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One-pot synthesis of echinus-like Fe-doped SnO2 with enhanced photocatalytic activity under simulated sunlight
Accepted Manuscript One-pot synthesis of echinus-like Fe-doped SnO2 with enhanced photocatalytic activity under simulated sunlight Jinhua Zhang, Jianfeng Ye, Hao Chen, Yang Qu, Qian Deng, Zhidong Lin PII:
S0925-8388(16)33530-7
DOI:
10.1016/j.jallcom.2016.11.063
Reference:
JALCOM 39558
To appear in:
Journal of Alloys and Compounds
Received Date: 19 July 2016 Revised Date:
2 November 2016
Accepted Date: 4 November 2016
Please cite this article as: J. Zhang, J. Ye, H. Chen, Y. Qu, Q. Deng, Z. Lin, One-pot synthesis of echinus-like Fe-doped SnO2 with enhanced photocatalytic activity under simulated sunlight, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.063. 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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Abstract
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Unique echinus-like Fe-doped SnO2 particles constructed from ultrathin nanorods with the 2~4 nm diameter were readily synthesized by one-pot solvothermal method,
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in which hydrochloric acid was used to control the hydrolysis rates of SnCl4 and FeCl3. The as-prepared sample were characterized by means of transmission electron
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microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), and N2 adsorption-desorption analysis, etc. The obtained echinus-like Fe-doped SnO2 composites exhibited enhanced activity for both photodegradation of Rhodamine B (RhB) and photoreduction of Cr(VI) under simulated sunlight irradiation, which could be largely attributed to the Eg decreased by Fe-doping, high specific surface area and porosity of structure.
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One-Pot Synthesis of Echinus-Like Fe-doped SnO2 with Enhanced Photocatalytic Activity under Simulated Sunlight
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Jinhua Zhang1, Jianfeng Ye1*, Hao Chen1∗, Yang Qu1, Qian Deng1 and Zhidong Lin2
1. Department of Chemistry, College of Science, Huazhong Agricultural University, Wuhan 430070, Wuhan 430073, China
Provincial Key Laboratory of Plasma Chemistry & Advanced Materials, Wuhan Institute of
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2
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Technology, Wuhan 430073, China
Abstract
Unique echinus-like Fe-doped SnO2 particles constructed from ultrathin nanorods with the 2~4 nm diameter were readily synthesized by one-pot solvothermal method,
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in which hydrochloric acid was used to control the hydrolysis rates of SnCl4 and FeCl3. The as-prepared sample were characterized by means of transmission electron
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microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), and N2 adsorption-desorption analysis, etc. The obtained echinus-like Fe-doped SnO2
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composites exhibited enhanced activity for both photodegradation of Rhodamine B (RhB) and photoreduction of Cr(VI) under simulated sunlight irradiation, which could be largely attributed to the Eg decreased by Fe-doping, high specific surface area and porosity of structure. Keywords: SnO2, echinus-like, Fe-doped, solvothermal, photocatalysts 1
Corresponding authors.
Jianfeng Ye, Tel.: +86 18671451628; fax: +86 27 87284018. e-mail:
[email protected] Hao Chen, Tel.: +86 13707165577; fax: +86 27 87284018. [email protected]
ACCEPTED MANUSCRIPT 1 Introduction SnO2 has attracted much attention because of its most fascinating tunable applications in sensors [1, 2], transistors [3], field emitters [4], lithium ion batteries
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[5-8], and photocatalysis [9, 10]. Generally, the key features for catalytic activity of SnO2 nanomaterials is morphology, particle size, specific surface area and dimension. In these regards, great efforts have been devoted to synthesizing SnO2 nanostructures
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with different morphologies and architectures, such as octahedra [11, 12], nanorods
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[13, 14], nanowires [15, 16], nanoribbons [17, 18], nanotubes [19], flower-like [20] and coral-like [21] structure. Although the strategy of shape control can improve photocatalytic activity of SnO2, the enhancement remains limited due to its low separation efficiency of photoinduced electron-hole pairs.
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Thus various strategies including ions doping [22, 23], synthesis of stannate nanomaterials [24], noble-metal deposition [25] and constructing hetero-junctions [26] have been studied. Doping defined as the intentional introduction of foreign atoms or
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ions of suitable elements into host lattice is a widely-applied approach to improve the
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electrical and optical properties. For example, doping with element Zn into the SnO2 has been paid much attention by several research groups in order to enhance their performance in dye-sensitized solar cells [27], degrading organic dyes [28], and lithium ion batteries [29]. In addition, there are also numerous experimental investigations on the improvement of optical and electrical properties of SnO2 nanomaterials by doping with other metals, such as Fe [30], Ni [31], Cu [32], Mo [33], Mn [34, 35], Cr [36] and V [37]. Among various dopants, Fe3+ is the most popular
ACCEPTED MANUSCRIPT because of its half-filled electronic configuration which is thought to help narrowing the band gap by formation of new intermediate energy levels and minimize the recombination rate of electron-hole pairs by capturing the photogenerated electrons
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[38, 39]. Therefore, it is expected that Fe-doped SnO2 nanostructures might exhibit enhanced photocatalytic activity.
Herein, we report a facile one-pot solvothermal route to prepare echinus-like
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Fe-doped SnO2 particles by using HCl to control the hydrolysis rates of SnCl4 and
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FeCl3 in ethanol. Echinus-like Fe-doped SnO2 is composed of ultrathin nanorod subunits with diameter of 2~4 nm and has a high specific surface area of 138.5m2 g-1. The obtained echinus-like Fe-doped SnO2 particles exhibite enhanced activity for both photodegradation of Rhodamine B (RhB) and photoreduction of Cr(VI) under
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simulated sunlight, which could be largely attributed to the narrow band gap (Eg) of Fe-doping SnO2 as well as high specific surface area and high porosity of
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Echinus-like Fe-doped SnO2 particles.
