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The Journal of Physical Chemistry

Ice-water Quenching Induced Ti3+ Self-doped TiO2 with Surface Lattice Distortion and the Increased Photocatalytic Activity

Baoshun Liu,a* Kai Cheng, Shengchao Nie,a Xiujian Zhao,a* Huogen Yu,b Jiaguo Yu,c Akira Fujishima,d Kazuya Nakata de

a

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan city, Hubei Province 430070, People’s Republic of China.

b

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan 430070, People’s Republic of China c

State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, People’s Republic of China

d

Research Institute for Science and Technology, Photocatalysis International Research Center, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan

e

Department of Applied Biological Science, Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan

ACS Paragon Plus Environment

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ABSTRACT: The present research reported a facile strategy to prepare Ti3+ self-doped TiO2 with increased photocatalytic activity. The TiO2 subjected to high temperature pre-annealing was directly thrown into ice-water for rapid quenching. It is interesting to see that the quenched samples show pale blue color due to the absorption in visible and near-IR region. The comprehensive analyses of X-ray diffraction, Raman spectroscopy, Fourier Transform Infrared Spectroscopy, field-emission scanning electron microscope, and Brunauer-Emmett-Teller (BET) show that the crystallinity, the morphologies, and the specific surface area are almost unchanged after the ice-water quenching. The spectroscopic analyses of UV-Vis diffusion reflectance spectra, photoluminescence spectra, and X-ray photoelectron spectra clearly show the change of electronic structure of TiO2 due to presence of Ti3+ ions induced by the ice-water quenching, which is further confirmed by the electron paramagnetic resonance analysis. No Ti3+ ions are generated if the pre-annealing temperature is below 800 °C. The energy band structure model involving the Ti3+ ions and the associated oxygen defects was proposed to explain the change of UV-Vis diffusion absorption. It is considered that the high concentration of oxygen defects at high pre-annealing temperatures can be partially frozen by the ice-water quenching, which then can denote the high concentration of excess electrons. Some excess electrons can be localized at Ti lattice sites, resulting in the presence of Ti3+ ions. More interestingly, it is also seen that the rapid ice- water quenching causes the distortion of surface lattice due to the interaction between hot TiO2 and water, which tends to be poly-crystalline and disordered for high preannealing temperature. The surface lattice distortion is considered to be correlated with the generation of oxygen defects during the ice-water quenching. The quenched samples show obviously increased photocatalytic activity for both methylene blue degradation and hydrogen evolution under UV light illumination. Although they do not have visible activity, loading amorphous Cu(OH)x nanoclusters can greatly increase their ability to degrade methylene blue under visible light illumination. It is also shown that the photocatalytic activity of ZnO can also be increased to some extent by the ice-water quenching. Therefore, the ice-water quenching could be the general method for increasing the photocatalytic activity of many materials.


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INTRODUCTION Driven by the increasing concerns on environmental and energy issues, the searches for pollutant removal and clean energy are attracting massive interests in recent years, with the photocatalysis being an important potential strategy to deal with them. 1-5 Among many types of materials, TiO2 is an important typical photocatalyst that were used for decomposing organic compounds 6-13 and generating hydrogen 11,12, so it is still necessary and significant to further the increase of its photocatalytic activity. Defect engineering was usually used to improve the photocatalytic activity due to the modulation of charge carrier kinetics.14-18 Foreign elements, such as Fe, V, Sn, N, C, and S had been doped to generate impurity states in TiO2.19-22 However, problems, such as the low thermal stability, inhomogeneity, and the increased recombination, still pose great limitations for this strategy. On the contrary, creating self-dopants in TiO2 was shown to be better to improve the photocatalytic activity. 23-27 For example, it was reported that the photocatalytic activities of TiO2 can be changed by tuning the ratio between oxygen vacancies on surface and in bulk,28 introducing Ti vacancies, 29 and making surface disorder. 30 Recently, reducing TiO2 to create the self-dopants, including O and Ti3+ defects, becomes interesting for increasing the photocatalytic activity and making use of the visible light. It was also shown that the combination of Ti3+ self-doped TiO2 with other materials, such as BiOCl, can effectively increase the photocatalytic activity. 31

Many methods, such as the thermal reduction, the electrochemical reduction, and the flame reduction, can be

used to generate the self-dopants in TiO2. For example, Zhou et al. used a solvothermal method to prepare stable blue Ti3+-self doped TiO2, which presented distinct visible activity for removing Rhodamine B; 24 Hamdy et al. heated the commercial P25 in reductants to prepare blue Ti3+ self-doped TiO2 with high selective oxidation of methylcyclohexane; 32 Feng et al. burned the ethanol-titanium isopropoxide to prepare Ti3+ self-doped TiO2 for visible generation of hydrogen. 33 Huang et al. reported the synthesis of Ti3+ self-doped TiO2 by the mild hydrothermal treatment of TiH2 in H2O2 aqueous solution, which showed higher photocatalytic activity for hydrogen production and methylene blue photooxidation. 34 However, these methods suffer from some disadvantages of harsh reaction conditions, high danger, and others. Because these methods mainly create the self-dopants on TiO2 surface, the defect species are unstable when exposing to water or air. Moreover, the photocatalytic activity of self-doped TiO2 differs for different preparation methods. Therefore, further efforts are still desirable to explore simpler and more general low cost methods for the preparation of Ti3+ self-doped TiO2. In the present research, we mainly investigated the ice-water quenching as a general methodology to implant self-dopants in TiO2 for increasing its photocatalytic activity. In the study performed by Supphasrirongjaroen et al., 35 it had been reported that the quenching can cause the Ti3+ self-dopants and the increased photocatalytic activity for TiO2 materials. In their study, TiO2 materials were quenched at 100 °C or room temperature after preannealing at 573 K. However, the present research got different conclusions that that the ice-water quenching cannot induce obvious change to the structure and photocatalytic activities of TiO2 if the pre-annealing


