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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.
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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: xingzipeng@hlju.edu.cn; hdzhuqi@126.com; zwchem@hotmail.com
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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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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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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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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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(284.6 eV). The surface morphology was collected via a field emission scanning
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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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20% methanol as the sacrificial agent. In order to remove the dissolved air in the
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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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the NaBH4 treatment, but the intensity of g-700-TR diffraction peaks has a slightly
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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 A
R
R
A
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50
60
2 theta (Degree)
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100
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Intensity (a.u.)
100
Eg
200
B 1g
300
157.4
153.2
147.6
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140
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Raman Shift (cm -1)
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700-TR
800-TR
g-700-TR
Eg
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R
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b
600-TR
700-TR
800-TR
g-700-TR
Intensity (a.u.)
Intensity (a.u.)
a
A 1g +B 1g
400
500
600
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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,
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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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464
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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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Binding energy (eV)
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172
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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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Binding energy (eV)
c
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464.2 eV
Ti4+ 2p1/2
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458.5 eV
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Ti 2p
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Intensity (a.u.)
Intensity(a.u.)
a
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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gray TiO2 displays an extended absorption band up to ca. 800 nm, which is mostly
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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
240
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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interface between the solution and the electrode. Based on previous literatures [50],
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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
1.0
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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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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
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g-700-TR
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Wavelength (nm)
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Z'' (ohm)
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b
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600-TR
700-TR
800-TR
g-700-TR
R2
R3
CPE1
0
50
100
150
Z' (ohm)
CPE1
200
250
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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the separation of photo-excited carriers and accelerating the electrons transport.
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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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TiO2. The photo-excited electrons can ulteriorly transport to the exterior 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.
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3.5
a
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30
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d
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600-TR
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H 2 production (µmol g -1)
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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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bandgap and inhibit the recombination between holes and electrons. Hence, 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.
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