2 Experimental
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2.1 Samples preparation
All the reagents were purchased from Shanghai Chemical Reagent Co. and used
without further purification. The fabrication of echinus-like Fe-doped SnO2 particles was simply achieved by using HCl to control the hydrolysis rates of SnCl4 and FeCl3 in ethanol. In a typical synthesis, 0.652 g SnCl4·5H2O and 0.038 g FeCl3·6H2O were added to the mixture of 200 µL hydrochloric acid (37%) and 32 mL ethanol under
ACCEPTED MANUSCRIPT continuous stirring. The obtained clear solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave, which was then heated at 180 °C for 12 h. After the autoclave was cooled down to room temperature, the product was collected by
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centrifugation, washed with ethanol and distilled water several times, and dried at 60 °C overnight. Fe3+ mole percent in the as-prepared nanomaterials was 7 at% Fe/SnO2
In addition, the pure SnO2, 3 at%, 5 at% and 10 at% Fe-doped SnO2 were
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synthesized by the above method, repectively. Here the as-prepared samples were
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named by SnO2, 3 at% Fe/SnO2, 5 at% Fe/SnO2, 7 at% Fe/SnO2, 10 at% Fe/SnO2, respectively. 2.2 Characterization
The obtained products were characterized by transmission electron microscopy
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(TEM, H-7650, 160 kV), high-resolution TEM (HRTEM, JEM-2100F, 200 kV), and powder X-ray diffraction (XRD, Bruker-AXS D8 Advance, Cu Kα). Nitrogen adsorption-desorption measurements were performed using Micromeritics ASAP
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2020. The pore size distribution was calculated from the adsorption branch of the
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sorption isotherms using the Brunauer-Joyner-Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) was performed on a VG Multilab 2000 spectrometer with monochromatized Al Kα radiation. The UV-vis diffuse reflectance spectra (DRS) were recorded on a Cary 5000 UV-vis-NIR spectrophotometer equipped with an integrating sphere using BaSO4 as a reference. 2.3 Photoactivity test for RhB degradation
ACCEPTED MANUSCRIPT The photoactivity of the as-prepared samples were evaluated by measuring photooxidation efficiency of Rhodamine B (RhB) in aqueous solution under stimulated sunlight. In a typical experiment, 50 mg photocatalyst was mixed with 50
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mL RhB solution (1×10-5 mol·L-1) in a quartz flask. Before irradiation, the suspension was magnetically stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. Then the suspension was exposed to the light irradiation from a 300 W
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Xenon lamp (PLS-SXE 300C Beijing Perfectlight Inc., China). At given irradiation
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time intervals, 3.0 mL of the suspension was taken out and the photocatalyst was separated from the solution by centrifugation. The remaining solution was analyzed by a Nicolet 300 evolution UV-vis spectrophotometer to evaluate the RhB concentration through monitoring the absorption peak at 554 nm.
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2.4 Photoactivity test for Cr(VI) reduction
The photoactivity of the as-prepared samples were also evaluated by measuring photoreduction efficiency of Cr(VI) in aqueous solution under stimulated sunlight
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irradiation. Briefly, 20 mg as-prepared photocatalyst were mixed with 50 mL K2Cr2O7
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aqueous solution (1×10-6 mol·L-1) and 0.01 M HCl in a beaker with a volume of 100 mL. Before irradiation, the suspension was magnetically stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. Then the suspension was exposed to the light irradiation from a 300 W Xenon lamp (PLS-SXE 300C). At given time intervals, 3 mL suspension were sampled on constant stirring, followed by centrifugation to remove precipitates. And the reduction effeciency of Cr(VI) was evaluated by the chromogenic reaction with diphenylcarbazide. A Nicolet 300
ACCEPTED MANUSCRIPT evolution UV-vis spectrophotometer was used to detect the hexavalent chromium content by recording the variation of the intensity of the absorption peak centered at 540 nm.
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3. Results and discussion
Fig. 1. XRD patterns of SnO2 and as-made Fe/SnO2 (a, b) and EDS patterns of
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echinus-like 7at% Fe/ SnO2 (c)
Fig. 1a shows the XRD pattern of the precipitate obtained after solvothermal reaction at 180 °C for 12 h. All diffraction peaks can be assigned to those of the
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tetragonal SnO2 phase (JCPDS No. 41-1445). The well-defined diffraction peaks of the spectrum suggest that the obtained products are highly crystallized under the
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solvothermal condition. No peak for crystalline iron oxide or any other crystalline compound is observed. As shown in Fig. 1b which is the magnification of X-ray diffraction pattern of the 2θ from 30° to 40°, the (101) peaks (at 33.9°) of Fe/SnO2 composite shifted about 1-2o with respect to the pure SnO2. The shifts are attributed to the Fe doping in the oxide lattice, moreover the half-width of the (101) plane increased with the increasing Fe doping percent. It should be owing to the Fe doping compressed the growth of SnO2 crystal grains, which could be observed according to the TEM images.
ACCEPTED MANUSCRIPT In order to verify the present of Fe doping in the doped samples, the element stoichiometric ratio of the as-prepared samples were analyzed by energy-dispersive spectroscopy [EDS]. (Fig. 1c). The EDS spectra clearly revealed that the emission
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peaks correspond to O, Sn and Fe, along with the silicon peak which might come from the glass substrate used to support the samples. Fe peak of the doped samples
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reveals that approximately 7at% of the Sn4+ sites are occupied by Fe3+ ions.
Fig. 2. TEM images (a, b), SAED pattern (c) and HRTEM image (d) of echinus-like
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7 at% Fe/SnO2.
