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S doped high thermostable anatase TiO2 nanorods as efficient visible-light-driven photocatalysts
Accepted Manuscript 3+ In-situ Ti /S doped high thermostable anatase TiO2 nanorods as efficient visiblelight-driven photocatalysts Meng Li, Zipeng Xing, Jiaojiao Jiang, Zhenzi Li, Junyan Kuang, Junwei Yin, Ning Wan, Qi Zhu, Wei Zhou PII:
S0254-0584(18)30715-6
DOI:
10.1016/j.matchemphys.2018.08.051
Reference:
MAC 20894
To appear in:
Materials Chemistry and Physics
Received Date: 3 November 2017 Revised Date:
29 June 2018
Accepted Date: 19 August 2018
Please cite this article as: M. Li, Z. Xing, J. Jiang, Z. Li, J. Kuang, J. Yin, N. Wan, Q. Zhu, W. Zhou, 3+ In-situ Ti /S doped high thermostable anatase TiO2 nanorods as efficient visible-light-driven photocatalysts, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.08.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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In-Situ Ti3+/S Doped High Thermostable Anatase
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TiO2 Nanorods as Efficient Visible-Light-Driven
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Photocatalysts
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Meng Lia, Zipeng Xinga,*, Jiaojiao Jianga, Zhenzi Lib, Junyan Kuanga, Junwei Yina,
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Ning Wana, Qi Zhua,*, Wei Zhoua,*
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a
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of the People’s Republic of China, Heilongjiang University, Harbin 150080, P. R.
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China
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Tel: +86-451-8660-8616, Fax: +86-451-8660-8240,
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Email: [email protected]; [email protected]; [email protected]
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b
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150086, P. R. China
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Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education
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Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin
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Abstract: In-situ Ti3+/S doped high thermostable anatase TiO2 nanorods using
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ethanediamine-modified TiOSO4 as precursor are synthesized under 700
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calcination, then combined with controllable in-situ solid-phase reaction method,
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calcined at 350 oC in argon. The outcomes declare that the obtained photocatalyst
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with a high crystallinity is effectively doped with S element and Ti3+ species, and
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synchronously possesses one-dimensional (1D) anatase nanorods structure with length
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of ~ 2-5 µm and width of ~ 0.5-1 µm. The S and Ti3+ co-doped 1D nanorod with a
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narrowed bandgap (2.56 eV) stretches the optical response range to visible-light. The
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visible-light-driven photocatalytic degradation efficiency of methyl orange and H2
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production rate for Ti3+/S-TiO2 nanorods are as high as 96% and 166 µmol h-1 g-1,
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showing about 6 times greater than 600-TR (TiO2 nanorods). This is be ascribed to the
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synergistic reaction of S and Ti3+ species co-doping narrows the bandgap and
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promotes the separation efficiency of photoexcited carriers, and the one-dimensional
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structure favors the transportation of photogenerated charge carriers. Hence, the
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prepared photocatalyst will have a great latent application prospect in fields of energy
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and environment.
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Keywords: Photocatalysis; TiO2 nanorods; Ti3+/S-doping; visible-light-driven
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photocatalyst; hydrogen evolution
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ACCEPTED MANUSCRIPT 1 Introduction
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In the past decades, semiconductor photocatalysts [1] have attracted a great deal of
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attention due to their potential applications in environmental remediation [2] and solar
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energy conversion [3]. As the most promising semiconductor photocatalyst, TiO2 has
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been extensively investigated in environmental cleaning and green energy production
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[4, 5], due to its safety, low cost [6], high chemical-stability [7], and good
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photoelectric performance [8] under ultraviolet (UV) light irradiation. However, as a
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representative broad bandgap energy semiconductor (3.2 eV for anatase) [9], TiO2
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mainly responses to UV light, about 3-5% of the entire solar energy [10], which
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severely restricts its actual utilization in the visible light. In addition, the low rate of
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electron transport and the high recombination rate of photoinduced electrons and
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holes [11] limit the promotion of solar power utilization efficiency. Accordingly, some
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strategies [12] have been proposed to tune the bandgap of TiO2 to extend its
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photoresponse to visible light region.
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It is well known that doping is an effective method to enhance the photocatalytic
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performance of TiO2 in visible light range, for example, transition metal elements
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doping(Cr, Mn, Fe, Ni, Ru, and Cu) or nonmetallic elements doping(B, C, N, S, and F)
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[13-16]. These dopants form a delocalized state or intra-band state in the bandgap and
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act as electron donor or acceptor [17, 18] to induce absorption in visible region.
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However, TiO2 doping with metal elements has been restricted owing to high
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recombination efficiency of electrons and holes, generation of a new auxiliary
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impurity level [19], and poisonous sensitization of dyes [20], so the amount of the
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ACCEPTED MANUSCRIPT doped elements need to be severely controlled not to debase the photocatalytic
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performance [21]. Therefore, nonmetal-doping is deemed as a more potential method
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to restrain the recombination between holes and electrons through producing a state of
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delocalization [22-25] in the TiO2 bandgap. And in these nonmetallic elements, the
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doping of sulfur element can inhibit the conversion of anatase phase to rutile phase
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and enhance the thermal stability of anatase TiO2. Based on previous research, it is
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suggested that the alliance of structural/morphology strategy [26] (1D nanomaterials
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[27], such as nanowires [28], nanorods [29], nanobelts [30], nanotube [31]) and
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nonmetal-doping [32], especially with S doped TiO2, can markedly improve the
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visible-light-driven photocatalytic performance of TiO2 material.
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In order to further expand the absorption of TiO2 nanomaterials in visible-light
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range, Mao et al. [33] prepare the black hydrogenated TiO2 nanomaterials with a
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narrowed bandgap (1.54 eV), exhibiting exceedingly excellent photocatalytic
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performance for degradation of organic contaminants. Hereafter, black TiO2 has
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aroused a great deal of concern. Many preparation methods consisting of high
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pressure hydrogenation, anodization, plasma assisted hydrogenation, aluminum
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reduction, and chemical oxidation [34-38] are proposed to synthesize black TiO2. In
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general, the excellent photocatalytic activity of black TiO2 is attributed to the highly
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efficient electron-hole pairs separation capability [39].
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In this paper, in-situ Ti3+/S doped high thermostable anatase TiO2 nanorods is
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effectively prepared by a simple direct calcination approach combined with an in-situ
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solid-phase reaction method. The synthesized gray Ti3+/S-TiO2 nanorods can retain
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ACCEPTED MANUSCRIPT anatase up to 700 oC. The bandgap of the as-prepared sample is reduced to 2.56 eV,
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exhibiting excellent photocatalytic properties for removal of methyl orange and H2
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production. A probable mechanism of Ti3+/S-TiO2 nanorods is also provided.
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2 Materials and Methods
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2.1 Materials
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Titanium oxysulfate (TiOSO4), and sodium borohydride (NaBH4, 98%), were
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purchased from Aladdin-Reagent-Company (China). Ethylenediamine (EDA), and
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anhydrous ethanol (EtOH), were purchased from Tianjin-Kermel-Chemical-Reagent
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Co. LTD (China). All reagents used in the experiment were analytical grade, and the
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deionized (DI) water was used throughout this study.