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temperature was lower 800 °C. Therefore, we mainly studied the effect of ice-quenching from the pre-annealing at temperatures higher than 800 °C, and obtained some different important results.35 Both of the normally-cooled and the quenched samples are of rutile phase after the pre-annealing due to the high temperature. More comprehensive analyses, including UV-Vis spectroscopy, photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), high-resolution transmittance electron microscope (HR-TEM), and electron paramagnetic resonance (EPR), were used to study the change of TiO2 structure induced by ice-water quenching. It was found that the ice-water quenching cannot cause the clear change on crystallinity, morphologies, and specific surface areas of TiO2, but the electron structure was obviously changed because of the generation of the Ti3+ self-dopants and the associated oxygen defects. More interestingly, the analysis of HR-TEM and selected area electron diffraction (SAED) showed that the ice-water quenching could also induce the crystalline lattice distortion and the poly-crystallization on surface layer of TiO2 particles. It was also considered that the creation of Ti3+ self-dopants should be related to the surface lattice distortion. Although the photocatalytic activity of the icewater quenched TiO2 samples is lower than the commercial P25 due to the rutile phase, the large particle size, and the low specific surface area, it is obviously higher than that of the normally-cooled samples. Because of the similar crystallinity, the morphologies, and the specific surface area, the increase of photocatalytic activity is mainly ascribed to the electronic structure change induced by ice-water quenching, i.e., the presence of Ti3+ selfdopants and oxygen vacancies. Although we cannot completely determine the function of lattice distortion, the lattice distortion should play a positive role through the cooperation with the defects. Due to low visible absorption induced by Ti3+ self-dopants, the quenched samples are visible-inactive. Grafting with Cu(OH)x nanoclusters can effectively increase the photocatalytic ability responded to visible light. In additional to TiO2, it was seen that the photocatalytic activity of ZnO can also be increased via ice-water quenching. Therefore, the icewater quenching from a high pre-annealing temperature could be a general strategy to increase the photocatalytic activity of oxide semiconductors by engineering the electronic and the surface crystalline structure.

EXPERIMENTAL Sample quenching: The commercial P25 powers were bought for use without further treatment. 3.0 g P25 materials were firstly subjected to the pre-annealing at 800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C for 2 h, respectively, and then were directly thrown into ice-water (ca. 4 °C) for rapid quenching. The quenched samples were filtered and then dried at 80 °C for 12 h for further use. In the meanwhile, the samples subjected to the same pre-annealing procedure were normally cooled to room temperature for comparison. The quenched TiO2 and the normally-cooled TiO2 were labeled as q-TiO2 and n-TiO2, respectively. Cu(OH)x nanocluster loading: The Cu(OH)x nanoclusters were loaded on the q-TiO2 and n-TiO2 samples by the thermal deposition-precipitation method according to the study. 36 1.0 g q-TiO2 and n-TiO2 powders were first dispersed in 5 mL of distilled water. 5 mL 0.01 M diluted Cu(NO3)2 aqueous solution acting as the source of Cu


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was added into the above TiO2 suspension, which meant Cu/Ti ratio of 0.5 wt%. The Cu-TiO2 suspension was heated at 90 °C and was stirred for 1 h. The suspension was then filtered and washed with sufficient amounts of distilled water. The obtained powders were dried at 110 °C for 24 h and subsequently ground into a fine powder using an agate mortar for further use. The Cu(OH)x nanocluster loaded q-TiO2 and n-TiO2 samples were labeled as Cu@q-TiO2 and Cu@n-TiO2, respectively. Characterization: The crystallinity of the q-TiO2 and n-TiO2 samples was checked by using a D/MAX-IIIA X-ray diffraction spectrometer in a 2θ mode from 10 to 80°. The comparison of crystallinity of q-TiO2 and n-TiO2 samples was also checked at room temperature by using a RENISHAW Raman Scattering spectroscopy equipped with an optical microscope. For excitation, the 514.5 nm line from an Ar+ ion laser was focused with an analyzing spot of about 1 µm on the sample under the microscope. Fourier transformation infrared (FT-IR) absorption spectra of the n-TiO2 and q-TiO2 samples were obtained on an FT-IR spectrometer (FT/IR-6100, JASCO, Japan). The image, highly-magnified images, and selected area electron diffractions (SAED) of the samples were observed using a field emission transmission electron microscope (FE-TEM; type: JEM2100F, JEOL, Tokyo, Japan). Morphologies of the n-TiO2 and q-TiO2 samples were observed with a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan), and the particle size was averaged on the low-magnification FESEM images by counting more than 100 particles. UV-vis diffusion spectra of TiO2 and Cu(OH)x loaded samples were measured by using a UV-Vis photo spectrometer within wavelength range from 200 nm to 800 nm (type: UV-3600, Shimadzu, Tokyo, Japan). The surface chemical compositions were checked with an X-ray photoelectron spectrometer (XPS; type: VG Multilab 2000, Thermo Scientific, Waltham, America), with an X-ray source working with the Al Kα radiation using the binding energy (284.6 eV) of C1s electrons as energy reference. The X-band electron paramagnetic resonance (EPR) spectra were recorded on a Bruker EMX spectrometer equipped with cylindrical quart tube operating at 100 kHz field modulation and the temperature of 100 K. The multipoint Brunauer–Emmett–Teller (BET) method was used to calculate the specific surface area of the samples using the adsorption data in the P/P0 range from 0.05 to 0.2. The pore size distributions of the TiO2 spheres were derived from the absorption data of the isotherms based on the Barrett–Joyner–Halenda (BJH) model.

Photocatalytic degradation of methylene blue: Firstly, 0.1 g of TiO2 samples were mixed with ethanol in φ100 mm glass containers, and then were ultrasonically dispersed for ca. 30 min. The ethanol was removed by heating at 70 °C, which was then subjected to strong UV light illumination for several hours to remove the surfaceadsorbed ethanol. 50 mL of methylene blue (MB) water solution (6 mg L−1) was slowly added into the glass container. The adsorption-desorption of MB on TiO2 surface was allowed to reach equilibrium by keeping in dark for one hour, and then the photocatalytic experiments were started. The photocatalytic activities of the TiO2 samples were evaluated by measuring the UV-Vis absorption spectra of the MB aqueous solution at different reaction intervals. A 15 W (TOSHIBA, 365 nm) UV fluorescent lamp was used as the light source. The UV light


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intensity was measured to be ca. 1.0 mW cm−2. Generally, the photocatalysis of low concentration MB aqueous solution follows the quasi-first order reaction according to eqn. (1).

