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Controlled synthesis of bismuth sulfide nanorods by hydrothermal method and their photocatalytic activity
Controlled synthesis of bismuth sulfide nanorods by hydrothermal method and their photocatalytic activity Ahmed Helal, Farid A. Harraz, Adel A. Ismail, Tarek M. Sami, I.A. Ibrahim PII: DOI: Reference:
S0264-1275(16)30517-2 doi: 10.1016/j.matdes.2016.04.043 JMADE 1687
To appear in: Received date: Revised date: Accepted date:
7 February 2016 31 March 2016 14 April 2016
Please cite this article as: Ahmed Helal, Farid A. Harraz, Adel A. Ismail, Tarek M. Sami, I.A. Ibrahim, Controlled synthesis of bismuth sulfide nanorods by hydrothermal method and their photocatalytic activity, (2016), doi: 10.1016/j.matdes.2016.04.043
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ACCEPTED MANUSCRIPT Controlled Synthesis of Bismuth Sulfide Nanorods by Hydrothermal Method and their Photocatalytic Activity
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Ahmed Helala, Farid A. Harraza , Adel A. Ismaila*, Tarek M. Samib, I. A. Ibrahima a
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Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development
Institute (CMRDI), P.O. 87 Helwan, Cairo 11421, Egypt. Tel: 0020-25010643, Fax: 0020-25010639.
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E. mail: [email protected] b
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Tabbin Institute for Metallurgical Studies, (TIMS), P.O.109 Helwan Cairo 11421, Egypt
ABSTRACT
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Bi2S3 nanorods with orthorhombic structure were successfully synthesized through hydrothermal method. Systematic experiments were accomplished to study the variable factors such as the Bi/S molar ratio, reaction time and reaction temperature, which have great impact on the structural morphologies of
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Bi2S3 and the photocatalytic performance. TEM and FE-SEM images reveal that the prepared Bi2S3 is
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flower-like built up from many nanorods with average 30-50 nm in diameter and 0.5-1µm length. The
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optimal conditions for the preparation of Bi2S3 nanorods were Bi/S molar ratio 1/2 for 20 h at 180 °C to obtain the highest photocatalytic activity of ~ 98% towards methylene blue (MB) degradation. It is also found that the determined k values for Bi2S3 nanorods prepared at Bi/S molar ratio 1/2 was higher 40 and 5.5 times than that the samples prepared at either low or high Bi/S molar ratios 1/0.5 and 1/5, respectively.
Keywords: Bi2S3 nanorods; Hydrothermal method ; Photocatalyst; MB photodegradation; UV illumination.
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ACCEPTED MANUSCRIPT 1. Introduction Metal sulfides are confirmed to be highly efficient photocatalysts because they have a broad light absorption and high charge, since photogenerated electrons and holes can speedily transfer to catalysts
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surface for oxidizing organic pollutants.[1-4] Metal sulfides have huge potential application such as photovoltaic cells, photodetectors, and photodegradation of organic pollutants under visible light and
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UV illumination.[1-4] Bi2S3 is a favorable semiconductor not only because of its chemical constancy
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and nontoxic components but also because of its large absorption coefficient about 105 cm-1.[5] In addition, Bi2S3 has n-type semiconductor and large electron mobility for different applications.[6] It has
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been largely employed in photovoltaic cells, [7,8] medicine contrast agent, [9,10] biomolecule detector, [11] and photocatalyst.[12-16] The bandgap energy and separation and efficient transport of charge
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carriers of the semiconductors are the main factors for successful utilization of semiconductor nanoparticles in photoelectron transmutation devices. The Bi2S3 band edges are valuable for separation of charge carriers, and generation a defect-free interface.[17-19] In photocatalysis application under
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visible light and UV illumination, when the semiconductors subjected to light with the energy > the
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bandgap energy, electrons in the valence band of the semiconductor materials are excited to the conduction band, leaving holes behind the valence band. The photogenerated charge carriers can
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expeditiously derive photocatalysis by reacting with organic pollutants onto the semiconductors surface.[18-22] The process of photoelectron transmutation to obtain high photonic efficiency is extremely affected by morphology, particle size, phases and crystallinity. In recent years, a lot of efforts to get the ease synthetic routes for Bi2S3 nanostructures such as nanodots, nanorods, nanoflowers, and nanowires have been done.[14-25] It can be prepared by microwave-assisted, hydrothermal method, hot injection technique, ionic liquid-assisted, or solvothermal method.[14-25] There are different factors influencing the as-prepared Bi2S3 nanocrystals such as the precursor concentration, Bi : S molar ratio, reaction temperature and reaction time, which eventually influence the energy-level structure and optical absorption.[24, 26-28] Although, the performance of Bi2S3 nanocrystals has been investigated in different applications[24], however, the effect of synthesis parameters on the morphology and phases of Bi2S3 nanocrystals and photocatalytic performances needs to be addressed. For this reason, in this contribution, one-step synthesis of Bi2S3 nanorods using thiourea as a source of sulfur was employed. The impact of experimental variables, such as the precursor concentration Bi : S molar ratio, reaction temperature and reaction time on the phase structure, morphology and photocatalytic performance of the prepared Bi2S3 photocatalyst have been systematically examined. The experimental findings indicated
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ACCEPTED MANUSCRIPT that the prepared Bi2S3 nanorods exhibited noteworthy photocatalytic activity for the photodegradation of MB at high concentration [50 ppm] at short time. The optimal conditions for the preparation of Bi2S3 nanorods were Bi/S molar ratio 1/2 for 20 h at 180 °C to obtain the highest photocatalytic activity of ~
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98% towards methylene blue (MB) degradation. The photocatalytic activity of the Bi2S3 nanorods prepared at Bi/S molar ratio 1/2 was superior to that of all synthesized samples and it exhibited high
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photocatalytic performance.
II.
