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Ti3+ Self-doped Blue TiO2(B) Single-Crystalline Nanorods for Efficient Solar-driven Photocatalytic Performance Yan Zhang, Zipeng Xing, Xuefeng Liu, Zhenzi Li, Xiaoyan Wu, Jiaojiao Jiang, Meng Li, Qi Zhu, and Wei Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09061 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 23, 2016

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ACS Applied Materials & Interfaces

Ti3+ Self-doped Blue TiO2(B) Single-Crystalline Nanorods for Efficient Solar-driven Photocatalytic Performance Yan Zhanga, Zipeng Xinga,*, Xuefeng Liua, Zhenzi Lib, Xiaoyan Wub, Jiaojiao Jianga, Meng Lia, Qi Zhua,*, Wei Zhoua,* a

Department of Environmental Science, School of Chemistry and Materials Science, Key

Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, P. R. China, b

Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin 150086, P.

R. China Tel: +86-451-8660-8616, Fax: +86-451-8660-8240, Email: [email protected]; [email protected]; [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Ti3+ self-doped blue TiO2(B) single-crystalline nanorods (b-TR) are fabricated via a simple sol-gelation method, cooperated with hydro-thermal treatment and subsequent in-situ treatment method, afterwards, annealed at 350 oC in Ar. The structures are characterized by Xray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (UV-vis), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The prepared b-TR with narrow bandgap possess single-crystalline TiO2(B) phase, Ti3+ self-doping and one-dimensional （ 1D ） rod-like nanostructure. In addition, the improved photocatalytic performance is studied by decomposition of Rhodamine B (RhB) and hydrogen evolution. The degradation rate of RhB by Ti3+ self-doped blue TiO2(B) singlecrystalline nanorods is ~ 6.9 and 2.1 times higher compared with titanium dioxide nanoparticles and pristine TiO2(B) nanorods under visible light illumination, respectively. The hydrogen evolution rate of b-TR is 26.6 times higher compared with titanium dioxide nanoparticles under AM 1.5 irradiation. The enhanced photocatalytic performances arise from the synergetic action of the special TiO2(B) phase, Ti3+ self-doping, and the 1D rod-shaped single-crystalline nanostructure, favoring the visible light utilization, the separation and transportation of photogenerated charge carriers. KEYWORDS:

TiO2(B);

Ti3+

self-doping;

single-crystalline

photocatalysis; hydrogen evolution


2

nanorod;

solar-driven

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1. INTRODUCTION Semiconductor photocatalysts have attracted enormous attention in photocatalytic hydrogen evolution,1 biocatalytic,2 dye-sensitized solar cells3 and pollutant degradation,4 because of their potential applications. Especially, titanium dioxide (TiO2) nanomaterials5-8 have been widely studied for their low toxicity, good chemical stability, and superior photocatalytic activity.9,10 However, the photocatalytic activity of TiO2 exists two major shortcomings, one is the bandgap is 3.2 eV, it is relatively wide, which limits its application in the visible light region, another is the fast recombination of photoinduced electrons and holes reduces photo-induced redox reaction.11,12 Therefore, it is urgent to extend the optical absorption of TiO2 to visible light and increase the separation efficiency of photoinduced charge carriers. To overcome these disadvantages, various efforts are made to improve visible light absorption of TiO2 including metal,13,14 non-metal15,16 and self-doping.17,18 The drawback of these approaches, nevertheless, is that the introduction of dopants, which are often as charge carrier recombination centers, lead to the decreased photocatalytic activity.19 The latest research suggests that, the introduction of crystal defects in TiO2 can extend the photo-absorbance into the visible light region by inhibiting the effect of recombination.20 Consequently, Mao et al.21 present a high pressure hydrogenation method to prepare Ti3+ self-doping TiO2.22,23 The introduction of Ti3+ in TiO2 is efficiently narrow the bandgap and inhibit the fast recombination of e--h+ pairs.24,25 The as-prepared black TiO2 shows excellent photocatalytic activities for dye degradation and hydrogen generation. In Mao's study, black TiO2 exhibits good photocatalytic performance, while the high-pressure preparation approach is unsiutable for practical application.26 What should be noted is that using short processing time and mild conditions such