ln k

t

(1)

where C(0) and C(t) are the initial and reactive concentrations of MB aqueous solution, and kapp is the quasi-first order constant. The kapp was calculated to compare the photocatalytic activity of q-TiO2 and n-TiO2 samples. Photocatalytic H2 production: To evaluate the photocatalytic activity of H2 production of the q-TiO2 and the nTiO2, the co-catalyst Pt was loaded on the TiO2 samples by impregnation of H2PtCl6 aqueous solution via the photo-reduction method. The photocatalytic H2 production experiments were performed at room temperature and atmospheric pressure in a 100 mL flask of three necks (sealed with silicone rubber septum). A light box with 4 LED lamps of 365 nm and 5 W was used as light source, and the distance between lamp with reactor was 5 cm. 0.05 g q-TiO2 and n-TiO2 powders was suspended in 80 mL aqueous solution (mixed with 20 mL methanol and 60 mL water). 67 µL H2PtCl6 aqueous solution (10 g/L) was added into the flask for Pt photo-deposition, and the nominal weight ratio of Pt to Ti loaded onto the above-prepared TiO2 was 0.5 wt%. The suspensions were stirred for 5 min, and then nitrogen was bubbled into the solution for 20 min to remove the dissolved oxygen. After stirred and irradiated for 30 min, 0.4 mL of gas was drawn from the reactor, and hydrogen was analyzed by gas chromatograph (GC-2014C, Shimadzu, Japan, TCD). The total irradiation time was 150 min, and the H2 concentration was measured in each 30 min.

Figure 1 (A) Digital pictures of q-TiO2 and n-TiO2 samples prepared from the commercial P25 powders after being subjected to different temperature pre-annealing; (B) Digital picture of just-quenched TiO2 sample on filter paper, which shows the blue color in the inner of the sample RESULTS AND DISCUSSION General Physical Characterization Figure 1 (A) shows the digital pictures of n-TiO2 and q-TiO2 samples pre-annealed at different temperatures. As compared to the n-TiO2 samples, it can be seen that the q-TiO2 samples tend to be pale blue when the pre-


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annealing temperature was higher than 900 °C. This color change indicates the presence of Ti3+ in TiO2 after icewater quenching. Figure 1 (B) shows the digital picture of just-quenched sample. It can be seen that the inner of sample is shallowly blue, whereas the surface is white. It was also found that this blue color tends to disappear, showing that the surface of quenched samples is unstable to resist oxidization. When the pre-annealing temperature was lower than 700 °C, the colors of q-TiO2 samples are almost same to that of the n-TiO2 samples. Figure 2 (A) shows the powder XRD patterns of n-TiO2 and q-TiO2 samples, with the diffraction plane indexes being labelled. Both of the n-TiO2 and q-TiO2 samples are of rutile crystalline phase, showing that the anatase phase of P25 materials were completely transferred to rutile after the pre-annealing above 800 °C. The corresponding magnification of (110) diffraction peaks are shown in Figure 2 (B) in order to see the possible change induced by ice-water quenching. We carefully compared the XRD peak’s width, intensity, and position between the n-TiO2 and q-TiO2 samples, and no obvious regular change can be found, indicating the less effect of ice-water quenching on the crystallinity. Moreover, the lattice constants of n-TiO2 and q-TiO2 samples were gotten by refining the XRD spectra, which are summarized in Table 1. Similarly, it is also seen that the ice-water quenching cannot cause the observable regular change to the lattice constants of TiO2.

Figure 2 (A) XRD patterns of q-TiO2 and n-TiO2 samples pre-annealed at different temperature; (B) Corresponding magnifications of the (110) diffraction peak of Figure 2 (A).

By considering that Raman spectroscopy is a more sensitive method to identify crystalline structure, the effect of ice-water quenching was also studied by using Raman spectroscopy. Although it was seen that the intensity of Raman peaks changes after the ice-water quenching, such regular change could not evidence the change of crystalline structure. As compared to the intensity, the change of width and position of Raman peaks should be more meaningful. Therefore, the normalized Raman spectra of n-TiO2 and q-TiO2 samples were plotted, as shown in Figure 3. Four Raman scattering peaks at 144 cm-1, 231 cm-1, 446 cm-1, and 608 cm-1 are observed, which are


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assigned to the Raman-active vibrations of rutile TiO2 structure. It can be clearly seen that the position and width of Raman peak of TiO2 does not show any change after ice-water quenching. In additional to the Raman spectra, the FT-IR spectra of n-TiO2 and q-TiO2 samples pre-annealed at 900 °C and 1000 °C were also measured, which is shown in Figure 3 (B). It can be seen that the IR absorptions from 524 cm-1 to 634 cm-1 arising from the IRactive vibration of TiO2 lattice did not present obvious change after ice-water quenching. If considerable defects and crystalline structure change can be caused by the ice-water quenching, the position and width of Raman peak and IR absorption should have a clear change. However, the comprehensive analysis of XRD, Raman scatterings, FT-IR absorptions did not evidence any observable clear change, so it is concluded that ice-water quenching cannot alter the TiO2 structure on the macroscopic level of crystallography. Therefore, it is possible that the icewater quenching may just cause some crystalline change on the surface layer, which cannot be detected by the XRD, the Raman scattering, and IR absorption techniques.

Figure 3 (A) Normalized Raman Scattering spectra of n-TiO2 (solid line) and q-TiO2 (dashed line) samples preannealed at different temperature; (B) FT-IR absorptions of n-TiO2 and q-TiO2 samples pre-annealed at 900 °C and 1000 °C, respectively


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Figure 4 FE-SEM images of n-TiO2 and q-TiO2 samples pre-annealed at different temperatures ((A): n-TiO2-800 °C; (B): q-TiO2-800 °C; (C): n-TiO2-900 °C; (d): q-TiO2-900 °C; (E): n-TiO2-1000 °C; (F): n-TiO2-1000 °C), the scale bar is 200 nm

Because the grains of these samples are larger than 100 nm, the Scherer equation cannot be used to estimate the average grain size. [] The FE-SEM images of different samples were used to observe the particle morphologies and sizes. Figure 4 shows the FE-SEM images of n-TiO2 and q-TiO2 samples pre-annealed at 800 °C, 900 °C, and 1000 °C, respectively. The FE-SEM images of the samples pre-annealed at 1100 °C and 1200 °C were shown in Figure S1. It can be seen that the morphologies of TiO2 particles are polyhedral or spherical. The particles of these samples were fused together due to high temperature pre-annealing, with the particle size increasing as the pre-annealing temperature increases. The ice-water quenching has less effect on the particle morphologies and sizes of TiO2 materials. The particle sizes of the n-TiO2 and q-TiO2 samples were estimated by averaging more than 100 particles in the low-magnification FE-SEM images, which are also summarized in Table 1. The specific surface areas (SBET) of n-TiO2 and q-TiO2 samples pre-annealed at 900 °C and 1000 °C were measured by using a multi-BET techniques, as shown in Table 1, which shows that the ice-quenching process cannot change the specific surface areas of TiO2 samples. In summary, it can be known that the above analyses show that the icewater quenching cannot alter the macroscopic crystallinity, morphologies, and specific surface areas of TiO2 materials.