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2.2 Preparation
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2.2.1 Preparation of S-TiO2 nanorods
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1 g of titanium oxysulfate were transferred to a porcelain boat and calcined at
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400 oC in air for 2 h (2 oC/min). The obtained samples were added to 50 mL of
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deionized water and then stirred for 0.5 h. At the same time, adding 10 mL of EDA to
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the solution till the pH reached 11. After that, the mixture were transferred to an oil
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bath with stirring and kept at 90 oC for 36 h. When natural cooling to 20 oC, the
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obtained solution were washed with deionized (DI) water and then dried at 60 oC for
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24 h. After that, the resulting products were annealed at 600, 700, and 800 oC for 2 h,
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respectively. The gained products were rinsed with deionized (DI) water for three
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times, and dried at 60 oC. Finally, the S-doped TiO2 nanorods were obtained (which
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were denoted as 600-TR, 700-TR, and 800-TR, respectively).
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2.2.2 Preparation of Ti3+/S-TiO2 nanorods 1.0 g of 700-TR and 1.0 g of NaBH4 were thoroughly mixed. Then they were
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calcined at 350 oC for 2 h in Ar at the speed of 5 oC min-1. After natural cooling to
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room temperature, the gray Ti3+/S-TiO2 nanorods (Scheme 1) were gained (marked as
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g-700-TR), rinsed with deionized (DI) water and anhydrous ethanol for several times
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to dislodge the unreacted sodium borohydride (NaBH4).
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Scheme 1. Schematic diagram for the formation of Ti3+/S-TiO2 nanorods.
2.3 Characterization
The Fourier transform infrared spectra (FT-IR), using KBr as diluting agent, was
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conducted via a PerkinElmer spectrum system. Electrochemical impedance
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spectroscopy (EIS) was observed with a CHI 760E electrochemical workstation
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(Chenhua, Shanghai) in a frequency region between 100 KHz and 10 MHz. The total
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organic carbon (TOC) removal was tested by TOC analysis equiped with analytic jena
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multi NIC 2100 analyzer. X-ray diffraction (XRD) was obtained with a Bruker D8
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Advance diffractometer by using Cu Kα radiation source (λ=1.5406 Å). Raman
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spectra were collected via a Jobin Yvon HR 800 micro-Raman spectrometer in the
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region of 100 cm-1 to 1000 cm-1 at 457.9 nm. X-ray photoelectron spectroscopy (XPS)
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measurements were obtained through a PHI-5700 ESCA instrument using Al-Kα
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X-ray source. Each binding energy was adjusted with surface adventitious carbon
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electron microscope (FE-SEM, Hitachi S-4800). Transmission electron microscopy
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(TEM) was observed with a JEM-2100 electron microscope (JEOL, Japan).
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UV-visible diffuse refection spectra (UV-vis DRS) were collected via a UV-vis
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spectrophotometer (UV-2550, Shimadzu).
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2.4 Photocatalytic test
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2.4.1 Photocatalytic degradation of methyl orange
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The catalytic performance of the the as-prepared samples was assessed by the
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photo-degradation of methyl orange (MO) at room temperature. A xenon lamp (300
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W) occupied with a cut-off filter (λ ≥ 420 nm) as a visible light source to achieve
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visible light induced photocatalysis. In the photocatalytic experiments, 25 mg of the
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samples was dispersed in 25 mL of methyl orange (MO) aqueous solution. Before
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illuminated, the solution was stirred in darkness for 0.5 h to achieve an
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adsorption-desorption balance. After reaction for 150 min under stirring, 2.0 mL of
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the suspension were immediately put into a plastic tube and centrifuged to dislodge
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the photocatalysts. Finally, the MO concentration was measured by UV-vis
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spectrophotometer at its specific wavelength (λ = 464 nm) for calculating the
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photocatalytic degradation rate of MO.
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2.4.2 Photocatalytic hydrogen generation
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Photocatalytic H2 generation was obtained by using CEL-SPH2N (AuLight,
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Beijing), which was an online photocatalytic H2 evolution system at room temperature.
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Typically, 50 mg of samples was added to 100 mL aqueous solution that contained
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water, a vacuum pump was connected with the system prior to the experiment. A 300
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W Xeon-lamp occupied with an AM 1.5G filter (Oriel, USA) was used as light source.
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Subsequently, the amount of H2 evolution was measured using a gas chromatography
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(GC) with the interval of each 1 h (molecular sieve 5 Å, N2 carrier, SP7800, TCD,
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Beijing Keruida, Ltd).
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3 Results and Discussion
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XRD is carried out to confirm the crystallinity and phase purity of the prepared
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photocatalysts. Generally, the calcination temperature greatly affects the crystalline
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phase composition and crystallinity of TiO2. As showed in Fig. 1a, it is obviously
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observed that the diffraction peaks located at 25.3, 37.1, 37.8, 38.7, 48.2, 53.9, 55.1,
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62.8, 69.0, 70.5, and 75.2 ° are perfectly corresponded to anatase phase (JCPDS
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#21-1272) for the 600-TR, 700-TR, and g-700-TR, without any other impurity or new
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phase. However, when the calcination temperature is up to 800 oC, the diffraction
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peaks located at 27.4, 36.1, 41.2, 54.3, 56.6, and 69.8 ° correspond well to rutile phase
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(JCPDS #21-1276), showing that a mixed phase containing anatase and rutile is
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gained for 800-TR. According to the literature [40], the photocatalytic performance of
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rutile titanium dioxide is less than that of anatase titanium dioxide. Therefore, the
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samples that we prepared are able to restrain the phase transition from anatase to rutile
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up to 700 oC. Furthermore, the intensity of XRD peaks become gradually stronger
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with increasing temperature, indicating the crystallinity of TiO2 is enhanced evidently.
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Compared with 700-TR, the g-700-TR still maintains the original crystal phase since
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weakening, which may be attributable to the production of Ti3+ species and oxygen
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vacancy, in virtue of disorder-led lattice strains and a slight reduction of crystallite
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size [41]. At the same time, the characteristic peaks of S have not been observed. On
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the one hand, it can’t be observed due to the low content of S. On the other hand, the
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S element may penetrate into the lattice of TiO2, which makes it undetectable.
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In addition, Raman technique is another powerful means to further investigate
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the phase compositions of 600-TR, 700-TR, 800-TR, and g-700-TR, respectively. As
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shown in Fig. 1b, the five characteristic peaks of 600-TR located at 147.6, 196.7,
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393.4, 507.9, and 631.2 cm-1 can be owing to six (3Eg+2B1g+A1g) Raman-active
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modes [42], which shows that anatase is the major phase. Notably, the strongest peak
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at 147.6 cm-1 is attributed to the O-Ti-O symmetric stretching modes. However,
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compared with XRD patterns, no diffraction peaks of 800-TR ascribe to rutile for the
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strong anatase peaks. It is clearly observed that the g-700-TR sample occurs a
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blue-shift at 157.4 cm-1, which is displayed in the illustration of Fig. 1b. As reported
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in previous literature [43], the shift of diffraction peaks is originated due to the
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presence of Ti3+ and oxygen vacancies in TiO2 lattice owing to the treatment of
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NaBH4. Meanwhile, the peaks of 700-TR and 800-TR at 153.2 and 154.8 slightly shift
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to larger wave numbers compared with the peaks of 600-TR. No peak ascribes to
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sulfur or sulfate species due to the lower doping amounts [44]. Raman spectra results
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are correlating well with the XRD results mentioned above.