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TEM characterizations were carried out in order to reveal the morphology of the as-prepared SnO2 products doped with Fe3+ ions (Fig. 2). The low-magnification TEM image (Fig. 2a) suggests the large-scale formation of echinus-like echinus-like particles predominantly 75~100 nm in diameter. An enlarged TEM image in Fig. 2b reveals that these as-prepared echinus-like particles are actually constructed by ultrathin nanorods with diameter of 2~4 nm, which abut against each other and form nanopores.
This
result
was
confirmed
by
the
corresponding
nitrogen
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respectively. Fig. 2c is a selected area electron diffraction (SAED) pattern corresponding to Fig. 2b, which exhibits sharp diffraction rings corresponding to the (110), (101), (200), (221), and (301) crystalline planes of the rutile-type SnO2,
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confirming the formation of polycrystalline SnO2. Fig. 2d displays an HRTEM image
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taken from the edge of a single nanopshere, in which two sets of lattice fringes with spaces of 0.33 and 0.16 nm can be attributed to the (110) and (002) planes of rutile-type SnO2, respectively. HRTEM image analysis also revealed that, the (110) lattice fringes are parallel to the axis of the nanorod subunits and the nanorods grow
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along the (001) direction.
In order to follow the morphology evolution of the echinus-like 7at% Fe/SnO2,
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the products obtained at different reaction stages were collected and characterized by TEM. The product obtained at 3 h showed as nanorods mostly and some of small
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aggregates (Fig. S1a). When the reaction time was increased to 7 h, echinus-like spherical particles appeared in addition to the dispersed nanorods. (Fig. S1b) And when the reaction time was further prolonged to 12 h, echinus-like particles were the predominant product. These observations demonstrate that the as-prepared echinus-like particles were actually assembled from nanorod subunits preformed in the reaction system.
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Fig. 3. Nitrogen adsorption and desorption isotherm (inset: pore size distribution) of
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echinus-like 7 at% Fe/SnO2.
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Fig. 4. XPS spectra of echinus-like 7 at% Fe/SnO2 (a) Sn 3d, (b) Sn 3p and Fe 2p, (c)
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Fe 3p, (d) O 1s.
To investigate the chemical state and binding energy of the elements for the obtained echinus-like 7 at% Fe/SnO2, XPS characterizations were employed, as shown in Fig. 4. The photoelectron peaks located at 486.7, 495.1 and 716.3 eV correspond to the binding energies of Sn 3d5/2, Sn 3d3/2 and Sn 3p3/2 of SnO2, respectively [39, 40]. The peak at 711.0 eV (Fig. 4b) are in agreement with the Fe 2p3/2 of Fe2O3, indicating that the oxide state of iron is trivalent [41]. In addition, the Fe 3p peak located at 55.5 eV
ACCEPTED MANUSCRIPT also can been seen in Fig.4c. The O 1s spectrum composed of an intense peak at 530.4 eV and an additional shoulder peak at 531.9 ev (Fig 4) can be attributed to the coordination of oxygen in the Sn–O–Sn and Sn–O–Fe modes, respectively [42, 43].
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Therefore, the XPS results further confirmed the formation of Fe-doped SnO2
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products.
Fig. 5. TEM images of SnO2 (a), 3 at% Fe/SnO2 (b), 5 at% Fe/SnO2 (c), 10 at% Fe/SnO2 (c) Besides the effects of Fe3+ doping on the morphology of SnO2 was investigated. The products with different doping concentrations were also characterized by TEM (Fig.
ACCEPTED MANUSCRIPT 5). It is noteworthy that tiny SnO2 nanocrystals less than 5 nm in diameter were obtained without imparting any Fe3+ ions into the SnO2 crystal lattice (Fig. 5a). Substituting ~3 at% Fe3+ ions for Sn4+ ions in the SnO2 crystal lattice can result in the
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formation of ultrathin nanorods in addition to some irregular loosely aggregates (Fig. 5b). Further increasing Fe3+ doping concentration to ~5 at% will acquire hollow spheres consisting of nanorods in addition to the dispersed nanorods (Fig. 5c), and
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echinus-like particles were the predominant product when the doping concentration of
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Fe3+ ions was increased to ~7 at%, clearly shown in Fig. 2. When the doping concentration of Fe3+ ions was increased to ~10 at%, dense aggregates with larger particle size appeared. (Fig. 5d), These results indicate that the substitution of Fe3+ ions for Sn4+ ions in the SnO2 crystal lattice can trigger the morphology
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transformation of SnO2 materials and choosing appropriate doping concentration is
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crucial for the formation of echinus-like Fe-doped SnO2 particles.
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undoped
SnO2
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Fig. 6. UV-vis diffuse reflectance spectra by echinus-like 7 at% Fe/SnO2 and nanocrystals
(a),
Photocatalytic
degradation
of
RhB
(b),
Photocatalytic reduction of Cr(VI) (c) and kinetic curves of the degradation of RhB
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by echinus-like Fe/SnO2 and undoped SnO2 nanocrystals (d).