Table 1 Summaries of Lattice constants, grain size, SBET of n-TiO2 and q-TiO2 after pre-annealing at different temperature


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n-TiO2

T (°°C)

Lattice constants

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q-TiO2 r (nm)

SBET 2

(m /g)

a (Å)

b (Å)

800

4.5825

2.9458

110

900

4.5830

2.9625

223

1000

4.6077

2.9722

387

1100

4.5994

2.9438

1200

4.5961

2.9575

Lattice constants

r (nm)

SBET (m2/g)

a (Å)

b (Å)

4.5825

2.9458

110

4.60

4.4222

2.9611

191

4.3

1.61

4.5908

2.9572

360

1.62

800

4.5840

2.9738

781

1364

4.5899

2.9580

1350

Spectroscopic analysis Figure 5 (A) shows the UV-visible DRS spectra of n-TiO2 and q-TiO2 samples pre-annealed at different temperatures. It was seen that the ice-water quenching almost has no effect on the UV-Vis DRS spectra when the pre-annealing temperature was 800 °C. When the pre-annealing temperature was higher than 900 °C, the UV-Vis DRS of q-TiO2 samples become clearly different from that of n-TiO2 samples. The q-TiO2 samples exhibit a broad absorption in the visible and near-IR region. Figure 5 (B) shows the absorption differences between the n-TiO2 and q-TiO2 samples, which were obtained by subtracting the DRS of n-TiO2 with that of q-TiO2. The absorptions in visible and NIR region show a regular increase with increasing the pre-annealing temperature for the q-TiO2 samples. The UV-Vis DRS of q-TiO2 samples are similar to that of Ti3+-self-doped TiO2, indicating that the icewater quenching can give rise to Ti3+ self-dopants in TiO2 lattice. Others reported that the absorption edge of Ti3+self-doped TiO2 showed red-shift as compared to the pristine TiO2. 29, 30 However, the present q-TiO2 samples do not present the observable red-shift, which may be due to the low concentration of Ti3+ defects, as indicated by the UV-Vis DRS. The result of UV-Vis DRS analysis is in good accordance with the digital pictures shown in Figure 1.


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Figure 5 (A) UV-Vis diffusion reflection spectra (DRS) of n-TiO2 and q-TiO2 samples pre-annealed at different temperatures; (B) UV-Vis DRS difference by subtracting the DRS of n-TiO2 with that of q-TiO2. Although the ice-water quenching cannot cause the clear change to the macroscopic crystallinity and the morphologies of TiO2 samples, the above UV-Vis analysis clearly shows that the electronic structures can be actually altered by the ice-water quenching. Therefore, the PL spectra were further measured to see the effect of ice-quenching. Figure 6 (A) shows the normalized PL spectra of n-TiO2 and q-TiO2 samples. For the samples preannealed at 800 °C and 900 °C, it can be seen that the normalized PL spectra almost show no difference after the ice-water quenching, also showing that the change of electronic structure induced by ice-queching is small, which is in accordance with the UV-Vis diffusion spectra. When the pre-annealing temperature is higher than 1000 °C, it is seen that the normalized intensity of the PL spectrum tails from 430 nm to 600 nm slightly decrease after icequenching. The differences of normalized PL spectra between the n-TiO2 and q-TiO2 samples are shown in Figure 6 (B), obtained by substracting the normalized PL spectra of n-TiO2 with that of q-TiO2, which clearly show the decrease of PL signals after the ice-water quenching. The PL signals in this region are usually assigned to the irrdiation recombination of excitions trapped at the intrinsic defects of TiO2, and such defects can be oxygen defects or Ti3+ defects. Combining with the UV-Vis results, it can be concluded that the slight decrease of normalized PL intensity should be due to the formation of self-dopants induced by the ice-water quenching. Therefore, the combined analysis of UV-Vis and PL spectroscopies shows that the ice-water queching can create the self-dopants in TiO2 lattice.


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Figure 6 (A) Normalized PL spectra of n-TiO2 samples (solid line) with that of q-TiO2 samples (dashed line); (B) Normalized PL differences between the n-TiO2 and q-TiO2 samples. Chen et al. showed that the hydrogenation of TiO2 surface has a great effect on its VB spectra. 30 The rapid icewater quenching may also cause the similar change because the treatment of TiO2 surface with cold water, so the VB XPS spectra of n-TiO2 and q-TiO2 samples pre-annealed at 900 °C and 1200 °C were measured, as shown in Figure 7 (A). Irrespective of the pre-annealing temperature, the slopes of the VB edges (labelled as black dashed lines) decrease for the q-TiO2 samples as compared to the n-TiO2 samples, showing that the ice-water quenching may widen the VB of surface layer of TiO2 and correspondingly reduce the density of states (DOS) of VB. In addition, it is seen that the VB edges shift upward for both q-TiO2 samples. The Ar+ ion etching does not change the slope of VB (red line in Figure 7 (A)), indicating that the quenching also changes the electronic structure of TiO2 bulk in additional to surface. It is seen that the VB edge shifts downward after Ar+ ion etching, implying that the VB edge position of TiO2 is determined by surface electronic structure, also according with the research. 30 The above results support that the interaction with cold water during ice-water quenching changes the surface structure of TiO2 to some extent, and more defects are created both on surface and in bulk. It is possible that the ice-water quenching can cause more surface hydroxyl groups (-OHs) due to the interaction with water. O1s corelevel XPS spectra of the n-TiO2 and q-TiO2 samples pre-annealed at 900 °C and 1200 °C are shown in Figure 7 (B) and (C), respectively. The O1s spectra were fitted by two Gaussian peaks at 529.6 eV and 531.7 eV, which are assigned to lattice oxygen and surface OHs, respectively. The amounts of OHs do not show obvious change after ice-water quenching (~ 20%), so the ice-water quenching cannot cause the more OHs on TiO2 surface. The FT-IR spectra (Figure 3 (B)) of n-TiO2 and q-TiO2 samples after pre-annealed at 900 °C and 1200 °C also show that that water amounts on TiO2 surface cannot be clearly altered by ice-water quenching, as the IR absorptions around 3400 cm-1, which are ascribed to the vibration of chemically-adsorbed water, do not present obvious change, in accordance with the XPS analysis. Moreover, XPS was also used to study the surface chemical


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composition of n-TiO2 and q-TiO2 samples. The Ti2p peaks of n-TiO2 and q-TiO2 samples have similar shapes before Ar+ ion etching (Figure S2). It can be known the surface of TiO2 samples is mainly composed of Ti4+, because the Ti3+ ions are susceptible to be oxidized by O2 in air, which is also in accordance with most of other studies, 32 but contradicted with the study performed by Supphasrirongjaroen et al..35 The Ti2p spectra of n-TiO2 also show no obvious change after Ar+ ion etching, whereas the Ti2p spectrum of q-TiO2 exhibited the evidence of Ti3+ ions, indicating that the ice-quenching process can create Ti3+ in TiO2 bulk, as shown in Figure 7 (D).