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II. Experimental 1. Materials
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Bi(NO3)3.5H2O and thiourea (NH2)2S were used as precursors for Bi and S, respectively. MB has been employed in the photocatalytic evaluation of the prepared photocatalysts. All chemicals in the present
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work were analytical grade reagents and they were purchased from Sigma-Aldrich. II. 2. Synthesis of Bi2S3 nanorods
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Bi2S3 nanorods with orthorhombic structure were prepared through the hydrothermal method. In a
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typical procedure, different weights of thiourea were added to 100 ml distilled water with magnetic stirring, then 4.0 g of Bi(NO3)3 was added to the thiourea solution to obtain Bi2S3 nanorods at different
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Bi:S ratios. After being stirred for 10 min at 60 °C, the solution was transferred to a Teflon-lined autoclave. The autoclave was transferred into an oven and maintained at various temperatures (100 -250 °C) for various reaction times (10-48 h). After cooling the autoclave to room temperature, the collected precipitates were washed with water and ethanol for three times, afterward, dried at 60 °C for 8 h.
II. 3. Characterization of Bi2S3 photocatalysts X-ray diffraction patterns were collected by powder X-ray diffraction (BRUKER D8 advanced Cu target λ 1.54 A° 40kv 40mA. All measurements were recorded in a 2θ range between 10° and 80°. Field emission-secondary electron microscope (FE-SEM) images were taken with a FE scanning electron microanalyzer (JEOL-6300F, 5 kV) attached with energy dispersive spectroscopy (EDS). Transmission electron microscopy (TEM) was performed at 200 kV with a JEOL JEM-2100F-UHR field-emission instrument equipped with a Gatan GIF 2001 energy filter and a 1k-CCD camera in order to obtain EEL spectra. X-ray photoelectron spectra (XPS) were measured using a VG Escalab 200R electron spectrometer equipped with a MgKα (hν = 1253.6 eV, 1 eV =1.6302 x 10-19 J) X-ray source powered at 100 W. The binding energies (BE) were calibrated relative to adventitious carbon (C-C/C-H) using the
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ACCEPTED MANUSCRIPT C1s peak at 284.8 eV. The nitrogen adsorption/desorption isotherms at 77 K were measured using a Quantachrome Autosorb equipment after degassing at 200 °C overnight. The sorption data were analyzed using the Barrett-Joyner-Halenda (BJH) model with Halsey equation[29]. Raman spectra were
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recorded by Raman Microscope spectrometer equipped with a laser beam emitting at 532 nm with a 100 mW output power. Fourier transforms infrared spectrometer (FT-IR; Perkin Elmer) spectrum was
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recorded in KBr dispersion in the range of 400 to 4000 cm-1. The bandgap energy of the prepared
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photocatalysts were recorded at 200-800 nm wavelength using diffuse reflectance spectroscopy with a JASCO V-570 UV-vis spectrophotometer equipped with a Labsphere integrating sphere diffuse
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reflectance accessory[30]. The diffuse reflectance mode (R) were recorded and transformed to the Kubelka-Munk function F(R) to separate the extent of light absorption from scattering light. Furthermore, the bandgap energy was calculated from the plot of the modified Kubelka-Munk function
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(F(R)E)2) versus the energy of the absorbed light E, according to the formula as follows [31]. 2 F(R)E2 = (1 R) h
2R
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2
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II. 4. Photocatalytic evaluation
The photocatalytic activity of Bi2S3 photocatalysts was examined by the photodegradation of MB under
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UV illumination. 0.1g of Bi2S3 photocatalyst was mixed with 200 ml MB [50 mg/L] aqueous solution in a quartz photoreactor with H2O circulation for photoreactor cooling. Prior to a photocatalytic reaction, the suspension was kept in dark with stirring at room temperature for 30 min to reach adsorption equilibrium. The above solution was photo-irradiated by using a 150 W medium pressure Hg lamp immersed into the photoreactor with the wavelength ranged between 280-360 nm. The MB concentration after equilibration was recorded to be the initial concentration (Co). It was withdrawn at regular intervals (Ct) from the upper part of the photoreactor. The photocatalyst was separated from the liquid phase by filtration through nylon syringe filters. The UV spectra of MB solution before and after illumination were measured using a JASCO V-570 UV-vis spectrophotometer. The photocatalytic activity of Bi2S3 photocatalysts for MB photodegradation was calculated as follows: % photocatalytic activity = (1- Ct)/ Co × 100 III. Results and discussion III. 1. Structural investigation of Bi2S3 Crystal structure and morphology of Bi2S3 synthesized by hydrothermal method without using either templates or capping agents at different Bi/S molar ratios, reaction times and temperature have been
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ACCEPTED MANUSCRIPT investigated. It was observed that the prepared samples were Bi2S3 nanorods. Additionally, the presence of other phases, owing to the thiourea contents added into Bi(NO3)3 solution, indicated that the Bi/S molar ratio is playing a vital role in determining the structure and morphology of the prepared Bi2S3.