The diffuse reflectance UV-visible spectra (DRUVS) of the as-prepared 7 at% Fe/SnO2 nanoparticle and undoped SnO2 nanocrystals as a reference material are shown in Fig. 6a. Compared with pure SnO2 nanocrystals, echinus-like 7 at% Fe-doped SnO2 shows an optical absorption threshold at ca. 420 nm in the visible range (red shift), which corresponds to the narrow band gap (Eg) of 2.96 eV. The decreasing of Eg in echinus-like Fe-doped SnO2 can be attributed to the doping of
ACCEPTED MANUSCRIPT Fe3+ ions. Therefore, owing to their structural characteristics, the as-prepared echinus-like Fe-doped SnO2 nanoparticles may exhibit enhanced photoactivity in degrading organic contaminants. To verify that, we measured their efficiency in
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photocatalytic decomposition of Rhodamine B (RhB) in aqueous solution under simulative sunlight irradiation. Fig. 6b shows the relative concentrations of RhB gradually decreases with the increasing of Fe doping content in SnO2. It is found that
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the obtained echinus-like Fe-doped SnO2 particles exhibit much higher photocatalytic
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efficiency in comparison with undoped SnO2 nanocrystals for the degradation of RhB. After 75 min irradiation, the degree of RhB degradation for echinus-like 7 at% Fe/SnO2 particles reached ~95%, while that for pure SnO2 nanocrystals only reached 44%. Fig. 5c reveals that the relationship between -ln(C/C0) and irradiation time is
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linear, hinting that the photocatalytic degradation of RhB follows pseudo-first order kinetics. The rate constant (k) for RhB degradation are 0.00656 min-1, 0.0219 min-1, 0.0322 min-1, 0.0367 min-1, 0.0421 min-1 in the presence of SnO2, 3 at% Fe/SnO2, 5
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at% Fe/SnO2, 7 at% Fe/SnO2, 10 at% Fe/SnO2, respectively. The rate constant (k) for is ~7 times than
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RhB degradation photocatalyzed by echinus-like 7 at% Fe/SnO2
that of undoped SnO2 nanocrystals, suggesting that the photooxidation activity of SnO2 materials can be significantly enhanced due to Fe-doped hollow nanosphere architechture. According to Fig. 6b and 6c, a conclusion could be drawn that the photocatalytic activity of Fe-doping SnO2 was enhanced with the increasing amount of Fe. However, 10 at% Fe/SnO2 displayed the lower photocatalytic efficiency towards RhB. These results indicated that a small amount of Fe doping could enhanced
ACCEPTED MANUSCRIPT the photocatalytic activity of as-made samples. For further investigation of the photocatalytic performance, the photocatalytic activity of echinus-like Fe-doped SnO2 particles was also evaluated by measuring the efficiency in photocatalytic reduction of
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Cr (VI) in aqueous solution. As shown in Fig. 6d, echinus-like 7 at% Fe/SnO2 particles exhibit the highest photocatalytic activity with 50% degradation efficiency in 2.5 h.
On the contrary, undoped SnO2 nanocrystals show much lower activity under
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the same light irradiation, which confirmed that Fe3+ doping can improve the
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photocatalytic efficiency of SnO2 in the simulative sunlight region.
4. Conclusions
In summary, echinus-like Fe-doped SnO2 echinus-like particles were readily
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synthesized via using HCl to control the hydrolysis rates of SnCl4 and FeCl3 in ethanol under one-pot solvothermal conditions. The echinus-like Fe-doped SnO2
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particles synthesized were assembled from ultrathin nanorods with diameter of 2~4 nm preformed in the reaction system and have a specific surface area high as 138.5 m2
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g-1. Owing to their novel structural characteristics, the obtained echinus-like Fe-doped SnO2 particles exhibited enhanced activity for both photodegradation of Rhodamine B (RhB) and photoreduction of Cr(VI) under simulated sunlight irradiation. This result may shed light on the growth mechanism of doped SnO2 materials, and open a new avenue toward large-scale synthesis of doped hollow architectures of functional materials with high specific surface area, high porosity, and promising applications in energy and environmental fields.
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Acknowledgements Financial support from Fundamental Research Funds for the Central University
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(Grants 2014QC006, 2662015QC042, 2662014BQ063), Natural Science Foundation of Hubei Province (Grants 2015CFB175), and National Natural Science Foundation
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of China (Grants 51572101) is gratefully acknowledged.
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Shukla A. Agarwal.
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[36] T. Indira Gandhi, R. Ramesh Babu, K. Ramamurthi, Structural, morphological, electrical and optical studies of Cr doped SnO2 thin films deposited by the spray pyrolysis technique, Mater. Sci. Semicond. Process. 16 (2013) 472-479. [37] J. Mazloom, F.E. Ghodsi, H. Golmojde, Synthesis and characterization of
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vanadium doped SnO2 diluted magnetic semiconductor nanoparticles with enhanced photocatalytic activities, J. Alloy. Compd. 639 (2015) 393-399.
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properties, J. Mater. Chem. A 2 (2014) 15875-15882.
ACCEPTED MANUSCRIPT [40] W. E. Morgan, J. R. Van Wazer, Binding Energy Shifts in the X-Ray Photoelectron Spectra of a Series of Rlelated Group IV-a Compounds, J. Phys. Chem. 77 (1973) 964-969.
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[42] T. Yamashita, P. Hayes, Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide
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materials, Applied Surface Science 254 (2008) 2441-2449.
[43] D. Shuttleworth, Preparation of Metal-Polymer Dlspersions by Plasma
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Techniques, An ESCA Investigation. J. Phys. Chem. 84 (1980) 1629-1634.
Figure Caption
Fig. 1. XRD (a, b) patterns of SnO2 and as-made Fe/ SnO2, and EDS (c) patterns of echinus-like 7 at% Fe/ SnO2 particles.
Fig. 2. TEM images (a, b), SAED pattern (c) and HRTEM image (d) of echinus-like
7 at% Fe/SnO2 particles.
ACCEPTED MANUSCRIPT Fig. 3. Nitrogen adsorption and desorption isotherm (inset: pore size distribution) of echinus-like 7 at% Fe/SnO2 particles.
Fe 2p, (c) Fe 3p, (d) O 1s.
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Fig. 4. XPS spectra of echinus-like 7 at% Fe/SnO2 particles: (a) Sn 3d, (b) Sn 3p and
Fig. 5. TEM images of as-prepared samples with different doping concentrations of
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Fe3+ ions: (a) 0, (b) 3%, (c) 5% and (d) 10%.