Figure 7 (A) Valence band XPS spectra of q-TiO2 and n-TiO2 samples pre-annealed at 900 °C and 1200 °C; (B) O1s high-resolution XPS spectra of n-TiO2 and q-TiO2 pre-annealed at 900 °C; (C) O1s high-resolution XPS spectra of n-TiO2 and q-TiO2 pre-annealed at 1200 °C; (D) Ti2p high-resolution XPS spectra of the q-TiO2 sample pre-annealed at 1200 °C before and after Ar+ ion etching; However, by considering that Ar+ ion bombardment can also induce the reduction of TiO2 and result in Ti3+ ions, 37

the XPS analysis cannot be used as the direct evidence of the presence of Ti3+ defects. EPR spectra recorded at

100 K were also measured to verify the existence of Ti3+ in the ice-quenched samples, as shown in Figure 8 (A)(D) for the n-TiO2 and q-TiO2 samples after pre-calcination at 900 °C, 1000 °C, 1100 °C, and 1200 °C, respectively. The n-TiO2 and q-TiO2 samples present the EPR signals at g=2.003 assigned to the electrons trapped in oxygen defects or adsorbed O2, showing that both of them are oxygen deficient TiO2. The oxygen deficient TiO2 surface tends to chemically adsorb O2 at oxygen vacancies. The excess electrons induced by oxygen defects


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can transfer to O2, resulting in the formation of O2•-,38 so the oxygen defects on TiO2 surface provide an important pathway for electron transfer to O2. The q-TiO2 samples present an intense paramagnetic signal at g≈1.95, whereas the n-TiO2 samples have not, which are assigned to the paramagnetic Ti3+ species in bulk as the XPS did not detect Ti3+ on surface. 30,32,33 it can be also seen that the intensity of this EPR signal greatly increases when the pre-annealing temperature is 1000 °C, so higher pre-annealing temperature can create more Ti3+ defects in TiO2. The further increase of pre-annealing temperature to 1100 °C to 1200 °C cannot obviously increase the amounts of Ti3+ ions. This result is in good agreement with the UV-Vis spectra and the PL spectra (Figure 5 and Figure 6). In summary, the EPR analysis shows that the n-TiO2 samples mainly contain oxygen vacancies, whereas the qTiO2 samples contain both oxygen vacancies and Ti3+ defects. The first-principle calculation indicates the localized electron can delocalized over several Ti sites at room temperature due to the strong electron-photon interaction, so the Ti3+ signals should decrease at room temperature. 39,40 It was observed that the Ti3+ signal in EPR spectra of room temperature decreases greatly due to the delocalization of electrons.

Figure 8 EPR spectra recorded at 100 K for a-TiO2 and q-TiO2 after pre-annealing at 900 °C (A), 1000 °C (B), 1100 °C, and 1200 °C (B), respectively. Ice-quenching induced surface lattice distortion Figure 9 shows the high-magnification TEM images of n-TiO2 and q-TiO2 samples pre-annealed at 900 °C and 1200 °C, with the low-magnification TEM images and the selected area electron diffractions (SAED) patterns being shown in corners. It can be seen from the TEM images that the ice-water quenching does not change the


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morphologies of TiO2 particles and their inter-connection, in accordance with the FE-SEM analysis. When the annealed temperature was 1200 °C, the TiO2 particles fused together to form the large particles. The interplanar spacing was also labeled, which shows the rutile phase of both q-TiO2 and n-TiO2 samples, in good agreement with the XRD and Raman analysis. Careful inspection showed that the surface lattice fringes of TiO2 exhibit obvious change after the ice-water quenching. For the samples annealed at 900 °C, it can be seen from Figure 9 (C) that the lattice fringes of q-TiO2 on surface are obviously distorted and tends to be disordered, whereas that of n-TiO2 are almost regular. Moreover, it can also be observed that the comparison between the SAED patterns of q-TiO2 and n-TiO2 samples shows that the q-TiO2 becomes a little disordered (Figure 9 (A) top-right corner and Figure 9 (B) left-down corner), whereas that of n-TiO2 shows the regular diffraction patterns. For the samples annealed at 1200 °C, it is seen that the lattice fringes of q-TiO2 become more distorted (Figure 9 (E) and (F)), which is also revealed by the SAED (Figure 9 (E), left-down corner). The lattice fringes and SAED of n-TiO2 sample pre-annealed at 1200 °C are still very regular (Figure 9 (D)). For the n-TiO2 and q-TiO2 samples preannealed at 1000 °C and 1100 °C, similar surface lattice distortion can also be seen, as shown in Figure S3. The TEM analysis clearly shows that the surface lattice distortion of TiO2 tends to become increased when the preannealing temperature increases. As shown in Figure 9 (F), after pre-annealing at 1200 °C, the surface lattice of q-TiO2 sample shows the obvious poly-crystallization. Therefore, it is known that the ice-water quenching can also alter the surface crystalline structure of TiO2 to distortion, in additional to the change of electronic structures. More intrinsic defects, such as oxygen vacancies, are inevitable to be created on TiO2 surface by accompanied with the disorder change of surface crystallinity. It was reported that the thin disordered layer on TiO2 surface can induce the up-shift of VB edge. 28 Therefore, it is possible that the surface lattice distortion and polycrystallization cause the up-shift of VB edge and the decrease of DOS of VB, as revealed by XPS analysis. During the ice-water quenching, the sudden contact of highly-hot TiO2 with ice-water can induce the fast shrinkage of surface layer, which should be the physical reason resulting in the lattice distortion and the polycrystallization of TiO2 surface.


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Figure 9 High-magnification TEM images of n-TiO2 and q-TiO2 samples after pre-annealing at 900 °C and 1200 °C, respectively ((A): n-TiO2-900 °C, (B): q-TiO2-900 °C, (C): magnification of Figure 5 (B), (D): n-TiO2-1200 °C, (E) q-TiO2-1200 °C, (F): magnification of Figure 5 (E)).