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A A A
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40 50 60 2 theta (Degree)
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Intensity (a.u.) 100
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157.4 153.2
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Raman Shift (cm -1)
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Intensity (a.u.)
Intensity (a.u.)
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400 500 600 -1 Raman Shift ( cm )
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Fig. 1. XRD patterns (a) and Raman spectra (b) of 600-TR, 700-TR, 800-TR, and g-700-TR,
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respectively. The inset of (b) is the magnified spectra between 100 and 200 cm-1.
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The structure and morphology of g-700-TR is discussed via SEM and TEM
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images, as revealed in Fig. 2. As observed from SEM image of Fig. 2a, the TiO2
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nanorods are around 2-5 µm long and 0.5-1 µm wide. HRTEM and inset of (Fig. 2b)
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TEM pattern further indicate that the g-700-TR is corresponding to nanorods structure.
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Simultaneously, it is obviously observed that the lattice fringe spacing is 0.352 nm,
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which agrees with the (101) crystal plane [45] of anatase TiO2, indicating the
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well-crystallinity of anatase TiO2 nanorods. It declares that the partial reduction does
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not change the TiO2 crystal phase. The outcomes are coincident with the XRD and
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Raman, and the TiO2 nanorods with high crystallinity are an ideal photocatalyst. In
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addition, it can be seen that an amorphous shell about 1-2 nm thick is formed after
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partial reduction treatment from Fig. 2b. The presence of surface disordered structure
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is mainly ascribed to the formation of defect states in the TiO2 bandgap owing to the
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existence of Ti3+ species and oxygen vacancies [46], which is responsible for the
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improved light absorption and photocatalysis.
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Fig. 2. SEM (a) and HRTEM image (b) of g-700-TR. The inset of (b) is the TEM image of
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g-700-TR.
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The XPS spectrum is further carried out to study the surface composition and
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chemical state of Ti, O, and S in g-700-TR, as revealed in Fig. 3. The full-scale XPS
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spectrum of g-700-TR is shown in Fig. 3a, displaying the presence of Ti elements, O
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elements, and S elements. Besides, Na element and B element are not observed,
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indicating that the NaBH4 have been washed away absolutely. Fig. 3b shows the Ti 2p
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XPS analysis of g-700-TR. The peaks located at 464.2 and 458.5 eV are
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corresponding to the characteristic Ti 2p1/2 and Ti 2p3/2 of Ti4+ in TiO2, respectively.
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And the other two peaks located at 463.0 and 457.9 eV are attributed to Ti3+ species
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[47], formed as a result of the partial reduction of Ti4+ in TiO2. Fig. 3c shows the XPS
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spectrum of O 1s. The peaks at 529.6 and 531.8 eV should be ascribed to Ti-O bonds
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and surface -OH groups, respectively. The high-resolution Ti 2p and O 1s XPS
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patterns suggest that both Ti3+ species and oxygen vacancies are effectively produced
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on the surface or in the bulk, which can reduce the bandgap of TiO2 and restrain the
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recombination between holes and electrons. Fig. 3d displays the XPS spectrum for S
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2p. The presence of S is confirmed by a peak centred at 168.5 eV, which can be
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assigned to the S6+ state. Hence, the S element might be S6+ in the lattice of g-700-TR.
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This is also similar to previous literature reports [48]. All the above results prove the
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successful generation of gray Ti3+/S-TiO2 nanorods.
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462 460 458 Binding energy (eV)
d
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S 2p
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Intensity (a.u.)
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170 169 168 167 Binding energy (eV)
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Fig. 3. Full-scale XPS spectrum (a), Ti 2p (b), O 1s (c), and S 2P (d) for g-700-TR.
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529.6 eV
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Intensity (a.u.)
Intensity(a.u.)
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The UV-vis DRS spectra in Fig. 4a is used to analyze the optical property and
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bandgaps of different photocatalysts. As displayed in Fig. 4a, it can be clearly seen
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that all the products present strong absorption in the ultraviolet region. Furthermore,
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the products display an increasing visible light absorption with color-change. The
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color of the S-doped samples is yellowish, and the color gradually deepens with the
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increasing calcination temperature (insets in Fig. 4a). The enhanced visible light
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absorption and color variation are ascribed to the existence of S in TiO2. However, the
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due to the existence of defect states in the TiO2 bandgap owing to the production of
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Ti3+ species and oxygen vacancies. The indirect bandgaps of the products are
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estimated in Fig. 4b. The bandgaps of 600-TR, 700-TR, 800-TR, and g-700-TR are
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3.08, 2.97, 2.88, and 2.56 eV, respectively. The narrow bandgap is propitious to the
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adsorption of visible light and enhances the utilization of photons. There is no color
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change in the next three months after the sample is prepared, indicating the high
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stability under ambient conditions.
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The FT-IR spectrum is used to research the functional groups of resultant
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samples. As displayed in Fig. 4c, the absorption peaks at around 1630 and 3348 cm-1
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can be assigned to the bending vibration of physically surface-adsorbed water
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molecular (H2O) and stretching vibrations of surface hydroxyl groups (-OH) on the
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surface of the TiO2, respectively. The spectra of all the products are analogous,
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presenting a broad and intense absorption peak in the range of 400-800 cm-1, which
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can be mostly assigned to the flexion vibration of Ti-O-Ti bonds and Ti-O bonds in
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the TiO2. Besides, the weak FT-IR absorption peaks at around 1051 cm-1 can be
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associated with S-O asymmetric stretch, implying the existence of Ti-O-S. This is in
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confirmation with the previous results shown by researchers [49]. The FT-IR results
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confirm that S6+ can successfully penetrate into the TiO2 structure, corresponding with
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the XPS results.
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Electrochemical impedance spectra (EIS) measurement is a powerful
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characterization technique to examine the electron-transport characteristics of the
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the smaller impedance arc radius in EIS plots represents the better charge transport.
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Fig. 4d shows the EIS patterns of 600-TR, 700-TR, 800-TR, and g-700-TR and the
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corresponding equivalent circuit in the inset. Comparatively, the g-700-TR has a
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much less depressed impedance semicircle arc than others. It implies that the
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interfacial electrons can be transported more faster, and photo-generated electrons and
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holes can be more effectively separated. Meanwhile, it suggests that Ti3+ plays a
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significant role in enhancing the conductivity of the materials, thereby improving the
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performance of the electrode. a
1.4
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1.2
Absorbance
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600-TR 700-TR 800-TR g-700-TR
1.2 1.0
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800-TR g-700-TR
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600-TR
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Transmittance (a.u.)
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1000
700
0.2 0.0 2.0
800
300
d
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2.97 eV 3.08 eV
3.0 3.5 Photon energy (eV)
100 R1
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3500
4.5
150
50
3000
4.0
600-TR 700-TR 800-TR g-700-TR
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800-TR
1500 2000 2500 Wavenumber (cm-1)
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g-700-TR
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600-TR 700-TR 800-TR g-700-TR
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R3
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100
150 Z' (ohm)
CPE1
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300
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Fig. 4. UV-vis diffuse reflectance spectra (a), determination of the indirect interband transition
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energies (b), FT-IR spectra (c) and Nyquist plots (d) for 600-TR, 700-TR, 800-TR, and g-700-TR,
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respectively. The inset of (d) is the equivalent circuit applied to fit the resistance data.