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XRD patterns of all prepared samples at different conditions (Bi/S molar ratios, reaction times and temperature) were examined to determine the structure of Bi2S3 (Fig.1). Fig. 1a exhibits the XRD
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patterns of samples that have been synthesized at different Bi/S molar ratios; 1/0.5, 1/1, 1/1.5, 1/2 and
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1/5. A low sulfur content of Bi/S molar ratios (1/0.5), (1/1), (1/1.5), the cannonite phase impurity with the main Bi2S3 phase is detected. The diffraction peaks are indexed as a cannonite Bi2O(OH)2(SO4)
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monoclinic phase, [32] which is corresponding to the standard diffraction pattern (JCPDS card no. 761102). The impurity cannonite phase disappears with increasing the sulfur contents at Bi/S molar ratios
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1/2 and 1/5. The different peaks can be indexed as the orthorhombic Bi2S3 structure (JCPDS Card No. 00-017-0320) with the major peaks attributed to (020), (130), (211), and (221) planes with unit cell constants of a = 11.15 Å, b =11.30 Å and c = 3.97 Å. Fig. 1b shows the effect of reaction time; 10 h, 20
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h, 36 h, 48 h on the crystalline phase at constant Bi/S molar ratio of (1/2). The patterns of the
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synthesized product indicate that Bi2S3 phase existed at all reaction times. All of the diffraction peaks can be indexed to an orthorhombic phase of Bi2S3. No impurities are detected in this pattern, which
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indicates that pure Bi2S3 can be obtained under the current synthesis conditions. However, with the increase of reaction time from 10 to 36 h, the crystallinity was increased but from the economic point of view and saving energy, the optimum reaction time is taken as 20 h. The XRD patterns of Bi2S3 samples obtained at different temperatures; 100 °C, 150 °C, 180 °C and 250 °C are shown in Fig.1c. The XRD pattern of the prepared sample at 100 °C has some impurity phases besides the Bi2S3 main phase. Also, the XRD pattern prepared at 100 °C shows broad and diffused peaks centered at around 2θ = 24.8 indicating the small size and poor degree of crystallinity (Fig.1c and Table 1).[5,7,24] The impurity phase disappeared with the increase of reaction temperature above 100 °C. All the diffraction peaks correspond well with the standard diffraction data and display a greater degree of crystallinity and an increase of crystal size as indicated by sharper diffraction peaks (Table 1). The suggested procedure is that the formed nanodots is unstable so it assembled in a line, and with extended heating time, the recrystallization taken place, resulting in nanorods formation. However, under a low temperature of 180 °C, the energy is not enough for complete crystallization and thus results in amorphous fringes in nanorods.[33] In general, all peaks can be listed to orthorhombic Bi2S3 and no impurity is detected in this pattern, indicating that the products are high purity. The XRD pattern is refined using Rietveld
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ACCEPTED MANUSCRIPT method (using Fullprof software) [34] to calculate the accurate unit cell dimensions, applying the space group number 62 (Pnma). The results of this refinement give the following lattice parameters, atomic positions and thermal parameters that are shown in Table S1. The goodness of fitting is the acceptable
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range. It is quite clear that our results are in good accordance with those reported in literature [35,36]. The 3D unit cell of Bi2S3 drawn using Diamond software [37] gives the orthorhombic crystal symmetry,
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where the Bi atoms are white spheres and S atoms are blue spheres as shown in Fig S1.
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The crystallinity of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h was also examined by Raman spectroscopy (Figure 2a). The Raman spectrum exhibits four vibrational peaks
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associated with the Bi-S bonds at 236, 413, 887 and 1348 cm-1, in agreement with the literature [38-40]. The broad peak was assigned at 236 cm-1 owing to the vibration modes of the Bi-S bond, which is
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agreement with the reported Bi2S3 values[41,42]. It is well known that the different morphologies can shift the wavenumbers due to the difference in the size (quantum size effect) and surface phonon modes [43]. This finding further confirms that the prepared Bi2S3 is the orthorhombic structure and it is
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harmonious with the obtained XRD results. Fig. 2b displays the FT-IR of the prepared Bi2S3 at Bi/S
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molar ratio (1/2) at 180 °C for 20 h. A broad band at 3200-3400 cm-1 is attributed to the stretching vibration of the O-H bond, while the band centered at 1635 cm-1 corresponds to the bending vibrations
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of H2O. It can also be found that the absorption peak at 2336 cm-1 appeared, which indicated a bond formation between Bi and S [44].
The morphology of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h was investigated by the SEM and TEM (Fig. 3). Fig. 3a reveals the FESEM image of Bi2S3 sample and it is a flower-like built up from many nanorods with an average 30-50 nm in diameter and 0.5-1µm long. [45] The Bi2S3 nanorods were further investigated by the TEM (Fig. 2b,c); TEM image (Fig. 2b) revealed that the size of the nanorods is ~ 50 nm in diameter and it contains many nanorods, evidencing the above SEM investigations and also suggesting that the flower-like pattern obtained is obviously different from the urchin like morphologies.[46- 48] The index of the spots in the SAED pattern reveals that the Bi2S3 nanorod is a single crystal [49, 50]. Figure 3c displays the TEM image of one Bi2S3 nanorod situated in the flower-like structure. It demonstrates that the Bi2S3 nanorods grow nearly exclusively in the [001] direction with nanorods that are on ~50 nm in diameter and ~0.5 µm long. Energy dispersive X-ray spectra (EDS) were examined to measure the stoichiometric composition of the Bi2S3 nanorods, suggesting that the optimum obtained Bi2S3 nanorods consisted of pure Bi2S3. The Bi/S molar ratio of
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ACCEPTED MANUSCRIPT Bi2S3 obtained from the peak areas of the EDS graph is 39.76: 60.24 and thus consistent to the prospective 2:3 ratio of the prepared Bi2S3. Nitrogen adsorption/desorption isotherms of Bi2S3 nanorods prepared at 180 °C for 20 h and Bi/S molar
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ratios of 1/2 and 1/5 are shown in Figure 4. At relative pressures p/p0 between 0.45 and 0.9, the sharpness of the inflection resulting from capillary condensation is attributed to a characteristic for
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mesoporous structure of the prepared Bi2S3 nanorods [51]. A typical reversible type IV adsorption
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isotherms are manifested in particular for Bi/S molar ratio 1/2 sample, however the 1/5 molar ratio Bi/S sample has a mesoporous structure with disordered state. The mesoporous Bi2S3 nanorods possess
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surface areas of 28.15 m2g-1 and large pore volumes of 0.036 cm3g-1 of Bi/S molar ratio 1/2 sample, which reduced to 9.2 m2g-1 and 0.016 cm3g-1 of Bi2S3 nanorods at Bi/S molar ratio 1/5 (see Fig. 4,
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inset). The pore size distribution of sample prepared at Bi/S molar ratio 1/2 is homogeneous with average 7.1 nm, however the pore size of the prepared at Bi/S molar ratio 1/5 is inhomogeneous and has two pore size values at 3.5 nm and 10.6 nm (see Fig. 4, inset). In general, the hysteresis loop can be
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explained by resulting from the voids between non-ordered particles and all pores can be regarded as
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irregular voids between Bi2S3 nanorods, in particular Bi2S3 sample at Bi/S molar ratio 1/5.