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Ti3+ formation and electronic structure It is considered that the ice-water quenching induced Ti3+ ions cannot be ascribed to reduction effect, because none reductants were used in additional to water. From the viewpoint of thermodynamic, water molecule are also impossible to be heatedly-decomposed as hydrogen during the ice-water quenching because the temperature is still lower than water decomposition temperature due to high O-H bonding energy. Therefore, it is highly possible that the Ti3+ generation should be attributed to a physical cause. The kinetic of oxygen vacancy formation in TiO2 should follow activation mechanism, so TiO2 may present high concentration of oxygen impurities at high temperature, even in the atmosphere of air. This high concentration of oxygen impurities can be partially frozen by the ice-water rapid quenching, while the normally-cooling process cannot. Theoretically, an oxygen vacancy has an effective charge of +2; thus, it is likely to present in the form of crystal neutral center (Ov•2Ti3+)0, singly ionized center (Ov+•Ti3+)+, and free vacancy (doubly ionized centers, Ov++). 41, 42 In the case of low concentration, oxygen vacancies mainly exist in the form of Ov++, with the electrons trapped at oxygen vacancies being thermally-activated to CB of TiO2, and none Ti3+ ions can be formed. However, for the high concertation, partial electrons trapped at oxygen vacancies can also transfer to the Ti4+ sites in additional to thermal activation to CB, which can result in the (Ov+•Ti3+)+ and (Ov•2Ti3+)0. For the n-TiO2 samples, the concentration of oxygen vacancies is not so high that the Ti3+ ions can be created because the oxygen vacancies generated at the temperature of pre-annealing was compensated with the oxygen in air. However, for the ice-water quenched samples, the high concentration of oxygen vacancies at high pre-annealing temperature are frozen to some extent, so their concentration should be higher than the normally-cooled samples, and the oxygen defect amount can be high enough to generate Ti3+ defects in TiO2 lattice. The UV-visible absorptions and EPR results show that the generation of oxygen vacancies in TiO2 lattice is highly temperature-dependent and should belong to activated mechanism. Our experiments show that the pre-annealing at temperatures lower than 800 °C may have not energy enough to activate the formation of oxygen defects in the atmosphere of air, as the UV-Visible absorptions of qTiO2 and n-TiO2 samples do not show much difference, which is in agreement with the formation mechanism of thermal defects. 41 According to the above analysis, the proposed energy band structure of q-TiO2 can be shown as Figure 10. According to the lowest principle of energy, the doubly ionized Ov++ species should locate just below the CB of TiO2, which form shallow donors yielding electrons in CB via the electron-phonon coupling effect. The singly ionized (Ov+•Ti3+)+ species locate beneath the energy level of OV++, and the electron trapped at an oxygen defect can be excited to CB. The neutral (Ov+•2Ti3+)0 should lie at the lowest position.


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Figure 10 Proposed energy band structure of Ti3+ self-doped q-TiO2 together with oxygen vacancies The energy levels of Ti3+ defects are associated with the chemical coordination and crystalline polymorphs. Theoretical calculations basing on DFT+U and B3LYP showed that the energy level of Ti3+ ions adjacent to oxygen defects locates at ~ 0.7 eV below the CB.43 It was also reported that the Ti3+ defects accompanied with oxygen defects are localized in the bandgap at ~ 0.8 to 1 eV below CB. 44 Disputes still exist for the physical reason for the blue color of Ti3+self-doped TiO2. Someone assigned the blue color to the electron transition from VB to Ti3+ defect band. 26 It was also assigned to the intra-band transition because that the Ti3+ defects could act as donors for yielding electrons in CB. 24 In addition, d-d transition from Ti3+ band-gap states to their resonant excited states is also considered for the blue color. Our analysis indicates that the (Ov+•Ti3+)+ and (Ov+•2Ti3+)0 species relating to Ti3+ defects are half-filled and fully-occupied, so the transition from VB to Ti3+ defects is largely inhibited. In addition, the Ti3+ defects locate deep ( > 0.7 eV below CB) within the band gap of TiO2. The electrons trapped at Ti3+ defects cannot be activated to CB as the thermal energy (~ 0.026 eV at room temperature) is much lower than it’s ionized energy, so the blue color of q-TiO2 may not induced by intra-band transition. The study of two-photo photoemission spectroscopy and DFT theoretical calculation supported that the distribution of Ti3+ gap states and the resonant excited states are relatively wide, so the d-d transition between them can result in the light absorption extending to visible light region. 25 In addition, the transition from the Ti3+ defects to the CB of TiO2 is possible under the illumination of near-infrared light. From this viewpoint, the d-d extended light absorption together with the near-IR absorption may result in the blue color of q-TiO2 samples, which is in agreement with the study 25.

Photocatalytic properties


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The photo-degradation of MB water solution was used to evaluate the difference between the photocatalytic activities of n-TiO2 and q-TiO2 samples. As shown in Figure S4, it was seen that the q-TiO2 samples pre-annealed at 900 °C shows obviously higher activity than the n-TiO2 samples. According to the quasi-first order mode, the dependences of ln (C(0)/C(t)) on time t were plotted in Figure 11 (A) and 11 (B) for the n-TiO2 and q-TiO2 samples, respectively. It is seen that the photocatalytic activities of both of n-TiO2 and q-TiO2 samples decrease with the increase of pre-annealing temperature, due to the great decrease of specific surface area. The comparison between Figure 11 (A) and (B) also shows that the q-TiO2 samples exhibit obviously higher photocatalytic activities than the n-TiO2 samples. Therefore, it is known that the ice-water quenching indeed increases the photocatalytic activities of TiO2 materials. The reaction constant (k) were gotten by linearly fit with the quasi-first order mode, with the dependences of the ratio of kque and knor (kque/knor) on the pre-annealing temperature being shown in Figure 11 (C). It is seen that the kque/knor firstly increases and then decreases with the increase of preannealing temperature. The q-TiO2 samples pre-annealed at 1000 °C showed the most increased photocatalytic activity as compared to the normally cooled samples. The EPR result and TEM analysis show that the increase of pre-annealing temperature increases the amount of Ti3+ defects and the surface lattice distortions, which may be the physiochemical reason for the increase of photocatalytic activity via the inhibition of recombination. However, the result also shows that too high pre-annealing temperature can generate more defects in TiO2 lattice and lead to the surface poly-crystallization, which may not effectively increase the photocatalytic activities. The stability of a photocatalyst is also important for practical application of TiO2 materials. In this study, a photo-stability test for the q-TiO2 sample pre-annealed at 900 °C was carried out for three cyclic runs of MB degradation. The result shows that the sample does not show significant decrease for the dye degradation after several runs, indicating the good stability of q-TiO2 samples (Figure 11 (D)). It is considered that the presence of Ti3+ ions cannot be directly related to the increase of photocatalytic activities because the direct interaction between O2 and surface Ti sites is not energetically favourable. As illustrated above, the Ti3+ ions correspond to the localization of the oxygen defect induced excess electrons at normal Ti lattice sites. The presence of Ti3+ ions indicates that more oxygen defects are created at TiO2 surface by the ice-water quenching. Therefore, the presence of oxygen defects and excess electrons favours the selective adsorption of O2 at oxygen vacancy sites, 45,46 which must be beneficial for the acceleration of the electron interfacial transfer to O2, and consequently leads to the increase of photocatalytic activities. The surface lattice distortion or poly-crystallization should be correlated with the creation of surface oxygen defects. Higher lattice distortion would cause more surface oxygen defects. In addition, the surface lattice distortion may increase the reactant adsorption on TiO2 surface due to high surface energy, which is also helpful for improving the photocatalytic activity. However, too more oxygen defects may accelerate the recombination of electrons and holes, resulting in the decrease of photocatalytic activities.