The photocatalytic performance of the as-obtained TiO2 is estimated via
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photo-degradation of MO under visible light. In this experiments, 0.5 h dark
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adsorption is done to ensure adsorption equilibrium of MO on the surface of catalyst.
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As illustrated in Fig. 5a, the degradation efficiency of MO for 600-TR, 700-TR, and
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800-TR are 39, 68, and 73% within 150 min of visible light irradiation, respectively.
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This lower degradation efficiency can be attributed to the quick recombination of
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photogenerated electrons and holes in 600-TR, 700-TR, and 800-TR. In particular, the
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g-700-TR shows an excellent degradation efficiency of MO, reaching up to ~ 96%. In
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order to further prove the activity of photocatalyst for the mineralization of MO, the
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TOC test is carried out. As shown in Fig. S1, 95% of initial TOC is removed from the
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MO aqueous solution by g-700-TR, indicating that g-700-TR has the highest activity
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toward MO mineralization (conversion to H2O and CO2).
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Moreover, the variations of ln(C0/C) versus visible light irradiation time with
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different samples are revealed in Fig. 5b. The degradation of MO with different
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samples conforms to the first-order reaction kinetics, satisfying ln (C0/C) =k·t (k is the
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first-order rate constant, C0 is concentration of MO solution after adsorption, C is the
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instantaneous concentration of MO solution after degradation). The first-order rate
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constants k values for 600-TR, 700-TR, 800-TR, and g-700-TR are estimated to be
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0.0032, 0.0073, 0.0086, and 0.0208, respectively. Identically, the g-700-TR shows the
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highest value around 6 times than 600-TR, which can be originated from the
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synergistic effects of the introduction of Ti3+ and rod-shaped nanostructure, promoting
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Meanwhile, the cycle measurement of MO degradation for g-700-TR under visible
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light irradiation is also evaluated by repeating for five cycles and the results are
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shown in Fig. S2, which suggests the good recyclability of the prepared catalyst.
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The photocatalytic performance of the as-obtained samples is also estimated by
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monitoring H2 generation in the existence of sacrificial agent (20%). As shown in Fig.
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5c, the g-700-TR sample demonstrates the most optimum photocatalytic H2 evolution
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capability with a H2 production rate of 166 µmol h−1 g−1, superior to 600-TR, 700-TR,
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and 800-TR (~ 27, 45, and 45 µmol h−1 g−1). The enhanced photocatalytic property is
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assigned to synergistic effect of the high crystallinity, the presence of S and Ti3+, and
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the rod-like nanostructure of g-700-TR, with a high separation and migration of
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electrons and holes. As revealed in Fig. 5d, the cycling test of hydrogen evolution
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reaction indicates an excellent stability of g-700-TR sample even after 25 h irradiation
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with AM 1.5.
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On account of the aforementioned analyses, a possible mechanisation of
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enhanced photocatalytic activity is proposed as illustrated in Fig. 5e. The introduction
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of S 2P state in the valence band (VB) of TiO2 forms a new impurity level through the
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upward shifting of the VB. Moreover, the Ti3+ species and oxygen vacancies can form
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a new isolated level near the conduction band (CB) edge in the TiO2 forbidden gap.
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The synergistic interaction narrows the bandgap to a lower state and effectively
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enhances the separation of photo-generated charge carriers. Under visible-light
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irradiation, the excited electrons in the VB can be transited to conduction band of
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photocatalyst and can subsequently be caught by dioxygen in the aqueous solution to
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generate superoxide anion radicals (•O2-) with high oxidation capacity, which can
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entirely mineralize the organic contaminant [51]. What’s more, the electrons can also
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react with water molecules or hydrogen ions to produce H2. Moreover, the
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photogenerated holes can react with water molecules (H2O) or hydroxide ions (H+) to
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produce •OH, which oxidize the pollutant into CO2, H2O, and other intermediates. 1.0
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Fig. 5. Photocatalytic degradation of MO under visible light irradiation (a), variations of ln(C0/C)
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versus visible light irradiation time with different samples (b) (C is the corresponding degradative
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concentration of MO and C0 is initial concentration of MO), Photocatalytic hydrogen evolution of
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different samples (c), the recyclability of g-700-TR under AM 1.5 irradiation (d), and Schematic
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illustration of the visible-light driven photocatalytic mechanism for g-700-TR nanorods (e).
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4 Conclusions
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In summary, in-situ Ti3+/S doped high thermostable anatase TiO2 nanorods are
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successfully synthesized by a simple direct calcination approach combined with an
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in-situ solid-phase reaction method. The synthesized gray Ti3+/S-TiO2 nanorods
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photocatalyst can retain anatase structure up to 700 oC with a high crystallinity.
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Meanwhile, the introduction of S and the Ti3+ evidently narrow the bandgap of TiO2.
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Evidently, the degradation efficiency of MO and the rate of H2 generation are 96%
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and 166 µmol h-1 g-1. In addition, the excellent photocatalytic capability of gray
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Ti3+/S-doped TiO2 is ascribed to the synergistic reaction of S and Ti3+ species
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co-doped and one-dimensional nanorods structure, which contributes to reduce the
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prepared novel gray Ti3+/S-doped high thermostable anatase TiO2 nanorod will be a
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promising photocatalyst for water purification and hydrogen evolution in future.
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Acknowledgments
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We gratefully acknowledge the support of this research by the National
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Natural Science Foundation of China (51672073), the Natural Science
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Foundation of Heilongjiang Province (B2018010 and H2018012), the
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Heilongjiang Postdoctoral Startup Fund (LBH-Q14135), the Postdoctoral
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Science Foundation of China (2017M611399), and the University Nursing
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Program for Young Scholars with Creative Talents in Heilongjiang Province
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(UNPYSCT-2015014 and UNPYSCT-2016018).
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Highlights In-situ Ti3+/S doped high thermostable TiO2 nanorods are fabricated successfully. The narrowed bandgap of Ti3+/S-TiO2 extends photoresponse to visible light
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region. It exhibits excellent pollutant degradation and H2 evolution in visible light range.
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It is ascribed to the synergy of S/Ti3+ co-doping and the 1D structure.
S0254-0584(18)30715-6
DOI:
10.1016/j.matchemphys.2018.08.051
Reference:
MAC 20894
To appear in:
Materials Chemistry and Physics
Received Date: 3 November 2017 Revised Date:
29 June 2018
Accepted Date: 19 August 2018
Please cite this article as: M. Li, Z. Xing, J. Jiang, Z. Li, J. Kuang, J. Yin, N. Wan, Q. Zhu, W. Zhou, 3+ In-situ Ti /S doped high thermostable anatase TiO2 nanorods as efficient visible-light-driven photocatalysts, Materials Chemistry and Physics (2018), doi: 10.1016/j.matchemphys.2018.08.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
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In-Situ Ti3+/S Doped High Thermostable Anatase
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TiO2 Nanorods as Efficient Visible-Light-Driven
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Photocatalysts
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Meng Lia, Zipeng Xinga,*, Jiaojiao Jianga, Zhenzi Lib, Junyan Kuanga, Junwei Yina,
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Ning Wana, Qi Zhua,*, Wei Zhoua,*
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a
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of the People’s Republic of China, Heilongjiang University, Harbin 150080, P. R.