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In order to evidence the oxidation state of Bi and S elements, XPS measurement was conducted for the Bi2S3 nanorods (Fig. 5a). The results exhibited two peaks assign at 158.5 and 163.6 eV, corresponding to the Bi 4f7/2 and Bi 4f 5/2 peaks of Bi3+ oxidation state (Fig. 5a). The peak centered at 161.2 is appeared to S 2p with S2- oxidation states.[24] Therefore, the valence of Bi atoms is +3, whereas for S atoms is -2, which is consistent with the charge neutrality calculated for the title compound. The XPS spectra also prove the stoichiometric composition of Bi2S3 that the atomic ratio of Bi/S is ~ 2:3 according to the calculation of the peak areas of Bi 4f and S 2s.
Ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectroscopy was recorded to measure the optical properties of the optimized prepared Bi2S3 nanorods as shown in Fig. 5b. The onset absorption wavelength of the sample is 100 nm, which is located in the infrared region, indicating a good absorption of visible light.[33] The band gap (Eg) of the optimized sample Bi2S3 is further explored by the extrapolation method based on Kubelka-Munk equation.[31] Fig. 5b (inset) plots the (αhν)2 against the photon energy curve, where α is the absorption coefficient and hν (h is Planck’s constant, ν is the frequency) represents the photon energy. The value of Eg was amounted to be 1.45 eV, which indicates
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ACCEPTED MANUSCRIPT that the obtained Bi2S3 photocatalyst is a relatively narrow band-gap semiconductor. Remarkably, the band gap is quite similar to the band gap of bulk Bi2S3 (1.3 eV).[52] In addition, the electron transition
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in bulk Bi2S3 occurs between the Bi-6s-S-3p and the Bi-6p states. The proposed mechanism for preparing Bi2S3 nanorods has been reported [15, 53]. In the present work,
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we suggest a crystal growth of Bi2S3 nanorods according the findings of SEM images (Fig. 6 and
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supporting information Fig. S2 and S3). Upon hydrothermal heating, the isomerization of thiourea may initially take place below 150 °C [Eq. (1)].[15] Then, the generated isothiourea has a strong nucleophilic
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substitution of the Bi3+ cation in Bi(NO3)3 solution, the formed S2- reacts with Bi3+ to obtain Bi2S3 nanorods. [Eq. (2)]. [15] It is well known that extend reaction time and high temperature are suitable for
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Bi2S3 nanorods formation as shown in Fig. 6 and supporting information S3. The formation of Bi2S3 nanorods boosted with the increase of reaction time to 48h. From Fig. 6, it is clearly seen that the precursors decompose into Bi2S3 crystallites when the temperature boosted to 180 °C. As the
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temperature boosted above 180°C, these crystallites of Bi2S3 nanorods self-assembled owing to reduce
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their surface energies [54, 55]. At 100 °C, the Bi2S3 crystallites are not completely formed because the thiourea is not decomposed resulting in a very low yield. At low concentration of thiourea, the
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formation of S2- anions is slow so a small quantity of Bi2S3 nuclei in the solution is produced (Fig. S2). At high thiourea content as in Bi/S molar ratio of 1/5 sample, the excess of adsorbed thiourea is not only attractive but also repulsive, which functioned together and are beneficial for the system to obtain the wanted balance among the nucleation, growth, and assembly.[53] S
S
NH2
C
Tautomerism
NH2
NH2
C
S
2 NH2
C
NH2
Isothiourea
+
Bi(NO3)3
(1)
NH2
S
Bi
C
NH + 2HNO3
Nucleophilic Substituation
2NH2
(2)
Bi2S3 nanorods growth
III. 2. Photocatalytic evaluation III. 2. 1. Impact of crystal structure and morphology of Bi2S3 on photocatalytic activity
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ACCEPTED MANUSCRIPT The photocatalytic performance of the synthesized Bi2S3 nanorods was evaluated for MB photodegradation. The degradation of the MB under UV illumination was followed by recording the absorption spectra using a UV/VIS spectrophotometer at varied time intervals (Fig. 7). Prior to the
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photocatalytic test runs, the photolysis of MB and the adsorption in dark was examined. The results indicate that the photolysis of MB was insignificant without Bi2S3 photocatalyst and its adsorption
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capacity was only 3%. MB can be degraded by either oxidative degradation of the MB molecule or by
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a two-electron reduction[56-58]. The photocatalytic evaluation of the optimized Bi2S3 photocatalyst is shown in Fig. 7. The absorption bands of MB centered at λ = 663 nm and λ = 291 nm steadily decrease
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upon boosting illumination time. Such an experimental finding indicates that the MB decoloration is notably completed under the UV-light illumination in the presence of Bi2S3 photocatalyst. The
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decoloration of MB dye is attributed to the oxidative degradation. It is clearly seen that the absorbance at 663 nm decreased from 8.41 to 0.16 after nearly 60 min illumination. The photocatalytic activity of Bi2S3 nanorods was 98%, in spite of the high concentration of MB [50 ppm].