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as in-situ reduction treatment is an effective method to improve photocatalytic and photoelectrochemical performances. As is well known, TiO2 is found in four main crystal phases: anatase,27 rutile,28 brookite29 and monoclinic TiO2(B).30-32 Among four types crystal phases of TiO2, TiO2(B) is generally considered to be the most active, which is widely used in many applications, including lithiumion batteries33-35 and photocatalysts

36,37

because of their long cycle life, high safety and

minimum environmental impact. TiO2(B) is first synthesized in 1980 by Marchand et al.38 through a pathway involving calcination of titanate H2Ti4O9 at 500 oC. Since then, the calcination method has been widely used for the preparation of TiO2(B) in various forms such as nanowires,39 nanorods,40 nanobelts41 and nanotubes.42 The morphology of titanium dioxide crystals

play

a

critical

role

in

influencing

the

photocatalytic

performance

and

photoelectrochemical performance.43 1D TiO2 nanostructure such as nanorod shows excellent photocatalytic and photoelectrochemical performances, which is able to provide direct transport route and by that means promote the transfer of electrons and restrict the recombination of e--h+ pairs when compares with the TiO2 nanoparticles.44 As previously reported, the bandgap of TiO2(B) is only 3.09 eV,45 which can improve photocatalytic activity with visible light illumination. However, the photoelectrochemical performance of TiO2(B) is seriously restricted by its extremely low electronic conductivity.46 In consequence, this problem may be solved by preparing TiO2(B) combined with 1D nanostructure. Based on the above discussion, we rationally design a facile, effective and environmentally friendly method to prepare Ti3+ self-doped blue TiO2(B) single-crystalline nanorods. The bandgap of the as-prepared sample is reduced to 2.61 eV. The greatly enhanced photocatalytic activity for degradation of RhB towards to visible light is examined. Furthermore, the


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photocatalytic hydrogen generation ability and photocurrent density are dramatically improved. The formation and photocatalytic mechanism of blue TiO2(B) single-crystalline nanorod are also determined. 2. EXPERIMENTAL 2.1. Materials. Except sodium hydroxide (NaOH) was purchased from Tianjin Kermel Chemical Reagent Co. LTD, China. Most of reagents were the same in our previous reports.47 2.2. Synthesis of Photocatalysts. 2.2.1. Synthesis of Titanium Dioxide Powers. The titanium dioxide powder was synthesied via the described sol-gelation approach. Briefly, 5 mL of TBOT was dispersed in 20 mL of anhydrous alcohol to form TBOT and anhydrous alcohol solution which was marked solution A. Simultaneously, another aqueous mixtures of 8 mL of DI water, 2 mL of HNO3 and 8 mL of anhydrous alcohol were with continuous mixing to prepare solution B. The solution B was stirred for half an hour at R.T. (20±2 oC) until formed transparent solution. Later, the clear solution B was slowly added drop by drop to solution A with strong stirring for 4 h to promote the hydrolysis. For gelation, the resulting solution was left 24 h at R.T. (20±2 oC), followed by drying at 60 oC for 12 h in an oven. Subsequently, the obtained powers were heated for 120 min at 500 oC with 2 oC·min-1 of heating-up rate. Therefore, the TiO2 nanoparticle was successfully obtained, which was denoted as TP. 2.2.2. Preparation of TiO2 (B) Single-crystalline Nanorods. Uncalcined 2 g of TiO2 was added in 45 mL of 10 M sodium hydroxide solution ultrasonically for half an hour. Thereafter, the suspension was then put into a Teflon-sealed autoclave of 100 mL and heated at 180 oC for 36 h in an oven. Down to R.T. (20 ±2 oC), the white resulting mixtures were collected, rinsed