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China
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Tel: +86-451-8660-8616, Fax: +86-451-8660-8240,
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Email: [email protected]; [email protected]; [email protected]
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b
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150086, P. R. China
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Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education
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Department of Epidemiology and Biostatistics, Harbin Medical University, Harbin
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Abstract: In-situ Ti3+/S doped high thermostable anatase TiO2 nanorods using
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ethanediamine-modified TiOSO4 as precursor are synthesized under 700
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calcination, then combined with controllable in-situ solid-phase reaction method,
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calcined at 350 oC in argon. The outcomes declare that the obtained photocatalyst
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with a high crystallinity is effectively doped with S element and Ti3+ species, and
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synchronously possesses one-dimensional (1D) anatase nanorods structure with length
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of ~ 2-5 µm and width of ~ 0.5-1 µm. The S and Ti3+ co-doped 1D nanorod with a
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narrowed bandgap (2.56 eV) stretches the optical response range to visible-light. The
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visible-light-driven photocatalytic degradation efficiency of methyl orange and H2
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production rate for Ti3+/S-TiO2 nanorods are as high as 96% and 166 µmol h-1 g-1,
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showing about 6 times greater than 600-TR (TiO2 nanorods). This is be ascribed to the
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synergistic reaction of S and Ti3+ species co-doping narrows the bandgap and
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promotes the separation efficiency of photoexcited carriers, and the one-dimensional
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structure favors the transportation of photogenerated charge carriers. Hence, the
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prepared photocatalyst will have a great latent application prospect in fields of energy
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and environment.
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Keywords: Photocatalysis; TiO2 nanorods; Ti3+/S-doping; visible-light-driven
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photocatalyst; hydrogen evolution
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ACCEPTED MANUSCRIPT 1 Introduction
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In the past decades, semiconductor photocatalysts [1] have attracted a great deal of
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attention due to their potential applications in environmental remediation [2] and solar
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energy conversion [3]. As the most promising semiconductor photocatalyst, TiO2 has
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been extensively investigated in environmental cleaning and green energy production
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[4, 5], due to its safety, low cost [6], high chemical-stability [7], and good
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photoelectric performance [8] under ultraviolet (UV) light irradiation. However, as a
44
representative broad bandgap energy semiconductor (3.2 eV for anatase) [9], TiO2
45
mainly responses to UV light, about 3-5% of the entire solar energy [10], which
46
severely restricts its actual utilization in the visible light. In addition, the low rate of
47
electron transport and the high recombination rate of photoinduced electrons and
48
holes [11] limit the promotion of solar power utilization efficiency. Accordingly, some
49
strategies [12] have been proposed to tune the bandgap of TiO2 to extend its
50
photoresponse to visible light region.
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It is well known that doping is an effective method to enhance the photocatalytic
52
performance of TiO2 in visible light range, for example, transition metal elements
53
doping(Cr, Mn, Fe, Ni, Ru, and Cu) or nonmetallic elements doping(B, C, N, S, and F)
54
[13-16]. These dopants form a delocalized state or intra-band state in the bandgap and
55
act as electron donor or acceptor [17, 18] to induce absorption in visible region.
56
However, TiO2 doping with metal elements has been restricted owing to high
57
recombination efficiency of electrons and holes, generation of a new auxiliary
58
impurity level [19], and poisonous sensitization of dyes [20], so the amount of the
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60
performance [21]. Therefore, nonmetal-doping is deemed as a more potential method
61
to restrain the recombination between holes and electrons through producing a state of
62
delocalization [22-25] in the TiO2 bandgap. And in these nonmetallic elements, the
63
doping of sulfur element can inhibit the conversion of anatase phase to rutile phase
64
and enhance the thermal stability of anatase TiO2. Based on previous research, it is
65
suggested that the alliance of structural/morphology strategy [26] (1D nanomaterials
66
[27], such as nanowires [28], nanorods [29], nanobelts [30], nanotube [31]) and
67
nonmetal-doping [32], especially with S doped TiO2, can markedly improve the
68
visible-light-driven photocatalytic performance of TiO2 material.
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In order to further expand the absorption of TiO2 nanomaterials in visible-light
70
range, Mao et al. [33] prepare the black hydrogenated TiO2 nanomaterials with a
71
narrowed bandgap (1.54 eV), exhibiting exceedingly excellent photocatalytic
72
performance for degradation of organic contaminants. Hereafter, black TiO2 has
73
aroused a great deal of concern. Many preparation methods consisting of high
74
pressure hydrogenation, anodization, plasma assisted hydrogenation, aluminum
75
reduction, and chemical oxidation [34-38] are proposed to synthesize black TiO2. In
76
general, the excellent photocatalytic activity of black TiO2 is attributed to the highly
77
efficient electron-hole pairs separation capability [39].
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In this paper, in-situ Ti3+/S doped high thermostable anatase TiO2 nanorods is
79
effectively prepared by a simple direct calcination approach combined with an in-situ
80
solid-phase reaction method. The synthesized gray Ti3+/S-TiO2 nanorods can retain
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ACCEPTED MANUSCRIPT anatase up to 700 oC. The bandgap of the as-prepared sample is reduced to 2.56 eV,
82
exhibiting excellent photocatalytic properties for removal of methyl orange and H2
83
production. A probable mechanism of Ti3+/S-TiO2 nanorods is also provided.
84
2 Materials and Methods
85
2.1 Materials
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Titanium oxysulfate (TiOSO4), and sodium borohydride (NaBH4, 98%), were
87
purchased from Aladdin-Reagent-Company (China). Ethylenediamine (EDA), and
88
anhydrous ethanol (EtOH), were purchased from Tianjin-Kermel-Chemical-Reagent
89
Co. LTD (China). All reagents used in the experiment were analytical grade, and the
90
deionized (DI) water was used throughout this study.
91
2.2 Preparation
92
2.2.1 Preparation of S-TiO2 nanorods
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1 g of titanium oxysulfate were transferred to a porcelain boat and calcined at
94
400 oC in air for 2 h (2 oC/min). The obtained samples were added to 50 mL of
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deionized water and then stirred for 0.5 h. At the same time, adding 10 mL of EDA to
96
the solution till the pH reached 11. After that, the mixture were transferred to an oil
97
bath with stirring and kept at 90 oC for 36 h. When natural cooling to 20 oC, the
98
obtained solution were washed with deionized (DI) water and then dried at 60 oC for
99
24 h. After that, the resulting products were annealed at 600, 700, and 800 oC for 2 h,
100
respectively. The gained products were rinsed with deionized (DI) water for three
101
times, and dried at 60 oC. Finally, the S-doped TiO2 nanorods were obtained (which
102
were denoted as 600-TR, 700-TR, and 800-TR, respectively).
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2.2.2 Preparation of Ti3+/S-TiO2 nanorods 1.0 g of 700-TR and 1.0 g of NaBH4 were thoroughly mixed. Then they were
105
calcined at 350 oC for 2 h in Ar at the speed of 5 oC min-1. After natural cooling to
106
room temperature, the gray Ti3+/S-TiO2 nanorods (Scheme 1) were gained (marked as
107
g-700-TR), rinsed with deionized (DI) water and anhydrous ethanol for several times
108
to dislodge the unreacted sodium borohydride (NaBH4).
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109 110 111
Scheme 1. Schematic diagram for the formation of Ti3+/S-TiO2 nanorods.