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Impact of crystal structure and morphology of the synthesized Bi2S3 at different Bi/S molar ratios,
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reaction times and temperatures on the photocatalytic activity of MB photodegradation was studied by details ( Fig.8). The photodegradation of MB over Bi2S3 nanorods at different Bi/S molar ratios 1/0.5,
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1/1, 1/1.5, 1/2 and 1/5 was evaluated (Fig. 8a). The results revealed that the photocatalytic activities of Bi2S3 at Bi/S molar ratios 1/0.5, 1/1 and 1/1.5 for MB photodegradation is insignificant and the photocatalytic activity is reached to 40% at Bi/S molar ratio 1/1.5 (Fig. 8a). This is explained by the photoactive Bi2S3 phase is not well formed and also there is other phase (as shown in XRD results) which might be served as a recombination center for the charge carriers. With boosting the Bi/S molar ratio up to 1/2, the photocatalytic activity was strongly improved and reached to 98% and then the photocatalytic activity decreased to 51% with the increase of Bi/S molar ratio to 1/5. This may be attributed to that the Bi2S3 phase at a Bi/S molar ratio of 1/2 is clearly formed with high crystallinity, however the Bi2S3 phase formed at a Bi/S molar ratio 1/5 showed a weak crystallization as clearly seen in Fig.1a and Table 1[59]. The optimum Bi/S ratio is accordingly taken at 1/2, giving the highest photocatalytic activity of 98%. The effect of different surface areas may be considered as a second reason. The surface area of Bi2S3 phase (28.15 m2g-1 ) obtained at Bi/S molar ratio 1/2 is higher three times than Bi2S3 phase (9.2 m2g-1) prepared at Bi/S molar ratio 1/5. Furthermore, such outstanding photocatalytic activity of Bi2S3 at Bi/S molar ratio 1/2 as compared with Bi2S3 at Bi/S molar ratio 1/5 can be explained by several effects, such as depress light scattering and •OH accumulated onto the
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ACCEPTED MANUSCRIPT mesoporous structure, [59-62] or a speedy transfer of the MB molecule to the active sites of Bi2S3 nanorods owing to the ease diffusion of the MB through the porous structure. With the boost of S content at a Bi/S molar ratio 1/5, the diameter and length of Bi2S3 nanorods was reduced, even
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becoming non-uniform shaped nanocrystals.[24] Fig. 8b exhibits the effect of Bi2S3 photocatalysts prepared at different reaction times 10 h, 20 h, 36 h, 48 h on the MB photodegradation. Also, the
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relationship of reaction time-crystallinity and temperature-crystallinity of the prepared Bi2S3 nanorods
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on their photocatalytic activity are listed in Table 1. The findings revealed that the photocatalytic activity of Bi2S3 gradually boosted with the increase reaction time from 10 to 20 h to reach 98%
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photocatalytic activity. When the reaction time was prolonged to 36 h and 48h, the photocatalytic activity of Bi2S3 decreased to 69% and 63% , respectively. Longer reaction time is not preferred and has
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an opposite effect on the photocatalytic activity [63]. This is explained by the Bi2S3 particle sizes are playing an important role for the photocatalytic activity. This is attributed to the fact that the crystalitte size of Bi2S3 decreased from 68 nm to 50 nm with the increase of reaction time from 10 to 20 h and then
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it boosted again to reach 68 nm with the increase of reaction time to 48 h. Fig. 8c and Table 1 show the
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effect of Bi2S3 obtained at different temperatures 100-250 °C on the MB photodegradation. The photocatalytic activity of the prepared Bi2S3 at 100 °C was 32% due to the Bi2S3 phase has a poor
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degree of crystallinity and cannonite phase impurity as clearly shown in XRD data. [5,7,24] The photogenerated electrons will be easily trapped at the surface because of crystal defects providing nonradiative pathways for recombination.[24] With increasing reaction temperature to reach 180 °C, the photocatalytic activity increased to 98% and then decreased again to 90% with increasing reaction temperature to 250 °C. The reason for such a phenomenon is the crystallite sizes of the prepared photocatalysts. The crystallite size of the sample prepared at 180 °C is 50 nm, which increased to 103 nm at 250 °C reaction temperature. Wang et al. reported that at the reaction temperature of 250 °C, the Bi2S3 nanorods are still remained, however the shape and size of Bi2S3 nanorods are completely changed and highly dispersed[26]. It is well known that too large Bi2S3 particles could also serve as recombination centers, detrimental the photocatalytic activity by prohibiting the active photogenerated charge transfer to the reactant species at the Bi2S3 surface. The second reason, the photocatalytic activity of the newly prepared photocatalyst at 180 ºC is slightly increased compared to the prepared sample at 250 ºC owing to the sample prepared at 180 ºC has large surface area 28 m2g-1, which reduced to 4 m2g-1 of the Bi2S3 nanorods prepared at 250 ºC. III. 2. 2. Performance of the optimized Bi2S3 nanorods on MB photodegradation
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ACCEPTED MANUSCRIPT Generally, the performance of the optimized Bi2S3 nanorods on MB photodegradation was crucially affected by the pH values, the amount of photocatalyst loading and the illumination time. Herein, we performed a series of experiments to systematically determine the pH value, photocatalyst loading and
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illumination time (Fig. 9). MB photodegradation at different pH values was conducted and the findings are shown in Fig. 9a. The results revealed that the photocatalytic activity was boosted with the increase
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of the pH value from 3 to 7 and there was no change in the photocatalytic activity at higher pH value
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above 7. It is clearly seen that MB dye adsorption capacity is influenced by the pH variation. In acidic media, Bi2S3 surface has positively charged and negatively charged in alkaline media, with a point of
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zero charge at pH 6.5-7.[64]. This can be ascribed by considering the electrostatic attraction that occurs between the negatively charged surface of Bi2S3 and MB, a cationic dye. At acidic pH, the adsorption of
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MB was reduced as a result of the presence of excess H+ ions contending with the MB dye cation's for adsorption sites. At high pH value, the positively charged sites were reduced and the negatively charged sites were boosted which favored the MB cationic dye removal[59, 65]. The optimum pH value is ~7 for
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the MB photodegradation which the photocatalytic efficiency is being maximum ~ 98%. The optimal
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amount of photocatalysts was determined by carrying out systematic experiments at pH 7 and varying of Bi2S3 photocatalyst loading 0.1 - 0.9 g/l (Fig. 9b). As revealed from Fig. 9b, the optimum
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photocatalyst loading for MB photodegradation is 0.5 g/l. It is clearly seen that the photocatalytic activity increases with the increment of Bi2S3 photocatalyst loading from 0.1 to 0.5 g/l due to an increase in the active sites of the Bi2S3 nanorods, which leads to increase the MB molecules adsorbed and the number of photons absorbed. The photocatalytic activity decreased behind the optimum amount 0.5 g/l. This phenomenon is likely related to the accumulation of Bi2S3 nanorods at a high photocatalyst loading causing a decrease in surface active sites and increase in light scattering and opacity of Bi2S3 nanorods, which led to a reduce in the passage of irradiation light through the Bi2S3 photocatalyst [66]. The effect of illumination time in the range 5-90 min was carried out at constant MB concentration and pH = 7.0 (Fig. 9c). The photocatalytic activity was found to increase with the increment of illumination time from 5 to 60 min. The results indicated that the photocatalytic activity reached a steady state at 60 min. Beyond 60 min illumination time, there was no significant observation for MB photodegration. In general, the MB was completely degraded within 60 min. Such data confirms the outstanding photocatalytic activity of the prepared Bi2S3 nanorods for accomplishing MB degradation at high concentration [50 ppm] and short illumination time.