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with 0.1 M dilute hydrochloric acid and DI water for several times, oven dried at 60 oC for 12 h, and then annealed for 120 min at 500 oC with 2 oC·min-1 of heating-up rate. Finally, the white TiO2(B) single-crystalline nanorod was successfully prepared, which was denoted as TR. 2.2.3. Preparation of Blue TiO2(B) Single-crystalline Nanorods. The blue TiO2(B) single-crystalline nanorod was prepared by NaBH4 reduction, the detail method is reported by previous studies.48 Finally, the blue TiO2(B) single-crystalline nanorod was successfully synthesized, which was denoted as b-TR. After 10 M NaOH hydrothermal, then with the treatment of 0.1 M dilute hydrochloric acid and DI water, Na+ ions can be displaced easily with a small part of hydrogen ions by dilute hydrochloric acid,49 the calcination process promotes the elimination of H2O which is formed by hydrogen ions and O2 and the rearrangement of TiO6 octahedra, leading to the formation of monoclinic TiO2(B) with one-dimensional rod-shape nanostructure.50 Ti4+ of TiO2 surface is reduced to Ti3+ by NaBH4, so the white TiO2(B) nanorods are turned to blue.51


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Figure 1. Diagrammatic sketch for the formation of blue TiO2(B) single-crystalline nanorods. 2.3. Characterization. The crystal structure of the TiO2(B) single-crystalline nanorod was investigated using X-ray diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation (λ=1.5406 Å). Raman measurements were identified by using a Jobin Yvon HR 800 microRaman spectrometer in the range of 100 cm-1 to 1000 cm-1 at 457.9 nm. XPS was conducted on a PHI-5700 ESCA instrument with Al-Kα X-ray source. Scanning electron microscopy (SEM) images were obtained with a Philips XL-30-ESEM-FEG instrument operating at 20 kV. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) measurements were carried out on a JEM-2100 microscope. ·OH radicals concentration was measured by the terephthalic acid photoluminescence (TA-PL) on a RF-5301PC fluorescence spectrophotometer. UV-vis diffuse refection spectra (DRS) were recorded on a UV-vis spectrophotometer (UV2550, Shimadzu) with an integrating sphere attachment, using BaSO4 as the reference.


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2.4. Photocatalytic Degradation. The photocatalytic activities of the as-prepared three samples were studied by the degradation of rhodamine B (RhB) dyes. A 350 W Xe light equipped with optical filter (λ≥420 nm) was as simulated visible light source. All of these tests were under R.T. (20±2 oC). The photocatalytic experiments were under visible light irradiation, 20 mg of the photocatalysts were added in 30 mL of the 10 mg·L-1 RhB solution, placed in a 50 mL of cylinder-shaped quartz glass kettle equipped with a water circulation equipment. Prior to illumination, the mixed solution was magnetically stirred for 30 min in the dark to reach an adsorption/desorption equilibrium. Every 30 min photocatalytic reaction during visible light illumination, the samples were centrifuged and separated, the absorbance of the centrifuged liquid samples was investigated using a Shimadzu Model UV 2550 spectro-photometer (λ = 553 nm) to calculate the photocatalytic degradation rate of RhB. 2.5. Photoelectrochemical Test. Photoelectrochemical measurements were investigated using an electrochemical workstation (CHI760E, Shanghai). An AM 1.5 solar power system as simulated sunlight source was used. A standard three-electrode system was composed of FTOglass electrode, an Ag/AgCl reference electrode, and platinum foil as opposite electrode. 1M KOH water solution was as the electrolyte. For the preparation of the photoelectrode, 100 mg of sample was dissolved in 2 mL of alcohol solution under the magnetic stirring. The mixture was dip-coated onto a 1×2 cm2 FTO-glass electrode to form a film and then calcined at 300 oC for 1 h in Ar to ensure good electrical contact. 2.6.

Photocatalytic

Hydrogen

Production.