2.3 Characterization
The Fourier transform infrared spectra (FT-IR), using KBr as diluting agent, was
113
conducted via a PerkinElmer spectrum system. Electrochemical impedance
114
spectroscopy (EIS) was observed with a CHI 760E electrochemical workstation
115
(Chenhua, Shanghai) in a frequency region between 100 KHz and 10 MHz. The total
116
organic carbon (TOC) removal was tested by TOC analysis equiped with analytic jena
117
multi NIC 2100 analyzer. X-ray diffraction (XRD) was obtained with a Bruker D8
118
Advance diffractometer by using Cu Kα radiation source (λ=1.5406 Å). Raman
119
spectra were collected via a Jobin Yvon HR 800 micro-Raman spectrometer in the
120
region of 100 cm-1 to 1000 cm-1 at 457.9 nm. X-ray photoelectron spectroscopy (XPS)
121
measurements were obtained through a PHI-5700 ESCA instrument using Al-Kα
122
X-ray source. Each binding energy was adjusted with surface adventitious carbon
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ACCEPTED MANUSCRIPT (284.6 eV). The surface morphology was collected via a field emission scanning
124
electron microscope (FE-SEM, Hitachi S-4800). Transmission electron microscopy
125
(TEM) was observed with a JEM-2100 electron microscope (JEOL, Japan).
126
UV-visible diffuse refection spectra (UV-vis DRS) were collected via a UV-vis
127
spectrophotometer (UV-2550, Shimadzu).
128
2.4 Photocatalytic test
129
2.4.1 Photocatalytic degradation of methyl orange
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The catalytic performance of the the as-prepared samples was assessed by the
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photo-degradation of methyl orange (MO) at room temperature. A xenon lamp (300
132
W) occupied with a cut-off filter (λ ≥ 420 nm) as a visible light source to achieve
133
visible light induced photocatalysis. In the photocatalytic experiments, 25 mg of the
134
samples was dispersed in 25 mL of methyl orange (MO) aqueous solution. Before
135
illuminated, the solution was stirred in darkness for 0.5 h to achieve an
136
adsorption-desorption balance. After reaction for 150 min under stirring, 2.0 mL of
137
the suspension were immediately put into a plastic tube and centrifuged to dislodge
138
the photocatalysts. Finally, the MO concentration was measured by UV-vis
139
spectrophotometer at its specific wavelength (λ = 464 nm) for calculating the
140
photocatalytic degradation rate of MO.
141
2.4.2 Photocatalytic hydrogen generation
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Photocatalytic H2 generation was obtained by using CEL-SPH2N (AuLight,
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Beijing), which was an online photocatalytic H2 evolution system at room temperature.
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Typically, 50 mg of samples was added to 100 mL aqueous solution that contained
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ACCEPTED MANUSCRIPT 20% methanol as the sacrificial agent. In order to remove the dissolved air in the
146
water, a vacuum pump was connected with the system prior to the experiment. A 300
147
W Xeon-lamp occupied with an AM 1.5G filter (Oriel, USA) was used as light source.
148
Subsequently, the amount of H2 evolution was measured using a gas chromatography
149
(GC) with the interval of each 1 h (molecular sieve 5 Å, N2 carrier, SP7800, TCD,
150
Beijing Keruida, Ltd).
151
3 Results and Discussion
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XRD is carried out to confirm the crystallinity and phase purity of the prepared
153
photocatalysts. Generally, the calcination temperature greatly affects the crystalline
154
phase composition and crystallinity of TiO2. As showed in Fig. 1a, it is obviously
155
observed that the diffraction peaks located at 25.3, 37.1, 37.8, 38.7, 48.2, 53.9, 55.1,
156
62.8, 69.0, 70.5, and 75.2 ° are perfectly corresponded to anatase phase (JCPDS
157
#21-1272) for the 600-TR, 700-TR, and g-700-TR, without any other impurity or new
158
phase. However, when the calcination temperature is up to 800 oC, the diffraction
159
peaks located at 27.4, 36.1, 41.2, 54.3, 56.6, and 69.8 ° correspond well to rutile phase
160
(JCPDS #21-1276), showing that a mixed phase containing anatase and rutile is
161
gained for 800-TR. According to the literature [40], the photocatalytic performance of
162
rutile titanium dioxide is less than that of anatase titanium dioxide. Therefore, the
163
samples that we prepared are able to restrain the phase transition from anatase to rutile
164
up to 700 oC. Furthermore, the intensity of XRD peaks become gradually stronger
165
with increasing temperature, indicating the crystallinity of TiO2 is enhanced evidently.
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Compared with 700-TR, the g-700-TR still maintains the original crystal phase since
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168
weakening, which may be attributable to the production of Ti3+ species and oxygen
169
vacancy, in virtue of disorder-led lattice strains and a slight reduction of crystallite
170
size [41]. At the same time, the characteristic peaks of S have not been observed. On
171
the one hand, it can’t be observed due to the low content of S. On the other hand, the
172
S element may penetrate into the lattice of TiO2, which makes it undetectable.
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In addition, Raman technique is another powerful means to further investigate
174
the phase compositions of 600-TR, 700-TR, 800-TR, and g-700-TR, respectively. As
175
shown in Fig. 1b, the five characteristic peaks of 600-TR located at 147.6, 196.7,
176
393.4, 507.9, and 631.2 cm-1 can be owing to six (3Eg+2B1g+A1g) Raman-active
177
modes [42], which shows that anatase is the major phase. Notably, the strongest peak
178
at 147.6 cm-1 is attributed to the O-Ti-O symmetric stretching modes. However,
179
compared with XRD patterns, no diffraction peaks of 800-TR ascribe to rutile for the
180
strong anatase peaks. It is clearly observed that the g-700-TR sample occurs a
181
blue-shift at 157.4 cm-1, which is displayed in the illustration of Fig. 1b. As reported
182
in previous literature [43], the shift of diffraction peaks is originated due to the
183
presence of Ti3+ and oxygen vacancies in TiO2 lattice owing to the treatment of
184
NaBH4. Meanwhile, the peaks of 700-TR and 800-TR at 153.2 and 154.8 slightly shift
185
to larger wave numbers compared with the peaks of 600-TR. No peak ascribes to
186
sulfur or sulfate species due to the lower doping amounts [44]. Raman spectra results
187
are correlating well with the XRD results mentioned above.
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A
A A A
R
R
A AA
A
AA
A
20
188
30
40 50 60 2 theta (Degree)
70
100
80
Intensity (a.u.) 100
Eg
200
B 1g
300
157.4 153.2
147.6
120
140
160
Raman Shift (cm -1)
180
200
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R R
600-TR 700-TR 800-TR g-700-TR
Eg
R R
154.8
b
600-TR 700-TR 800-TR g-700-TR
Intensity (a.u.)
Intensity (a.u.)
a
A 1g +B 1g
400 500 600 -1 Raman Shift ( cm )
Eg
700
800
Fig. 1. XRD patterns (a) and Raman spectra (b) of 600-TR, 700-TR, 800-TR, and g-700-TR,
190
respectively. The inset of (b) is the magnified spectra between 100 and 200 cm-1.