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ACCEPTED MANUSCRIPT The kinetics of MB degradation using Bi2S3 nanorods can be expressed by the first-order equation [13, 67]. This rate constant is employed to calculate the MB photodegradation rates by the formula as follows: ln (Co/C) = kt, where Co and C are the MB concentration at time 0 and t min, respectively, k is
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the first-order reaction rate constant. Fig. 10a presented the photodegradation kinetics of MB vs. the illumination time with employing Bi2S3 nanorods obtained at different Bi/S molar ratios 1/0.5, 1/1,
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1/1.5, 1/2 and 1/5. The k values estimated from the slopes are summarized in the inset of Fig. 10a. The
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results indicate a rather perfect correlation to the pseudo-first order reaction kinetic for all prepared photocatalysts. It has been observed that the MB photodegradation using Bi2S3 nanorods prepared at
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Bi/S molar ratio 1/2, proceeds much more rapidly than all obtained photocatalysts. It is also found that the determined k value for Bi2S3 nanorods prepared at the optimum Bi/S molar ratio 1/2 was higher 40
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and 5.5 times than that prepared at low and high molar ratios 1/0.5 and 1/5, respectively. It is clearly seen that the photocatalytic activity of the Bi2S3 nanorods prepared at Bi/S molar ratio 1/2 was superior to that of all synthesized samples and it exhibited the highest photocatalytic performance. The suggested
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mechanism to explain the superior photocatalytic activity of Bi2S3 nanorods is shown in scheme 1. When
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photons with higher energies than 1.45 eV are attacked the surface of Bi2S3 nanorods; the electrons in the valence band are excited to the conduction band leaving hole behind (equations 1&2 and Scheme 1).
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The thus generated charge carriers (photoelectrons and holes) take part in the Bi2S3 nanorods/H2O interface redox reactions. The photoholes transfer from the bulk to the surface of the Bi2S3 nanorods where they react with adsorbed H2O or
-
OH, creating adsorbed •OH radicals. Simultaneously, the
photogenerated electrons reduced the adsorbed O2 to form O2•¯ radicals (equation 3) [68-71]. The produced O2•¯ radicals onto the surface of Bi2S3 nanorods might be reacted with 2H+ to give H2O2 and hence •OH and ¯OH were generated (see equations 4&5). The adsorbed MB molecules can be easily degraded to CO2 and O2, when subjected to oxidizing agent species O2•¯ and •OH radicals. (equations 6&7)[72]. Bi2S3 (ecb- + hvb+)
Bi2S3 + light hvb+ + ¯OHs
•
OHs
-
Bi2S3 (e ) + O2 •¯
+
O2 + 2H H2O2 + e-
MB + •OHs
(2)
Bi2S3 + O2
•¯
(3)
H2O2
eq. •
OH
(4)
+ ¯OH
(5)
MB• + H2O
MB + Bi2S3)h) + •OHs
(1)
(6)
degradation products
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To find out the advantage of Bi2S3 nanorods as efficient photocatalysts and their viability, re-use of the Bi2S3 nanorods was evaluated for the MB photodegradation for five cycles (Fig. 10b). The recycling
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experiments demonstarted that the optimized Bi2S3 photocatalyst was fully balanced during that solidliquid heterogeneous photocatalysis since no significant decrease in photocatalytic activity in the five
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cycles was noticed. However, the photocatalytic efficiency was slightly decreased from 98% to 95%
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after 5 cycles. The results exhibit a good potential applications owing to the good recyclability of the Bi2S3 nanorods based photocatalysts. A more detailed conception of the photocatalytic activity of Bi2S3
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nanorods under visible light and their doping with Fe2O3 and Cr2O3 is being under way and will be reported elsewhere.
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IV. Conclusions
In summary, by controlling the synthetic parameters of the hydrothermal technique, Bi2S3 flower-like structures consisting of nanorods are obtained. The photocatalytic performance of Bi2S3 nanorods was
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optimized by controlling the affecting preparation parameters such as Bi/S molar ratio, reaction time and
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reaction temperature. The highest catalytic activity towards MB photodegradation was achieved at reaction temperature of 180 °C for 20 h at Bi/S molar ratio 1/2. The photocatalytic properties
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demonstrated that the as-prepared Bi2S3 nanorods exhibited a remarkable photocatalytic activity ~ 98% at pH ~ 7 for 60 min illumination time in the presence of 0.5 g/l photocatalyst loading.
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References
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H2O
OH
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CO +
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Scheme 1. Schematic of the suggested mechanism to explain the outstanding photocatalytic activity of Bi2S3 nanorods for photodegradation of MB.
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H2O/ OH O 2 H +
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CO 2
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Figure captions Fig. 1. (a) XRD patterns obtained from Bi2S3 nanorods prepared at different Bi/S molar ratios (1/0.5), (1/1), (1/1.5), (1/2) and (1/5) for 20 h. reaction time at 180 ºC reaction temperature, (b) Effect of reaction time 10h, 20h, 36h and 48h on Bi2S3 prepared at Bi/S molar ratio (1/2) and 180 ºC reaction temperature; (c) Effect of reaction temperature of Bi2S3 nanorods prepared at 100 ºC, 150 ºC, 180 ºC and 250 ºC on its structure phase at Bi/S molar ratio (1/2) for 20 h reaction time.