Photocatalytic

hydrogen

production

measurements were carried out in an on-line photocatalytic H2 production installation (AuLight, Beijing, CEL-SPH2N) R.T. (20±2 oC). Typically, 50 mg of photocatalysts were dispersed in a mixture of DI water and methanol, which was 4:1 by volume, placed in sealed-gas circulation


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reaction cell with a magnetic mixer. Before the start of the reaction, the system was vacuumized completely to eliminate O2 and CO2 dissolved in aqueous solution. Afterwards, the mixed solution was irradiated by a 300 W xenon light equipped with an AM 1.5G filter (Oriel, USA). The generated H2 was quantitatively analyzed every 1 h by a gas periodically measured using an on-line gas chromatography (SP7800, TCD, molecular sieve 5Å, N2 carrier, Beijing Keruida Limited). 3. RESULTS AND DISCUSSION 3.1 Characterization of blue TiO2(B) single-crystalline nanorods. To identify crystal structure and crystalline phase of the different samples greatly affect photocatalytic and photoelectrochemical properties. The XRD measurements are carried out to analyze the crystal phase of three samples in Figure 2. It is distinctly to observe that anatase is the main phase of TP. The diffraction peaks at 25.3, 37.8, 48.1, 54.1 and 54.9o are attributed to the (101), (004), (200), (105), and (211) lattice planes of the anatase TiO2, respectively.52 Notably, the crystal phase of TR is totally different from TP. The main diffraction peaks are 14.2, 24.9, 28.6, 43.9 and 48.4o, which correspond well to (001), (110), (002), (003) and (020) crystal faces of the TiO2(B) crystalline phase, respectively. The results are consistent with those have been reported in previous researches.53 In terms of b-TR, it is obviously observed that a decrease and abroad (110) diffraction peak intensity, revealing that the particle size is gradually reduced owing to disorder-elicited lattice strains. The change of crystalline structure can be assigned to the generation of Ti3+ and oxygen vacancy (Ov) by the treatment of NaBH4, as discussed below. Obviously, the results of the phase composition from Raman spectra are consistent with the results of XRD.


9


TR b-TR TP

40

(022)

(020)

(003)

(112)

(401)

30

(200)

(004) 20

(002)

(110)

(001) 10

(105) (211)

(101)

Intensity (a.u.)

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

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50

60

2 theta (Degree) Figure 2. XRD patterns of TP, TR and b-TR, respectively. Raman test is used to further study the crystal structure of TiO2(B) nanorod. As shown in Figure 3, the Raman spectra of TP sample displays five (3Eg+ 2B1g+ A1g) Raman-active modes which locate at around 144, 197, 397, 516 and 639 cm-1, respectively, suggesting that it is the anatase phase in TP.54 Evidently, five characteristic Raman peaks of TR and b-TR locate at around 121, 192, 250, 401 and 640 cm-1 are due to the vibration of TiO2(B),55 which can be clearly observed. Compared with the Raman spectra of TR, it is clearly observed that the Raman spectrum of b-TR shows a clearly blue shift. According to previous studies, the shifts of Raman peaks are attributed to the generation of Ti3+ and Ov in TiO2 lattice.56 Obviously, the results of the phase composition from Raman spectra are correspond to that of the XRD.


10

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TR b-TR TP

Eg

Intensity (a.u.)

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


100

B1g

Eg

200

300

400

Eg

A1g+ B1g

500

600

Raman shift (cm

-1

700

800

)

Figure 3. Raman spectra of TP, TR and b-TR, respectively. The chemical valence state and surface component of b-TR are analyzed by XPS. Figure 4a displays that Ti 2p3/2 and Ti 2p1/2 of the b-TR are observed at 464.1 and 458.9 eV, and these two peaks assigned to the Ti4+ in b-TR. The formation of Ti3+ species in the TiO2 are confirmed by the appearance of the signals at 457.9 and 463.6 eV. These Ti3+ species are created as a result of the Ti4+ reduction of TiO2 with the treatment of NaBH4. As shown in Figure 4b, the main peak at 529.5 eV can be assigned to the O lattice of TiO2 and the binding energy of 531.6 eV can be ascribed to lattice oxygen (Ti-O) and hydroxyl groups (-OH).57 According to the XPS spectra of O 1s and Ti 2p, the result suggests that there are Ti3+ defects and oxygen vacancy in the b-TR, which can contribute to narrow the bandgap and greatly promote the separation of photoinduced electrons and holes.58


11


a

b

Ti 2p

Ti4+(2p3/2)

O 1s 529.5 eV

458.9 eV Ti4+(2p1/2)

Ti3+(2p3/2)

3+

Ti (2p1/2)

457.9 eV

456

Intensity (a.u.)