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Fig. 6. UV-vis diffuse reflectance spectra by echinus-like 7 at% Fe/SnO2 and undoped SnO2 nanocrystals (a), Photocatalytic degradation of RhB (b), Photocatalytic reduction of Cr(VI) (c) and kinetic curves of the degradation of RhB by echinus-like Fe/SnO2 and undoped SnO2 nanocrystals (d).
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Fig. S1. TEM images of the precipitates obtained at 3h (a) and 7h (b).
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1. Unique echinus-like Fe-doped SnO2 hollow nanospheres were synthesized with simple solvothermal treatment. 2. As doping concentration of Fe3+ ions was 7 at%, echinus-like Fe-doped SnO2 was formed. 3. Echinus-like Fe-doped SnO2 exhibited enhanced photoactivity to photodegrade rhodamine B and photoreduct Cr(VI).
S0925-8388(16)33530-7
DOI:
10.1016/j.jallcom.2016.11.063
Reference:
JALCOM 39558
To appear in:
Journal of Alloys and Compounds
Received Date: 19 July 2016 Revised Date:
2 November 2016
Accepted Date: 4 November 2016
Please cite this article as: J. Zhang, J. Ye, H. Chen, Y. Qu, Q. Deng, Z. Lin, One-pot synthesis of echinus-like Fe-doped SnO2 with enhanced photocatalytic activity under simulated sunlight, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.063. 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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Abstract
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Unique echinus-like Fe-doped SnO2 particles constructed from ultrathin nanorods with the 2~4 nm diameter were readily synthesized by one-pot solvothermal method,
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in which hydrochloric acid was used to control the hydrolysis rates of SnCl4 and FeCl3. The as-prepared sample were characterized by means of transmission electron
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microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), and N2 adsorption-desorption analysis, etc. The obtained echinus-like Fe-doped SnO2 composites exhibited enhanced activity for both photodegradation of Rhodamine B (RhB) and photoreduction of Cr(VI) under simulated sunlight irradiation, which could be largely attributed to the Eg decreased by Fe-doping, high specific surface area and porosity of structure.
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One-Pot Synthesis of Echinus-Like Fe-doped SnO2 with Enhanced Photocatalytic Activity under Simulated Sunlight
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Jinhua Zhang1, Jianfeng Ye1*, Hao Chen1∗, Yang Qu1, Qian Deng1 and Zhidong Lin2
1. Department of Chemistry, College of Science, Huazhong Agricultural University, Wuhan 430070, Wuhan 430073, China
Provincial Key Laboratory of Plasma Chemistry & Advanced Materials, Wuhan Institute of
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2
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Technology, Wuhan 430073, China
Abstract
Unique echinus-like Fe-doped SnO2 particles constructed from ultrathin nanorods with the 2~4 nm diameter were readily synthesized by one-pot solvothermal method,
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in which hydrochloric acid was used to control the hydrolysis rates of SnCl4 and FeCl3. The as-prepared sample were characterized by means of transmission electron
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microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), and N2 adsorption-desorption analysis, etc. The obtained echinus-like Fe-doped SnO2
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composites exhibited enhanced activity for both photodegradation of Rhodamine B (RhB) and photoreduction of Cr(VI) under simulated sunlight irradiation, which could be largely attributed to the Eg decreased by Fe-doping, high specific surface area and porosity of structure. Keywords: SnO2, echinus-like, Fe-doped, solvothermal, photocatalysts 1
Corresponding authors.
Jianfeng Ye, Tel.: +86 18671451628; fax: +86 27 87284018. e-mail:
[email protected] Hao Chen, Tel.: +86 13707165577; fax: +86 27 87284018. [email protected]
ACCEPTED MANUSCRIPT 1 Introduction SnO2 has attracted much attention because of its most fascinating tunable applications in sensors [1, 2], transistors [3], field emitters [4], lithium ion batteries
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[5-8], and photocatalysis [9, 10]. Generally, the key features for catalytic activity of SnO2 nanomaterials is morphology, particle size, specific surface area and dimension. In these regards, great efforts have been devoted to synthesizing SnO2 nanostructures
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with different morphologies and architectures, such as octahedra [11, 12], nanorods
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[13, 14], nanowires [15, 16], nanoribbons [17, 18], nanotubes [19], flower-like [20] and coral-like [21] structure. Although the strategy of shape control can improve photocatalytic activity of SnO2, the enhancement remains limited due to its low separation efficiency of photoinduced electron-hole pairs.
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Thus various strategies including ions doping [22, 23], synthesis of stannate nanomaterials [24], noble-metal deposition [25] and constructing hetero-junctions [26] have been studied. Doping defined as the intentional introduction of foreign atoms or
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ions of suitable elements into host lattice is a widely-applied approach to improve the
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electrical and optical properties. For example, doping with element Zn into the SnO2 has been paid much attention by several research groups in order to enhance their performance in dye-sensitized solar cells [27], degrading organic dyes [28], and lithium ion batteries [29]. In addition, there are also numerous experimental investigations on the improvement of optical and electrical properties of SnO2 nanomaterials by doping with other metals, such as Fe [30], Ni [31], Cu [32], Mo [33], Mn [34, 35], Cr [36] and V [37]. Among various dopants, Fe3+ is the most popular
ACCEPTED MANUSCRIPT because of its half-filled electronic configuration which is thought to help narrowing the band gap by formation of new intermediate energy levels and minimize the recombination rate of electron-hole pairs by capturing the photogenerated electrons
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[38, 39]. Therefore, it is expected that Fe-doped SnO2 nanostructures might exhibit enhanced photocatalytic activity.