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Figure 11 (A) Time dependences of ln(C(0)/C(t)) in the course of MB photocatalytic degradation by using n-TiO2 samples pre-annealed at different temperatures; (B) Time dependences of ln(C(0)/C(t)) in the course of MB photocatalytic degradation by using q-TiO2 samples pre-annealed at different temperatures; (C) Dependence of kque/knon on the pre-annealing temperature; (D) Cyclic runs for the photocatalytic MB degradation on q-TiO2 sample subjected to 900 °C pre-annealing. However, our experiments saw that the q-TiO2 and n-TiO2 samples are almost inactive under the visible light illumination, which may be due to the weak visible light absorption. Liu et al. saw that the Ti3+-self doped rutile TiO2 are also visible-inactive, in good agreement with our results.21 They showed that the Ti3+ self-doped TiO2 can present obvious visible activity after grafted with amorphous Cu(OH)x amorphous nanoclusters, owing to the synergistic action between Ti3+ defects and Cu(OH)x nanoclusters.23 Inspired by this research, the same technique was used to load Cu(OH)x amorphous nanoclusters on surface of q-TiO2 and n-TiO2 samples pre-annealed at 900 °C. The XPS analysis shows that the Cu exists mainly in the form of Cu+ (Cu(I)), and ratio of Cu to TiO2 on surface is near 1.2 at. % for both q-TiO2 and n-TiO2 samples (Figure S5). The TEM images (Figure S6) of Cu(I)@q-TiO2 and Cu(I)@n-TiO2 show the existence of amorphous nanoclusters of several nanometers, which are assigned to the Cu(I) hydrated Cu(OH)x amorphous nanoclusters on TiO2 surface. The photocatalytic


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decomposition of MB under visible light illumination is shown in Figure 12 (A). It can be seen that the quasifirst-order reaction rate constant (kque and knon) of the Cu(I)@q-TiO2 and the Cu(I)@n-TiO2 are 0.032 min-1 and 0.022 min-1, respectively. The samples without grafting with the Cu(OH)x amorphous nanoclusters exhibited the neglected photocatalytic activity for degrading MB under the visible light illumination. According to the research 21, the visible light responded photocatalytic activity of Cu(I)@TiO2 can be ascribed to the direct electron transition from VB of TiO2 to Cu(OH)x amorphous nanoclusters. The function of Cu(OH)x on the increase of visible photocatalytic activity is not the main aim of this research, which will be investigated in further studies.

Moreover, the photocatalytic activities of the q-TiO2 and n-TiO2 sanples for water photocatalytic reduction to produce hydrogen were also studied under the illumination of UV light illumination. After loading 0.05 g of sample with 0.5 wt% Pt, the q-TiO2 and n-TiO2 samples were placed into a methanol-containing aqueous solutions in a closed gas vessel. Figure 12 (B) shows the time course of H2 evaluation for q-TiO2 and n-TiO2 samples pre-annealed at 900 °C and 1000 °C. It can be seen that the photocatalytic reactions exhibit a stable H2 release rate. The H2 evaluation efficiency is increased by 46 % and 52 % through ice-water quenching, respectively, for the samples subjected to pre-annealing at 900 °C and 1100 °C.

Figure 12 (A) Time Dependences of ln (C(0)/C(t)) for the photocatalytic degradation of MB aqueous solution by Cu(I)@q-TiO2 and Cu(I)@n-TiO2 samples under visible light illumination; (B) Time-course of hydrogen evolution on the q-TiO2 and n-TiO2 samples pre-annealed at 900 °C and 1000 °C under the illumination of UV light. Proposed effect of Ti3+ defects and surface distortion on photocatalysis of q-TiO2 samples