Intensity (a.u.)

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

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465.1 eV

463.6 eV

458

460

462

464

466

538

531.6 eV

536

Binding energy (eV)

534

532

530

528

526

524

Binding energy (eV)

Figure 4. XPS spectra of the b-TR: (a) Ti 2p, and (b) O 1s. The morphology and crystal structure of the b-TR are displayed in Figure 5. As observed from TEM images and inset of the SEM in Figure 5a, the length and width of b-TR are about 0.5-2 µm and 100-350 nm, respectively. It is clearly seen that the nanorods with smooth and clean surfaces are uniform. Notably, such good morphology only comes forth in the case of using alkali as solvent.59 As shown in Figure 5b, the clear 0.62 and 0.36 nm can be observed, which agree well with the (001) and (110) spacing of TiO2(B) phase, suggesting that the b-TR is well-crystallized. The b-TR sample corresponding to TiO2(B) phase is shown in HRTEM (Figure 5b), and well-developed single-crystalline in nature is displayed in inset of (Figure 5a) selected area electron diffraction (SAED) pattern. By measuring the lattice fringes of the HRTEM image, it is clearly to observe that the nanorod grows along the (110) direction. Significantly, the result indicates that the in-situ reduction can not change the TiO2(B) crystal phase. In addition, the surface disordered shells with a thickness of ca. 1 nm are formed at certain conditions, which can be clearly observed in the b-TR. The presence of surface disorder suggests the modification of the TiO2(B), which is correspond to the results of XRD.


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Figure 5. TEM (a) and HRTEM (b) images of b-TR. The insets of (a) are the corresponding SEM and SAED pattern. The photocatalytic performance is greatly determined by the optical absorption property.60 The optical properties and the bandgap values are examined with UV-vis diffuse reflectance spectra in Figure 6a. It can be clearly seen that TP with a wide bandgap only exhibits strong absorption in the ultraviolet range. Because it is reported that the bandgap of TiO2(B) phase is narrower compared with anatase TiO2. Thus the TR and b-TR show visible light absorption. Compared with TR, b-TR sample exhibits strong visible light absorption and even extends to the near-infrared region. The increased visible light absorption can be ascribed to the generation of Ti3+ and Ov.61 The photographic image (insets in Figure 6a) also shows a distinct color among three samples, the color of TP and TR are white, but b-TR is dark blue. The indirect bandgap values of the samples can be calculated from Figure 6b, which are 3.20 and 3.09 eV, respectively. However, we conclude that the bandgap value of b-TR is only 2.61 eV, which can be confirmed that b-TR with a narrower bandgap is more active in the visible light region than


13


the white ones. Consequently, the substantially enhanced light absorption may be beneficial for improving photocatalytic performance.

1.2

a

1.2

TP TR b-TR

1.0 0.8

TP TR b-TR

0.8

0.6 0.4

0.6 0.4

2.61 eV 3.09eV

0.2 0.0 200

b

1.0

(αhν)1/2

Absorbance (a.u.)

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

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3.20 eV

0.2 300

400

500

600

700

800

Wavelength (nm)

0.0 1.5

2.0

2.5

3.0

3.5

4.0

4.5

Photon energy (eV)

Figure 6. UV-vis diffuse reflectance absorption spectra (a) and the corresponding calculated bandgap (b) for TP, TR and b-TR samples. To explain the excellent photocatalytic performances, three types of photocatalysts are estimated by the degradation of RhB, fluorescence intensity and cycling experiments, respectively. Before visible light irradiation, dark adsorption experiment is done to ensure adsorption equilibrium of RhB on the catalyst surface. As displayed in Figure 7a, the degradation efficiency of the TP and TR are only 14.06% and 46.44%, respectively. These decreased degradation rates are due to the fast recombination of photoinduced electrons and holes in TP and TR. Evidently, the b-TR shows excellent degradation efficiency of RhB, reaching up to 97.01% after 150 min of visible light irradiation. The high visible light photocatalytic activity of b-TR arises from the synergistic effects of the color and the rod-shaped nanostructure. It is suggested that the b-TR exhibits exceptionally better photocatalytic activity than others. The photocatalytic activity

with three samples on RhB and phenol removal under solar light

illumination is also given in Figure S1 and Figure S2. The b-TR shows wonderful degradation