Herein, we report a facile one-pot solvothermal route to prepare echinus-like
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Fe-doped SnO2 particles by using HCl to control the hydrolysis rates of SnCl4 and
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FeCl3 in ethanol. Echinus-like Fe-doped SnO2 is composed of ultrathin nanorod subunits with diameter of 2~4 nm and has a high specific surface area of 138.5m2 g-1. The obtained echinus-like Fe-doped SnO2 particles exhibite enhanced activity for both photodegradation of Rhodamine B (RhB) and photoreduction of Cr(VI) under
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simulated sunlight, which could be largely attributed to the narrow band gap (Eg) of Fe-doping SnO2 as well as high specific surface area and high porosity of
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Echinus-like Fe-doped SnO2 particles.
2 Experimental
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2.1 Samples preparation
All the reagents were purchased from Shanghai Chemical Reagent Co. and used
without further purification. The fabrication of echinus-like Fe-doped SnO2 particles was simply achieved by using HCl to control the hydrolysis rates of SnCl4 and FeCl3 in ethanol. In a typical synthesis, 0.652 g SnCl4·5H2O and 0.038 g FeCl3·6H2O were added to the mixture of 200 µL hydrochloric acid (37%) and 32 mL ethanol under
ACCEPTED MANUSCRIPT continuous stirring. The obtained clear solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave, which was then heated at 180 °C for 12 h. After the autoclave was cooled down to room temperature, the product was collected by
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centrifugation, washed with ethanol and distilled water several times, and dried at 60 °C overnight. Fe3+ mole percent in the as-prepared nanomaterials was 7 at% Fe/SnO2
In addition, the pure SnO2, 3 at%, 5 at% and 10 at% Fe-doped SnO2 were
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synthesized by the above method, repectively. Here the as-prepared samples were
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named by SnO2, 3 at% Fe/SnO2, 5 at% Fe/SnO2, 7 at% Fe/SnO2, 10 at% Fe/SnO2, respectively. 2.2 Characterization
The obtained products were characterized by transmission electron microscopy
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(TEM, H-7650, 160 kV), high-resolution TEM (HRTEM, JEM-2100F, 200 kV), and powder X-ray diffraction (XRD, Bruker-AXS D8 Advance, Cu Kα). Nitrogen adsorption-desorption measurements were performed using Micromeritics ASAP
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2020. The pore size distribution was calculated from the adsorption branch of the
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sorption isotherms using the Brunauer-Joyner-Halenda (BJH) method. X-ray photoelectron spectroscopy (XPS) was performed on a VG Multilab 2000 spectrometer with monochromatized Al Kα radiation. The UV-vis diffuse reflectance spectra (DRS) were recorded on a Cary 5000 UV-vis-NIR spectrophotometer equipped with an integrating sphere using BaSO4 as a reference. 2.3 Photoactivity test for RhB degradation
ACCEPTED MANUSCRIPT The photoactivity of the as-prepared samples were evaluated by measuring photooxidation efficiency of Rhodamine B (RhB) in aqueous solution under stimulated sunlight. In a typical experiment, 50 mg photocatalyst was mixed with 50
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mL RhB solution (1×10-5 mol·L-1) in a quartz flask. Before irradiation, the suspension was magnetically stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. Then the suspension was exposed to the light irradiation from a 300 W
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Xenon lamp (PLS-SXE 300C Beijing Perfectlight Inc., China). At given irradiation
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time intervals, 3.0 mL of the suspension was taken out and the photocatalyst was separated from the solution by centrifugation. The remaining solution was analyzed by a Nicolet 300 evolution UV-vis spectrophotometer to evaluate the RhB concentration through monitoring the absorption peak at 554 nm.
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2.4 Photoactivity test for Cr(VI) reduction
The photoactivity of the as-prepared samples were also evaluated by measuring photoreduction efficiency of Cr(VI) in aqueous solution under stimulated sunlight
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irradiation. Briefly, 20 mg as-prepared photocatalyst were mixed with 50 mL K2Cr2O7
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aqueous solution (1×10-6 mol·L-1) and 0.01 M HCl in a beaker with a volume of 100 mL. Before irradiation, the suspension was magnetically stirred in the dark for 30 min to achieve the adsorption-desorption equilibrium. Then the suspension was exposed to the light irradiation from a 300 W Xenon lamp (PLS-SXE 300C). At given time intervals, 3 mL suspension were sampled on constant stirring, followed by centrifugation to remove precipitates. And the reduction effeciency of Cr(VI) was evaluated by the chromogenic reaction with diphenylcarbazide. A Nicolet 300
ACCEPTED MANUSCRIPT evolution UV-vis spectrophotometer was used to detect the hexavalent chromium content by recording the variation of the intensity of the absorption peak centered at 540 nm.
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3. Results and discussion
Fig. 1. XRD patterns of SnO2 and as-made Fe/SnO2 (a, b) and EDS patterns of
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echinus-like 7at% Fe/ SnO2 (c)
Fig. 1a shows the XRD pattern of the precipitate obtained after solvothermal reaction at 180 °C for 12 h. All diffraction peaks can be assigned to those of the
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tetragonal SnO2 phase (JCPDS No. 41-1445). The well-defined diffraction peaks of the spectrum suggest that the obtained products are highly crystallized under the
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solvothermal condition. No peak for crystalline iron oxide or any other crystalline compound is observed. As shown in Fig. 1b which is the magnification of X-ray diffraction pattern of the 2θ from 30° to 40°, the (101) peaks (at 33.9°) of Fe/SnO2 composite shifted about 1-2o with respect to the pure SnO2. The shifts are attributed to the Fe doping in the oxide lattice, moreover the half-width of the (101) plane increased with the increasing Fe doping percent. It should be owing to the Fe doping compressed the growth of SnO2 crystal grains, which could be observed according to the TEM images.