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The structure and morphology of g-700-TR is discussed via SEM and TEM
192
images, as revealed in Fig. 2. As observed from SEM image of Fig. 2a, the TiO2
193
nanorods are around 2-5 µm long and 0.5-1 µm wide. HRTEM and inset of (Fig. 2b)
194
TEM pattern further indicate that the g-700-TR is corresponding to nanorods structure.
195
Simultaneously, it is obviously observed that the lattice fringe spacing is 0.352 nm,
196
which agrees with the (101) crystal plane [45] of anatase TiO2, indicating the
197
well-crystallinity of anatase TiO2 nanorods. It declares that the partial reduction does
198
not change the TiO2 crystal phase. The outcomes are coincident with the XRD and
199
Raman, and the TiO2 nanorods with high crystallinity are an ideal photocatalyst. In
200
addition, it can be seen that an amorphous shell about 1-2 nm thick is formed after
201
partial reduction treatment from Fig. 2b. The presence of surface disordered structure
202
is mainly ascribed to the formation of defect states in the TiO2 bandgap owing to the
203
existence of Ti3+ species and oxygen vacancies [46], which is responsible for the
204
improved light absorption and photocatalysis.
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Fig. 2. SEM (a) and HRTEM image (b) of g-700-TR. The inset of (b) is the TEM image of
207
g-700-TR.
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The XPS spectrum is further carried out to study the surface composition and
209
chemical state of Ti, O, and S in g-700-TR, as revealed in Fig. 3. The full-scale XPS
210
spectrum of g-700-TR is shown in Fig. 3a, displaying the presence of Ti elements, O
211
elements, and S elements. Besides, Na element and B element are not observed,
212
indicating that the NaBH4 have been washed away absolutely. Fig. 3b shows the Ti 2p
213
XPS analysis of g-700-TR. The peaks located at 464.2 and 458.5 eV are
214
corresponding to the characteristic Ti 2p1/2 and Ti 2p3/2 of Ti4+ in TiO2, respectively.
215
And the other two peaks located at 463.0 and 457.9 eV are attributed to Ti3+ species
216
[47], formed as a result of the partial reduction of Ti4+ in TiO2. Fig. 3c shows the XPS
217
spectrum of O 1s. The peaks at 529.6 and 531.8 eV should be ascribed to Ti-O bonds
218
and surface -OH groups, respectively. The high-resolution Ti 2p and O 1s XPS
219
patterns suggest that both Ti3+ species and oxygen vacancies are effectively produced
220
on the surface or in the bulk, which can reduce the bandgap of TiO2 and restrain the
221
recombination between holes and electrons. Fig. 3d displays the XPS spectrum for S
222
2p. The presence of S is confirmed by a peak centred at 168.5 eV, which can be
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assigned to the S6+ state. Hence, the S element might be S6+ in the lattice of g-700-TR.
224
This is also similar to previous literature reports [48]. All the above results prove the
225
successful generation of gray Ti3+/S-TiO2 nanorods.
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0
464
462 460 458 Binding energy (eV)
d
O 1s
457.9 eV Ti3+ 2p3/2
456
454
S 2p
168.5 eV
Intensity (a.u.)
Intensity (a.u.)
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532 530 528 Binding energy (eV)
526
172
171
170 169 168 167 Binding energy (eV)
166
165
Fig. 3. Full-scale XPS spectrum (a), Ti 2p (b), O 1s (c), and S 2P (d) for g-700-TR.
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529.6 eV
534
463.0 eV Ti3+ 2p1/2
S 2p
400 200 Binding energy (eV)
c
536
464.2 eV Ti4+ 2p1/2
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458.5 eV Ti4+ 2p3/2
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Ti 2p
b
O 1s
Intensity (a.u.)
Intensity(a.u.)
a
The UV-vis DRS spectra in Fig. 4a is used to analyze the optical property and
230
bandgaps of different photocatalysts. As displayed in Fig. 4a, it can be clearly seen
231
that all the products present strong absorption in the ultraviolet region. Furthermore,
232
the products display an increasing visible light absorption with color-change. The
233
color of the S-doped samples is yellowish, and the color gradually deepens with the
234
increasing calcination temperature (insets in Fig. 4a). The enhanced visible light
235
absorption and color variation are ascribed to the existence of S in TiO2. However, the
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237
due to the existence of defect states in the TiO2 bandgap owing to the production of
238
Ti3+ species and oxygen vacancies. The indirect bandgaps of the products are
239
estimated in Fig. 4b. The bandgaps of 600-TR, 700-TR, 800-TR, and g-700-TR are
240
3.08, 2.97, 2.88, and 2.56 eV, respectively. The narrow bandgap is propitious to the
241
adsorption of visible light and enhances the utilization of photons. There is no color
242
change in the next three months after the sample is prepared, indicating the high
243
stability under ambient conditions.
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The FT-IR spectrum is used to research the functional groups of resultant
245
samples. As displayed in Fig. 4c, the absorption peaks at around 1630 and 3348 cm-1
246
can be assigned to the bending vibration of physically surface-adsorbed water
247
molecular (H2O) and stretching vibrations of surface hydroxyl groups (-OH) on the
248
surface of the TiO2, respectively. The spectra of all the products are analogous,
249
presenting a broad and intense absorption peak in the range of 400-800 cm-1, which
250
can be mostly assigned to the flexion vibration of Ti-O-Ti bonds and Ti-O bonds in
251
the TiO2. Besides, the weak FT-IR absorption peaks at around 1051 cm-1 can be
252
associated with S-O asymmetric stretch, implying the existence of Ti-O-S. This is in
253
confirmation with the previous results shown by researchers [49]. The FT-IR results
254
confirm that S6+ can successfully penetrate into the TiO2 structure, corresponding with
255
the XPS results.
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Electrochemical impedance spectra (EIS) measurement is a powerful
257
characterization technique to examine the electron-transport characteristics of the
13
ACCEPTED MANUSCRIPT interface between the solution and the electrode. Based on previous literatures [50],
259
the smaller impedance arc radius in EIS plots represents the better charge transport.
260
Fig. 4d shows the EIS patterns of 600-TR, 700-TR, 800-TR, and g-700-TR and the
261
corresponding equivalent circuit in the inset. Comparatively, the g-700-TR has a
262
much less depressed impedance semicircle arc than others. It implies that the
263
interfacial electrons can be transported more faster, and photo-generated electrons and
264
holes can be more effectively separated. Meanwhile, it suggests that Ti3+ plays a
265
significant role in enhancing the conductivity of the materials, thereby improving the
266
performance of the electrode. a
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1.2
Absorbance
1.0 0.8
600-TR 700-TR 800-TR g-700-TR
1.2 1.0
700-TR
800-TR g-700-TR
(αhν)1/2
0.8
600-TR
0.6
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0.4 0.2
267 1051
1630
Transmittance (a.u.)
500
1000
700
0.2 0.0 2.0
800
300
d
600-TR
2.97 eV 3.08 eV
3.0 3.5 Photon energy (eV)
100 R1
0
3500
4.5
150
50
3000
4.0
600-TR 700-TR 800-TR g-700-TR
200
700-TR
2.88 eV
2.5
250
800-TR
1500 2000 2500 Wavenumber (cm-1)
2.56 eV
3348
g-700-TR
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0.4
Z'' (ohm)
300
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268
b
1.4
600-TR 700-TR 800-TR g-700-TR
R2
R3
CPE1
0
50
100
150 Z' (ohm)
CPE1
200
250
300
269
Fig. 4. UV-vis diffuse reflectance spectra (a), determination of the indirect interband transition
270
energies (b), FT-IR spectra (c) and Nyquist plots (d) for 600-TR, 700-TR, 800-TR, and g-700-TR,
14
ACCEPTED MANUSCRIPT 271
respectively. The inset of (d) is the equivalent circuit applied to fit the resistance data.