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efficiency of RhB and phenol under the solar light illumination. Compared with TP and TR, bTR exhibits excellent degradation efficiency of RhB which is up to 98.56%. Similarly, among three samples, b-TR shows the highest degradation efficiency for phenol removal (99.12%). The data is also plotted in order to calculate the first-order reaction rate constants (k). As illustrated in Figure 7b, reaction rate constant k of TP, TR and b-TR for RhB removal are calculated to be 0.0016, 0.0053 and 0.0146 min-1, respectively. Moreover, the reaction constants k of b-TR is about 8.8 times higher than that of TP. Meanwhile, the reaction constants k of b-TR for RhB and phenol removal is 0.0250 and 0.0366, respectively, which is the highest in degradating RhB and phenol in Figure S1 and Figure S2. It is apparent that b-TR shows an increased photocatalytic activity. 1.0

2.5

a

ln(C0/C)

C/C0

Dark

0.6 0.4 0.2

0

30

60

1.5 1.0 RhB

TP TR b-TR

RhB

0.0

b TP TR b-TR

2.0

0.8

0.5

90

120

150

0.0

180

0

30

150

c

90

120

150

d

1.0

TP TR b-TR

1st 2nd 3rd 4th

0.8

C/C0

100

60

Irradiation time (min)

Time (min)

Intensity (a.u.)

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


50

0.6 0.4 0.2

0 375

400

425

450

475

500

525

Wavelength(nm)

0.0

0

100

200

300

400

Time(min)


15

500

600

700


Figure 7. Evaluation of photocatalytic activity under visible light irradiation, (a) photocatalytic degradation RhB of three samples, (b) the linear relationship of ln(C0/C) versus irradiation time with three samples, (c) fluorescence intensity under 1 h by using TA solution for different samples, (d) and four cycling tests of RhB photocatalytic degradation of b-TR . In order to explain the relationship between the photoinduced carriers separation and photocatalytic enhancement of the prepared samples, the fluorescence intensities (FL) tested by terephthalic acid (TA) fluorescence technique are used to examine the generation of hydroxyl radicals (·OH) every one hour under visible light illumination. Figure 7c exhibits the comparison of FL among TP, TR and b-TR. It can be easily observed that maximal yield of ·OH is produced by b-TR during the photoreaction, the photooxidation capability of the b-TR for ·OH formation higher than that of the TP and TR, which is consistent with the results of photocatalytic degradation of RhB. Furthermore, as revealed in Figure 7d, cycling measurements imply that the high photocatalytic efficiency of the b-TR during RhB degradation is effectively maintained after four cycles, indicating that the b-TR is stable under visible light irradiation. 100

60 a

TR TP b-TR

60 40 20 0 -0.8

-0.4

-0.2

0.0

0.2

0.4

TP TR b-TR

40 30 20 10 0

-0.6

b

50

Current (µA cm -2)

80

Current ( µAcm -2 )

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

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0.6

60

90

120

150

180

210

Time (s)

Potential (V) vs Ag/AgCl

Figure 8. Photoelectrochemical performances of the TP, TR and b-TR: (a) linear sweep voltammetry under AM 1.5, (b) chronoamperometry tests with AM 1.5.


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The photoelectrochemical behavior of three samples is researched by linear sweep voltammetry with AM 1.5 illumination, and the results are presented in Figure 8a. In comparison to TP and TR, b-TR shows a remarkable improvement in photoresponse with photocurrent densities of 56 µA cm-2, which is 28 times higher than that of TP (2 µA cm-2). The results demonstrate efficient charge separation in b-TR under AM 1.5. Higher photocurrent densities indicate that better photogenerated charge generation, separation, and transfer produced in the bTR, which can be ascribed to the rod-shape single-crystalline structure and the formation of Ti3+ and oxygen vacancy.62 Figure 8b shows that the photocurrent density of three photocatalysts in IT curves. The b-TR shows unprecedented high photocurrent density of 45 µA cm-2 under AM 1.5. The photocurrent response of the b-TR is about 15 times as high as that of TP, indicating photoexcited electrons and holes can be separated more efficiently and have a longer lifetime in b-TR. 7x109 6x10