ACCEPTED MANUSCRIPT In order to verify the present of Fe doping in the doped samples, the element stoichiometric ratio of the as-prepared samples were analyzed by energy-dispersive spectroscopy [EDS]. (Fig. 1c). The EDS spectra clearly revealed that the emission
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peaks correspond to O, Sn and Fe, along with the silicon peak which might come from the glass substrate used to support the samples. Fe peak of the doped samples
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reveals that approximately 7at% of the Sn4+ sites are occupied by Fe3+ ions.
Fig. 2. TEM images (a, b), SAED pattern (c) and HRTEM image (d) of echinus-like
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7 at% Fe/SnO2.
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TEM characterizations were carried out in order to reveal the morphology of the as-prepared SnO2 products doped with Fe3+ ions (Fig. 2). The low-magnification TEM image (Fig. 2a) suggests the large-scale formation of echinus-like echinus-like particles predominantly 75~100 nm in diameter. An enlarged TEM image in Fig. 2b reveals that these as-prepared echinus-like particles are actually constructed by ultrathin nanorods with diameter of 2~4 nm, which abut against each other and form nanopores.
This
result
was
confirmed
by
the
corresponding
nitrogen
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respectively. Fig. 2c is a selected area electron diffraction (SAED) pattern corresponding to Fig. 2b, which exhibits sharp diffraction rings corresponding to the (110), (101), (200), (221), and (301) crystalline planes of the rutile-type SnO2,
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confirming the formation of polycrystalline SnO2. Fig. 2d displays an HRTEM image
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taken from the edge of a single nanopshere, in which two sets of lattice fringes with spaces of 0.33 and 0.16 nm can be attributed to the (110) and (002) planes of rutile-type SnO2, respectively. HRTEM image analysis also revealed that, the (110) lattice fringes are parallel to the axis of the nanorod subunits and the nanorods grow
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along the (001) direction.
In order to follow the morphology evolution of the echinus-like 7at% Fe/SnO2,
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the products obtained at different reaction stages were collected and characterized by TEM. The product obtained at 3 h showed as nanorods mostly and some of small
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aggregates (Fig. S1a). When the reaction time was increased to 7 h, echinus-like spherical particles appeared in addition to the dispersed nanorods. (Fig. S1b) And when the reaction time was further prolonged to 12 h, echinus-like particles were the predominant product. These observations demonstrate that the as-prepared echinus-like particles were actually assembled from nanorod subunits preformed in the reaction system.
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Fig. 3. Nitrogen adsorption and desorption isotherm (inset: pore size distribution) of
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echinus-like 7 at% Fe/SnO2.
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Fig. 4. XPS spectra of echinus-like 7 at% Fe/SnO2 (a) Sn 3d, (b) Sn 3p and Fe 2p, (c)
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Fe 3p, (d) O 1s.
To investigate the chemical state and binding energy of the elements for the obtained echinus-like 7 at% Fe/SnO2, XPS characterizations were employed, as shown in Fig. 4. The photoelectron peaks located at 486.7, 495.1 and 716.3 eV correspond to the binding energies of Sn 3d5/2, Sn 3d3/2 and Sn 3p3/2 of SnO2, respectively [39, 40]. The peak at 711.0 eV (Fig. 4b) are in agreement with the Fe 2p3/2 of Fe2O3, indicating that the oxide state of iron is trivalent [41]. In addition, the Fe 3p peak located at 55.5 eV
ACCEPTED MANUSCRIPT also can been seen in Fig.4c. The O 1s spectrum composed of an intense peak at 530.4 eV and an additional shoulder peak at 531.9 ev (Fig 4) can be attributed to the coordination of oxygen in the Sn–O–Sn and Sn–O–Fe modes, respectively [42, 43].
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Therefore, the XPS results further confirmed the formation of Fe-doped SnO2
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products.
Fig. 5. TEM images of SnO2 (a), 3 at% Fe/SnO2 (b), 5 at% Fe/SnO2 (c), 10 at% Fe/SnO2 (c) Besides the effects of Fe3+ doping on the morphology of SnO2 was investigated. The products with different doping concentrations were also characterized by TEM (Fig.
ACCEPTED MANUSCRIPT 5). It is noteworthy that tiny SnO2 nanocrystals less than 5 nm in diameter were obtained without imparting any Fe3+ ions into the SnO2 crystal lattice (Fig. 5a). Substituting ~3 at% Fe3+ ions for Sn4+ ions in the SnO2 crystal lattice can result in the
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formation of ultrathin nanorods in addition to some irregular loosely aggregates (Fig. 5b). Further increasing Fe3+ doping concentration to ~5 at% will acquire hollow spheres consisting of nanorods in addition to the dispersed nanorods (Fig. 5c), and
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echinus-like particles were the predominant product when the doping concentration of
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Fe3+ ions was increased to ~7 at%, clearly shown in Fig. 2. When the doping concentration of Fe3+ ions was increased to ~10 at%, dense aggregates with larger particle size appeared. (Fig. 5d), These results indicate that the substitution of Fe3+ ions for Sn4+ ions in the SnO2 crystal lattice can trigger the morphology
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transformation of SnO2 materials and choosing appropriate doping concentration is
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crucial for the formation of echinus-like Fe-doped SnO2 particles.
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undoped
SnO2
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Fig. 6. UV-vis diffuse reflectance spectra by echinus-like 7 at% Fe/SnO2 and nanocrystals
(a),
Photocatalytic
degradation
of
RhB
(b),
Photocatalytic reduction of Cr(VI) (c) and kinetic curves of the degradation of RhB
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by echinus-like Fe/SnO2 and undoped SnO2 nanocrystals (d).