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Some studies reported the visible activities of Ti3+ self-doped TiO2 for both dye degradation 24 and water reduction to hydrogen. 29 These studies showed that the optical absorption edge of Ti3+ self-doped TiO2 presents an obvious red-shift because of the high concentration of Ti3+ defects. The DFT theoretical calculation showed that the high concentration of Ti3+ defects in bulk TiO2 is responsible for band narrowing. However, the optical absorptions of the present q-TiO2 samples do not show obvious red-shift because the amount of Ti3+ defects created by ice-water quenching is limited, which is one reason that the q-TiO2 samples are visible inactive. Our result is in good agreement with the report of Liu et al., as they also did not observe obvious activity for the Ti3+self doped TiO2 that did not exhibit optical red-shift. According to the above analysis, for the low concentration of Ti3+ defects, three types of oxygen defects exist within the band gap. The doubly ionized OV++ defects are empty, which allows the electron transition from VB to them. However, the concentration of empty oxygen defects may be too low to produce a large optical red-shift, which cannot result in the great visible absorption. The (Ov+•Ti3+)+ and (Ov+•2Ti3+)0 are half-filled and fully-filled, so the electron transition from VB to such gap states is limited, which also cannot lead to the visible light activity. Although the localized Ti3+ electrons are possible to undergo resonant absorption via d-d transition, the oxidative ability of the holes at Ti3+ energy levels may be too negative to produce photocatalytic activity. Suffering from these problems, the pure q-TiO2 samples do not show visible activity. When Cu(I) amorphous nanoclusters were grafted on the TiO2 surface, we agree with the mechanism advanced by liu et al. 23 that the cooperation between Ti3+ defects and Cu(I) amorphous nanoclusters leads to visible activity. Firstly, the d-d transition of Ti3+ defects induced by visible light illumination can empty some Ti3+ defects, which allows the electron transition from VB of TiO2 to the Ti3+ defects. The electrons can quickly transfer to Cu(I) amorphous nanoclusters for further multi-electron transfer. In addition, the direct charge transfer from VB to Cu(I) amorphous nanoclusters is also proposed for visible light activity. Therefore, the synergetic effect between Ti3+ defects and Cu(I) amorphous nanoclusters enhances the visible-responded photocatalytic activities for ice-quenched samples. The possible synergetic effect between Ti3+ defects and Cu(I) nanoclusters will not be carefully discussed in this research. The function of Ti3+ defects on UV light photocatalysis is still in dispute. It is generally considered that Ti3+ defects are localized and locate deeply in the band gap of rutile TiO2 (> 0.7 eV below CB edge). The Ti3+ defects formed within the particle bulk mainly act as recombination centers and reduce the photocatalytic activity. 46,47 For example, it was shown that the Ti3+self-doped rutile TiO2 materials had very low activity for acetone decomposition under UV light. 23 However, it was also reported that the increased photocatalytic activity of rutile TiO2 for H2 reduction, and the oxygen vacancies that associated with Ti3+ defects were believed to play a significant role.48 The transient infrared (IR) spectroscopy study showed that the increased activity for H2 reduction is related to an increase in the density of photo-excited electrons, which means that Ti3+ defects and oxygen vacancies can reduce the recombination loss. 49 Recently, the study of EPR and electron conductivity


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showed that the increased photocatalytic activity for H2 reduction is closely related to the increased electron conduction, partially to the band bending at the interface between electrolyte and TiO2. 26In the present research, we also observed that the MB degradation and hydrogen generation both increases after introducing some Ti3+ defects and oxygen defects through the ice-water quenching. Although the Ti3+ defects mainly locate in TiO2 bulk,50 which cannot act as the catalytic centers for interfacial transfer of electrons, but the Fermi level of q-TiO2 should shift upward by Ti3+ self-doping, which correspondingly increases the electron transport. In addition, although we cannot confirm the detailed effect of surface lattice distortion on the increased photocatalytic activity, the lattice distortion and the crystallinity disorder must be correlated with the formation of oxygen and Ti3+ defects, so suitable lattice distortion may create more photocatalytic catalytic centers, finally resulting in the increase of photocatalytic activity.

CONCLUSION In summary, the Ti3+ self-doped rutile TiO2 materials with surface lattice distortion can be prepared by ice-water quenching, which shows obviously increased photocatalytic activity for both MB photo-degradation and hydrogen evolution from water under the UV light illumination. The Ti3+ ion formation is ascribed to the high concentration of oxygen vacancies induced by the ice-water quenching. It is also considered that the creation of oxygen vacancies is correlated to the surface lattice distortion. The increase of photocatalytic activity is believed to be related to presence of Ti3+ defects, surface distortion, and the associated oxygen defects, which was explained from the viewpoint of energy band structure change induced by the ice-water quenching. In addition, it is also seen that q-TiO2 samples are visible-inactive because they cannot absorb much visible light or the photo-excited electrons at Ti3+ defect level cannot transfer outside. Grafting Cu(I) amorphous nanoclusters on the q-TiO2 surface can improve the visible activity due to the synergetic action between Ti3+ defects and Cu(I) nanoclusters. The icequenching reported here should be an extensive general method to create defects in oxides, which must have the important effect on the performances of other oxides, in additional to TiO2. As example, the commercial ZnO powders were also quenched in ice water. The differences of the light absorption and the photocatalytic activity were measured, as shown in Figure S7 and Figure S8. It was seen that the ice-water quenching indeed changes the light absorption of ZnO due to the generation of oxygen defects, and accordingly the photocatalytic activity was also slightly increased.

ASSOCIATED CONTENT Supporting Information. Additional supporting figures (FE-SEM images, Ti2p XPS spectra, MB photodegradation, Cu2p XPS spectra, TEM images of Cu@TiO2, UV-Visible absorption of normally-cooled and


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quenched ZnO, Photocatalytic activity of normally-cooled and quenched ZnO) were shown in the supporting information

AUTHOR INFORMATION Corresponding Author B. Liu ([email protected]); X. Zhao ([email protected] )

ACKNOWLEDGMENT B. Liu acknowledges the Fundamental Research Funds for the NSFC (No. 51772230, No. 51461135004) and the Japan Society for the Promotion of Science (JSPS) for an Invitation Fellowship for Foreign Researchers (ID: L16531).

REFERENCES 1. Fujishima, A.; K. Honda. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37-38. 2. Yamaguchi, Y.; Shimodo, T.; Usuki, S.; Torigoe, K.; Terashima, C.; Katsumata, K.; Ikekita, M.; Fujishima, A.; Sakai, H.; Nakata, K. Different Hollow and Spherical TiO2 Morphologies Have Distinct Activities for the Photocatalytic Inactivation of Chemical and Biological Agents. Photochem. Photobiol. Sci. 2016, 15, 988994. 3. Kuma, S. G.; Devi, L. G. Review on Modified TiO2 Photocatalysis under UV/visible Light: Selected Results and Related Mechanisms on Interfacial Charge Carrier Transfer Dynamics. J. Phys. Chem. A 2011, 115,13211-13241. 4. Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114, 9919-9986. 5. Paul, T.; Miller, P. L.; Strathmann, T. J. Visible-Light-Mediated TiO2 Photocatalysis of Fluoroquinolone Antibacterial Agents. Environ. Sci. Technol. 2017, 41, 4720-4727. 6. Sotelo-Vazquez, C.; Noor, N.; Kafizas, A.; Quesada-Cabrera, R.; Scanlon, D. O.; Taylor A., Durrant, J. R.; Parkin, I. P. Multifunctional P-doped TiO2 Films: A New Approach to Self-Cleaning, Transparent Conducting Oxide Materials. Chem. Mater. 2015, 27, 3234-3242.


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IceâWater Quenching Induced Ti3+ Self-doped TiO

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IceâWater Quenching Induced Ti3+ Self-doped TiO