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Fig. 2. (a) Raman spectroscopy of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h; (b) FT-IR of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h.
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Fig. 3. (a) SEM image of Bi2S3 nanorods, (b) TEM image of a typical Bi2S3nanorod; Inset: the index of the spots in the SAED pattern, (c) TEM image of a typical selected one Bi2S3 nanorod and (d) Energy dispersive X-ray spectra of the Bi2S3 nanorods. Fig. 4. Nitrogen adsorption/desorption isotherms and (inset) BJH pore size distribution plot for Bi2S3 nanorods at molar ratios of Bi/S (1/2) and (1/5).
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Fig. 5. (a) XPS spectra of Bi2S3 nanorods of Bi4f and S2p regions. (b) Ultraviolet−visible−near-infrared (UV-vis-NIR) absorption spectrum of Bi2S3 nanorods. (Inset) Plot of (αhν)2 vs. hν for the absorption spectrum.
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Fig. 6. Typical SEM images of Bi2S3 nanorods prepared at different reaction temperatures 100 ºC (a) , 150 ºC (b), 180 ºC (c) and 250 ºC (d). Fig. 7. Absorbance vs. wavelength as a function of illumination time for the photodegradation of MB on Bi2S3 nanorods hydrothermally prepared at optimum conditions: (Bi/S molar ratio 1/2 at 180 °C for 20 h) and the photocatalytic conditions of: (MB concentration [50 ppm], volume of MB = 200 ml, photocatalyst loading = 0.5 g/l). Fig. 8. Change in concentration vs. illumination time in the presence of Bi2S3 nanorods photocatalysts for MB photodegradation (a) at different Bi/S molar ratios, (b) reaction times and (c) reaction temperatures. Fig. 9. Change in concentration vs. illumination time in the presence of Bi2S3 nanorods photocatalysts for MB photodegradation at different pH values (a), the amount of photocatalyst loadings (b) and illumination times (c). Fig. 10. (a) The variation of ln (Co/C) vs. illumination time for MB photodegradation using Bi2S3 nanorods photocatalysts prepared at Bi/S molar ratio 1/2 and 180 °C for 20 h). (b) Repeated cycles up to 5 times of MB photodegradation over the optimized Bi2S3 nanorods.
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Figure captions Fig. 1. (a) XRD patterns obtained from Bi2S3 nanorods prepared at different Bi/S molar ratios (1/0.5), (1/1), (1/1.5), (1/2) and (1/5) for 20 h. reaction time at 180 ºC reaction temperature, (b) Effect of reaction time 10h, 20h, 36h and 48h on Bi2S3 prepared at Bi/S molar ratio (1/2) and 180 ºC reaction temperature; (c) Effect of reaction temperature of Bi2S3 nanorods prepared at 100 ºC, 150 ºC, 180 ºC and 250 ºC on its structure phase at Bi/S molar ratio (1/2) for 20 h reaction time.
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Fig. 2. (a) Raman spectroscopy of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h; (b) FT-IR of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h.
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Fig. 3. (a) SEM image of Bi2S3 nanorods, (b) TEM image of a typical Bi2S3nanorod; Inset: the index of the spots in the SAED pattern, (c) TEM image of a typical selected one Bi2S3 nanorod and (d) Energy dispersive X-ray spectra of the Bi2S3 nanorods. Fig. 4. Nitrogen adsorption/desorption isotherms and (inset) BJH pore size distribution plot for Bi2S3 nanorods at molar ratios of Bi/S (1/2) and (1/5).
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Fig. 5. (a) XPS spectra of Bi2S3 nanorods of Bi4f and S2p regions. (b) Ultraviolet−visible−near-infrared (UV-vis-NIR) absorption spectrum of Bi2S3 nanorods. (Inset) Plot of (αhν)2 vs. hν for the absorption spectrum.
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Fig. 6. Typical SEM images of Bi2S3 nanorods prepared at different reaction temperatures 100 ºC (a) , 150 ºC (b), 180 ºC (c) and 250 ºC (d). Fig. 7. Absorbance vs. wavelength as a function of illumination time for the photodegradation of MB on Bi2S3 nanorods hydrothermally prepared at optimum conditions: (Bi/S molar ratio 1/2 at 180 °C for 20 h) and the photocatalytic conditions of: (MB concentration [50 ppm], volume of MB = 200 ml, photocatalyst loading = 0.5 g/l). Fig. 8. Change in concentration vs. illumination time in the presence of Bi2S3 nanorods photocatalysts for MB photodegradation (a) at different Bi/S molar ratios, (b) reaction times and (c) reaction temperatures. Fig. 9. Change in concentration vs. illumination time in the presence of Bi2S3 nanorods photocatalysts for MB photodegradation at different pH values (a), the amount of photocatalyst loadings (b) and illumination times (c). Fig. 10. (a) The variation of ln (Co/C) vs. illumination time for MB photodegradation using Bi2S3 nanorods photocatalysts prepared at Bi/S molar ratio 1/2 and 180 °C for 20 h). (b) Repeated cycles up to 5 times of MB photodegradation over the optimized Bi2S3 nanorods.
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CO +
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Research highlight Bi2S3 nanorods with orthorhombic structure were successfully synthesized.
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Bi2S3 is nanorods with average 30-50 nm in diameter and 0.5-1µm length.
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The conditions for synthesis Bi2S3 were Bi/S molar ratio 1/2 for 20 h at 180 °C.
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Bi2S3 nanorods exhibit photoactivity ~ 98% at pH ~ 7 for 60 min illumination
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time.
k values for Bi2S3 nanorods was higher 40 and 5.5 time than other prepared
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S0264-1275(16)30517-2 doi: 10.1016/j.matdes.2016.04.043 JMADE 1687
To appear in: Received date: Revised date: Accepted date:
7 February 2016 31 March 2016 14 April 2016
Please cite this article as: Ahmed Helal, Farid A. Harraz, Adel A. Ismail, Tarek M. Sami, I.A. Ibrahim, Controlled synthesis of bismuth sulfide nanorods by hydrothermal method and their photocatalytic activity, (2016), doi: 10.1016/j.matdes.2016.04.043
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.