The diffuse reflectance UV-visible spectra (DRUVS) of the as-prepared 7 at% Fe/SnO2 nanoparticle and undoped SnO2 nanocrystals as a reference material are shown in Fig. 6a. Compared with pure SnO2 nanocrystals, echinus-like 7 at% Fe-doped SnO2 shows an optical absorption threshold at ca. 420 nm in the visible range (red shift), which corresponds to the narrow band gap (Eg) of 2.96 eV. The decreasing of Eg in echinus-like Fe-doped SnO2 can be attributed to the doping of
ACCEPTED MANUSCRIPT Fe3+ ions. Therefore, owing to their structural characteristics, the as-prepared echinus-like Fe-doped SnO2 nanoparticles may exhibit enhanced photoactivity in degrading organic contaminants. To verify that, we measured their efficiency in
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photocatalytic decomposition of Rhodamine B (RhB) in aqueous solution under simulative sunlight irradiation. Fig. 6b shows the relative concentrations of RhB gradually decreases with the increasing of Fe doping content in SnO2. It is found that
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the obtained echinus-like Fe-doped SnO2 particles exhibit much higher photocatalytic
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efficiency in comparison with undoped SnO2 nanocrystals for the degradation of RhB. After 75 min irradiation, the degree of RhB degradation for echinus-like 7 at% Fe/SnO2 particles reached ~95%, while that for pure SnO2 nanocrystals only reached 44%. Fig. 5c reveals that the relationship between -ln(C/C0) and irradiation time is
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linear, hinting that the photocatalytic degradation of RhB follows pseudo-first order kinetics. The rate constant (k) for RhB degradation are 0.00656 min-1, 0.0219 min-1, 0.0322 min-1, 0.0367 min-1, 0.0421 min-1 in the presence of SnO2, 3 at% Fe/SnO2, 5
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at% Fe/SnO2, 7 at% Fe/SnO2, 10 at% Fe/SnO2, respectively. The rate constant (k) for is ~7 times than
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RhB degradation photocatalyzed by echinus-like 7 at% Fe/SnO2
that of undoped SnO2 nanocrystals, suggesting that the photooxidation activity of SnO2 materials can be significantly enhanced due to Fe-doped hollow nanosphere architechture. According to Fig. 6b and 6c, a conclusion could be drawn that the photocatalytic activity of Fe-doping SnO2 was enhanced with the increasing amount of Fe. However, 10 at% Fe/SnO2 displayed the lower photocatalytic efficiency towards RhB. These results indicated that a small amount of Fe doping could enhanced
ACCEPTED MANUSCRIPT the photocatalytic activity of as-made samples. For further investigation of the photocatalytic performance, the photocatalytic activity of echinus-like Fe-doped SnO2 particles was also evaluated by measuring the efficiency in photocatalytic reduction of
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Cr (VI) in aqueous solution. As shown in Fig. 6d, echinus-like 7 at% Fe/SnO2 particles exhibit the highest photocatalytic activity with 50% degradation efficiency in 2.5 h.
On the contrary, undoped SnO2 nanocrystals show much lower activity under
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the same light irradiation, which confirmed that Fe3+ doping can improve the
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photocatalytic efficiency of SnO2 in the simulative sunlight region.
4. Conclusions
In summary, echinus-like Fe-doped SnO2 echinus-like particles were readily
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synthesized via using HCl to control the hydrolysis rates of SnCl4 and FeCl3 in ethanol under one-pot solvothermal conditions. The echinus-like Fe-doped SnO2
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particles synthesized were assembled from ultrathin nanorods with diameter of 2~4 nm preformed in the reaction system and have a specific surface area high as 138.5 m2
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g-1. Owing to their novel structural characteristics, the obtained echinus-like Fe-doped SnO2 particles exhibited enhanced activity for both photodegradation of Rhodamine B (RhB) and photoreduction of Cr(VI) under simulated sunlight irradiation. This result may shed light on the growth mechanism of doped SnO2 materials, and open a new avenue toward large-scale synthesis of doped hollow architectures of functional materials with high specific surface area, high porosity, and promising applications in energy and environmental fields.
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Acknowledgements Financial support from Fundamental Research Funds for the Central University
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(Grants 2014QC006, 2662015QC042, 2662014BQ063), Natural Science Foundation of Hubei Province (Grants 2015CFB175), and National Natural Science Foundation
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of China (Grants 51572101) is gratefully acknowledged.
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Figure Caption
Fig. 1. XRD (a, b) patterns of SnO2 and as-made Fe/ SnO2, and EDS (c) patterns of echinus-like 7 at% Fe/ SnO2 particles.
Fig. 2. TEM images (a, b), SAED pattern (c) and HRTEM image (d) of echinus-like
7 at% Fe/SnO2 particles.
ACCEPTED MANUSCRIPT Fig. 3. Nitrogen adsorption and desorption isotherm (inset: pore size distribution) of echinus-like 7 at% Fe/SnO2 particles.
Fe 2p, (c) Fe 3p, (d) O 1s.
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Fig. 4. XPS spectra of echinus-like 7 at% Fe/SnO2 particles: (a) Sn 3d, (b) Sn 3p and
Fig. 5. TEM images of as-prepared samples with different doping concentrations of
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Fe3+ ions: (a) 0, (b) 3%, (c) 5% and (d) 10%.
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Fig. 6. UV-vis diffuse reflectance spectra by echinus-like 7 at% Fe/SnO2 and undoped SnO2 nanocrystals (a), Photocatalytic degradation of RhB (b), Photocatalytic reduction of Cr(VI) (c) and kinetic curves of the degradation of RhB by echinus-like Fe/SnO2 and undoped SnO2 nanocrystals (d).
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Fig. S1. TEM images of the precipitates obtained at 3h (a) and 7h (b).
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1. Unique echinus-like Fe-doped SnO2 hollow nanospheres were synthesized with simple solvothermal treatment. 2. As doping concentration of Fe3+ ions was 7 at%, echinus-like Fe-doped SnO2 was formed. 3. Echinus-like Fe-doped SnO2 exhibited enhanced photoactivity to photodegrade rhodamine B and photoreduct Cr(VI).