The photocatalytic performance of the as-obtained TiO2 is estimated via
273
photo-degradation of MO under visible light. In this experiments, 0.5 h dark
274
adsorption is done to ensure adsorption equilibrium of MO on the surface of catalyst.
275
As illustrated in Fig. 5a, the degradation efficiency of MO for 600-TR, 700-TR, and
276
800-TR are 39, 68, and 73% within 150 min of visible light irradiation, respectively.
277
This lower degradation efficiency can be attributed to the quick recombination of
278
photogenerated electrons and holes in 600-TR, 700-TR, and 800-TR. In particular, the
279
g-700-TR shows an excellent degradation efficiency of MO, reaching up to ~ 96%. In
280
order to further prove the activity of photocatalyst for the mineralization of MO, the
281
TOC test is carried out. As shown in Fig. S1, 95% of initial TOC is removed from the
282
MO aqueous solution by g-700-TR, indicating that g-700-TR has the highest activity
283
toward MO mineralization (conversion to H2O and CO2).
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Moreover, the variations of ln(C0/C) versus visible light irradiation time with
285
different samples are revealed in Fig. 5b. The degradation of MO with different
286
samples conforms to the first-order reaction kinetics, satisfying ln (C0/C) =k·t (k is the
287
first-order rate constant, C0 is concentration of MO solution after adsorption, C is the
288
instantaneous concentration of MO solution after degradation). The first-order rate
289
constants k values for 600-TR, 700-TR, 800-TR, and g-700-TR are estimated to be
290
0.0032, 0.0073, 0.0086, and 0.0208, respectively. Identically, the g-700-TR shows the
291
highest value around 6 times than 600-TR, which can be originated from the
292
synergistic effects of the introduction of Ti3+ and rod-shaped nanostructure, promoting
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294
Meanwhile, the cycle measurement of MO degradation for g-700-TR under visible
295
light irradiation is also evaluated by repeating for five cycles and the results are
296
shown in Fig. S2, which suggests the good recyclability of the prepared catalyst.
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The photocatalytic performance of the as-obtained samples is also estimated by
298
monitoring H2 generation in the existence of sacrificial agent (20%). As shown in Fig.
299
5c, the g-700-TR sample demonstrates the most optimum photocatalytic H2 evolution
300
capability with a H2 production rate of 166 µmol h−1 g−1, superior to 600-TR, 700-TR,
301
and 800-TR (~ 27, 45, and 45 µmol h−1 g−1). The enhanced photocatalytic property is
302
assigned to synergistic effect of the high crystallinity, the presence of S and Ti3+, and
303
the rod-like nanostructure of g-700-TR, with a high separation and migration of
304
electrons and holes. As revealed in Fig. 5d, the cycling test of hydrogen evolution
305
reaction indicates an excellent stability of g-700-TR sample even after 25 h irradiation
306
with AM 1.5.
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On account of the aforementioned analyses, a possible mechanisation of
308
enhanced photocatalytic activity is proposed as illustrated in Fig. 5e. The introduction
309
of S 2P state in the valence band (VB) of TiO2 forms a new impurity level through the
310
upward shifting of the VB. Moreover, the Ti3+ species and oxygen vacancies can form
311
a new isolated level near the conduction band (CB) edge in the TiO2 forbidden gap.
312
The synergistic interaction narrows the bandgap to a lower state and effectively
313
enhances the separation of photo-generated charge carriers. Under visible-light
314
irradiation, the excited electrons in the VB can be transited to conduction band of
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ACCEPTED MANUSCRIPT TiO2. The photo-excited electrons can ulteriorly transport to the exterior of
316
photocatalyst and can subsequently be caught by dioxygen in the aqueous solution to
317
generate superoxide anion radicals (•O2-) with high oxidation capacity, which can
318
entirely mineralize the organic contaminant [51]. What’s more, the electrons can also
319
react with water molecules or hydrogen ions to produce H2. Moreover, the
320
photogenerated holes can react with water molecules (H2O) or hydroxide ions (H+) to
321
produce •OH, which oxidize the pollutant into CO2, H2O, and other intermediates. 1.0
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3.5
a
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C/C0
ln(C0/C)
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0.4
30
60
90 120 Time (min)
150
g-700-TR
c
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Rate of H2 production (µmol h-1g-1)
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50
700-TR
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30
60
90 Time (min)
120
150
d
800 700 600 500 400 300
800-TR
200
600-TR
0
323
0.0
180
H 2 production (µmol g -1)
322
0
1.5
0.5
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2.0
1.0
600-TR 700-TR 800-TR b-700-TR
0.2
b
600-TR 700-TR 800-TR g-700-TR
3.0
100 0
Photocatalyst
17
0
5
10
15 Time (h)
20
25
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Fig. 5. Photocatalytic degradation of MO under visible light irradiation (a), variations of ln(C0/C)
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versus visible light irradiation time with different samples (b) (C is the corresponding degradative
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concentration of MO and C0 is initial concentration of MO), Photocatalytic hydrogen evolution of
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different samples (c), the recyclability of g-700-TR under AM 1.5 irradiation (d), and Schematic
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illustration of the visible-light driven photocatalytic mechanism for g-700-TR nanorods (e).
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4 Conclusions
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In summary, in-situ Ti3+/S doped high thermostable anatase TiO2 nanorods are
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successfully synthesized by a simple direct calcination approach combined with an
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in-situ solid-phase reaction method. The synthesized gray Ti3+/S-TiO2 nanorods
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photocatalyst can retain anatase structure up to 700 oC with a high crystallinity.
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Meanwhile, the introduction of S and the Ti3+ evidently narrow the bandgap of TiO2.
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Evidently, the degradation efficiency of MO and the rate of H2 generation are 96%
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and 166 µmol h-1 g-1. In addition, the excellent photocatalytic capability of gray
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Ti3+/S-doped TiO2 is ascribed to the synergistic reaction of S and Ti3+ species
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co-doped and one-dimensional nanorods structure, which contributes to reduce the
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prepared novel gray Ti3+/S-doped high thermostable anatase TiO2 nanorod will be a
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promising photocatalyst for water purification and hydrogen evolution in future.
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Acknowledgments
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We gratefully acknowledge the support of this research by the National
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Natural Science Foundation of China (51672073), the Natural Science
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Foundation of Heilongjiang Province (B2018010 and H2018012), the
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Heilongjiang Postdoctoral Startup Fund (LBH-Q14135), the Postdoctoral
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Science Foundation of China (2017M611399), and the University Nursing
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Program for Young Scholars with Creative Talents in Heilongjiang Province
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(UNPYSCT-2015014 and UNPYSCT-2016018).
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Highlights In-situ Ti3+/S doped high thermostable TiO2 nanorods are fabricated successfully. The narrowed bandgap of Ti3+/S-TiO2 extends photoresponse to visible light
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region. It exhibits excellent pollutant degradation and H2 evolution in visible light range.
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It is ascribed to the synergy of S/Ti3+ co-doping and the 1D structure.