9

5x109

1/C2(F2)

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


TR TP b-TR

4x109 3x109 2x109 1x109 0 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

Potential E (V vs Ag/AgCl) Figure 9. The Mott-Schottky plots of TP, TR and b-TR, respectively. The results of Mott-Schottky (M-T) analysis for three samples are presented in Figure 9. The positive slopes of the M-T demonstrate that samples are n-type semiconductors. The result


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indicates that b-TR shows the smaller slope in M-T plot than TP and TR, indicating a higher charge carrier density.63 That is to say, b-TR possesses better conductivity, which promotes charge separation and migration. Because of the introduction of Ti3+ and Ov suppress recombination of charge, in that way leaving more charge carriers to generate activity components and facilitate the decomposition of pollutants, thus to enhance photocatalytic and photoelectrochemical performances. The photocatalytic performances are evaluated by measuring the photocatalytic hydrogen evolution rate under AM 1.5 irradiation. In Figure 10a, both TP and TR with the hydrogen evolution rates of 5.6 and 40.8 µmol h-1 g-1 are more fewer photoactivity than b-TR. However, the fastest hydrogen evolution rate of 149.2 µmol h-1 g-1 comes from b-TR sample, which is about 26.6 times higher than that of TP. It indicates that the synergetic effect of Ti3+ and the rodshaped single-crystalline nanostructure are rather important for the enhanced photocatalytic property. The enhanced photocatalytic property is ascribed to the efficient separation and migration of electron-hole pairs. The stability of b-TR photocatalytic activities are tested by the recycling hydrogen evolution reaction. As shown in Figure 10b, after five recycles last 25 h, the H2 evolution rates still keep highly stable under the AM 1.5 irradiation, which reveal a good stability.


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180

Rate of H 2 production (µmol h -1 g -1 )

160

a

140 120 100 80 60

TR

40 20

b

800

b-TR

700 -1 H 2 production (µmol g )

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


TP

600 500 400 300 200 100 0

0

0

5

10

15

20

25

Time (h)

Photocatalyst

Figure 10. The photocatalytic hydrogen evolution for different samples under AM 1.5 irradiation (a), and the recyclability of b-TR in H2 evolution under AM 1.5 irradiation (b). Based on the above experimental results, the mechanism of enhanced photocatalytic activity is displayed in Figure 11. There are two types of Ti3+, surface Ti3+ and lattice Ti3+. Different kinds of Ti3+ ions play different roles in photocatalytic mechanism. The surface Ti3+ will react with O2 to generate ·O2, ·HO2 and ·OH to participate in oxidation and reduction reactions. The formation of lattice Ti3+ localizes states under conduction band (CB) of TiO2, so it promotes to absorb visible light.64 Under solar light irradiation, the photogenerated electrons can be migrated to the CB of TiO2 and then the multiple-e- reduction process occurs, inducing indirectly to the generation of ·OH. Furthermore, the photogenerated holes in the valance band (VB) of TiO2 can have a reaction with the surface hydroxyl ion or water adsorption to form ·OH which is caused the degradation of organic pollutant. So the ·OH is responsible for the degradation of RhB. The generated electrons in the CB on b-TR surface can react with hydrogen ion to produce hydrogen.


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Figure 11. Schematic illustration of the solar light photocatalytic mechanism for the b-TR single-crystalline nanorod photocatalyst. 4. CONCLUSIONS In summary, we present a sol-gelation, hydrothermal technique and subsequent reduction treatment route for preparing Ti3+ self-doped blue TiO2(B) single-crystalline nanorods. Characterization results showed that the photocatalyst was 1D rod-shape structure, and the oxygen vacancies was generated in TiO2 because of the reduction of Ti4+ centers to Ti3+. The photocatalytic activities and photoelectrochemical performances had been verified by different property tests. The degradation rate of RhB reached 97.01%, photocurrent density was 56 µA cm-2 , and photocatalytic hydrogen evolution was up to 149.2 µmol h-1 g-1. Therefore, the enhanced photocatalytic and photoelectrochemical performances were attributed to the special TiO2(B) phase, Ti3+ self-doping, and the rod-shaped single-crystalline nanostructure, which could narrow the bandgap and prevent the fast recombination of electrons and holes. Therefore, it is believed that this approach is feasible, which can be referenced for designing other photocatalysts for solar energy utilization and water puriﬁcation in future.


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AUTHOR INFORMATION Corresponding Author *Tel.:

+86-451-8660-8616.

Fax:

+86-451-8660-8240.

Email:

[email protected];

[email protected]; [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We gratefully acknowledge the support of this research by the National Natural Science Foundation of China (21376065, 51672073, 81302511, 81573134, and 21106035), the Natural Science Foundation of Heilongjiang Province (QC2012C001, QC2013C079, and E201456), the Heilongjiang Postdoctoral Startup Fund (LBHQ14135), the Program for New Century Excellent Talents in University of Heilongjiang Province (1253-NCET-020), the University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2015014) and the Innovative Science Research Project of Harbin Medical University (2016JCZX13). REFERENCES (1) Zhou, W.; Li, W.; Wang, J. Q. ; Qu, Y.; Yang, Y.; Xie, Y.; Zhang, K. F.; Wang, L.; Fu, H. G.; Zhao, D. Y. Ordered Mesoporous Black TiO2 as Highly Efficient Hydrogen Evolution Photocatalyst. J. Am. Chem. Soc. 2014, 136, 9280-9283.


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(64) Fu, R. R.; Gao, S. M.; Xu, H.; Wang, Q. Y.; Wang, Z. Y.; Huang, B. B. Dai. Y. Fabrication

of

Ti3+

Self-doped

TiO2(A)Nanoparticle/TiO2(R)Nanorod

Heterojunctions with Enhanced Visible-light-driven Photocatalytic Properties. RSC Adv. 2014, 4, 37061-37069.


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List of figure captions Figure 1. Diagrammatic sketch for the formation of the blue TiO2(B) single-crystalline nanorod.

Figure 2. XRD pattern of TP, TR and b-TR, respectively.

Figure 3. Raman spectra of TP, TR and b-TR, respectively.

Figure 4. XPS spectra of the b-TR: (a) Ti 2p, and (b) O 1s.

Figure 5. TEM (a) and HRTEM (b) images of b-TR. The insets of (a) are the corresponding SEM and SAED pattern.

Figure 6. UV-vis diffuse reflectance absorption spectra (a) and the indirect interband transition energies (b) for TP, TR and b-TR samples, respectively.

Figure 7. Evaluation of photocatalytic activity under visible light irradiation, (a) photocatalytic degradation RhB of three samples, (b) the linear relationship of ln(C0/C) versus irradiation time with three samples, (c) fluorescence intensity under 1 h by using TA solution for different samples, (d) and four cycling tests of RhB photocatalytic degradation of b-TR.


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Figure 8. Photoelectrochemical performances of the TP, TR and b-TR: (a) linear sweep voltammetry under AM 1.5, (b) chronoamperometry tests with AM 1.5.

Figure 9. The Mott-Schottky plots of TP, TR and b-TR, respectively.

Figure 10. The photocatalytic hydrogen evolution for different samples under AM 1.5 irradiation (a), and the recyclability of b-TR in H2 evolution under AM 1.5 irradiation (b).

Figure 11. Schematic explaintion for the mechanism of the b-TR single-crystalline nanorod with solar light illumination.


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Table of Contents

Ti3+ self-doped blue TiO2(B) single-crystalline nanorods are fabricated via sol-gel method, hydrothermal reaction and in-situ chemical reduction, which exhibit excellent solar-driven photocatalytic performance due to the synergistic effect of Ti3+ selfdoping and 1D TiO2(B) single-crystalline rod-like nanostructure favoring light harvesting, and separation and transportation of photogenerated charge carriers.

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