ACCEPTED MANUSCRIPT Controlled Synthesis of Bismuth Sulfide Nanorods by Hydrothermal Method and their Photocatalytic Activity
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Ahmed Helala, Farid A. Harraza , Adel A. Ismaila*, Tarek M. Samib, I. A. Ibrahima a
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Nanostructured Materials and Nanotechnology Division, Central Metallurgical Research and Development
Institute (CMRDI), P.O. 87 Helwan, Cairo 11421, Egypt. Tel: 0020-25010643, Fax: 0020-25010639.
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E. mail: [email protected] b
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Tabbin Institute for Metallurgical Studies, (TIMS), P.O.109 Helwan Cairo 11421, Egypt
ABSTRACT
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Bi2S3 nanorods with orthorhombic structure were successfully synthesized through hydrothermal method. Systematic experiments were accomplished to study the variable factors such as the Bi/S molar ratio, reaction time and reaction temperature, which have great impact on the structural morphologies of
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Bi2S3 and the photocatalytic performance. TEM and FE-SEM images reveal that the prepared Bi2S3 is
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flower-like built up from many nanorods with average 30-50 nm in diameter and 0.5-1µm length. The
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optimal conditions for the preparation of Bi2S3 nanorods were Bi/S molar ratio 1/2 for 20 h at 180 °C to obtain the highest photocatalytic activity of ~ 98% towards methylene blue (MB) degradation. It is also found that the determined k values for Bi2S3 nanorods prepared at Bi/S molar ratio 1/2 was higher 40 and 5.5 times than that the samples prepared at either low or high Bi/S molar ratios 1/0.5 and 1/5, respectively.
Keywords: Bi2S3 nanorods; Hydrothermal method ; Photocatalyst; MB photodegradation; UV illumination.
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ACCEPTED MANUSCRIPT 1. Introduction Metal sulfides are confirmed to be highly efficient photocatalysts because they have a broad light absorption and high charge, since photogenerated electrons and holes can speedily transfer to catalysts
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surface for oxidizing organic pollutants.[1-4] Metal sulfides have huge potential application such as photovoltaic cells, photodetectors, and photodegradation of organic pollutants under visible light and
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UV illumination.[1-4] Bi2S3 is a favorable semiconductor not only because of its chemical constancy
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and nontoxic components but also because of its large absorption coefficient about 105 cm-1.[5] In addition, Bi2S3 has n-type semiconductor and large electron mobility for different applications.[6] It has
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been largely employed in photovoltaic cells, [7,8] medicine contrast agent, [9,10] biomolecule detector, [11] and photocatalyst.[12-16] The bandgap energy and separation and efficient transport of charge
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carriers of the semiconductors are the main factors for successful utilization of semiconductor nanoparticles in photoelectron transmutation devices. The Bi2S3 band edges are valuable for separation of charge carriers, and generation a defect-free interface.[17-19] In photocatalysis application under
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visible light and UV illumination, when the semiconductors subjected to light with the energy > the
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bandgap energy, electrons in the valence band of the semiconductor materials are excited to the conduction band, leaving holes behind the valence band. The photogenerated charge carriers can
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expeditiously derive photocatalysis by reacting with organic pollutants onto the semiconductors surface.[18-22] The process of photoelectron transmutation to obtain high photonic efficiency is extremely affected by morphology, particle size, phases and crystallinity. In recent years, a lot of efforts to get the ease synthetic routes for Bi2S3 nanostructures such as nanodots, nanorods, nanoflowers, and nanowires have been done.[14-25] It can be prepared by microwave-assisted, hydrothermal method, hot injection technique, ionic liquid-assisted, or solvothermal method.[14-25] There are different factors influencing the as-prepared Bi2S3 nanocrystals such as the precursor concentration, Bi : S molar ratio, reaction temperature and reaction time, which eventually influence the energy-level structure and optical absorption.[24, 26-28] Although, the performance of Bi2S3 nanocrystals has been investigated in different applications[24], however, the effect of synthesis parameters on the morphology and phases of Bi2S3 nanocrystals and photocatalytic performances needs to be addressed. For this reason, in this contribution, one-step synthesis of Bi2S3 nanorods using thiourea as a source of sulfur was employed. The impact of experimental variables, such as the precursor concentration Bi : S molar ratio, reaction temperature and reaction time on the phase structure, morphology and photocatalytic performance of the prepared Bi2S3 photocatalyst have been systematically examined. The experimental findings indicated
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ACCEPTED MANUSCRIPT that the prepared Bi2S3 nanorods exhibited noteworthy photocatalytic activity for the photodegradation of MB at high concentration [50 ppm] at short time. The optimal conditions for the preparation of Bi2S3 nanorods were Bi/S molar ratio 1/2 for 20 h at 180 °C to obtain the highest photocatalytic activity of ~
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98% towards methylene blue (MB) degradation. The photocatalytic activity of the Bi2S3 nanorods prepared at Bi/S molar ratio 1/2 was superior to that of all synthesized samples and it exhibited high
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photocatalytic performance.
II.
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II. Experimental 1. Materials
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Bi(NO3)3.5H2O and thiourea (NH2)2S were used as precursors for Bi and S, respectively. MB has been employed in the photocatalytic evaluation of the prepared photocatalysts. All chemicals in the present
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work were analytical grade reagents and they were purchased from Sigma-Aldrich. II. 2. Synthesis of Bi2S3 nanorods
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Bi2S3 nanorods with orthorhombic structure were prepared through the hydrothermal method. In a
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typical procedure, different weights of thiourea were added to 100 ml distilled water with magnetic stirring, then 4.0 g of Bi(NO3)3 was added to the thiourea solution to obtain Bi2S3 nanorods at different
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Bi:S ratios. After being stirred for 10 min at 60 °C, the solution was transferred to a Teflon-lined autoclave. The autoclave was transferred into an oven and maintained at various temperatures (100 -250 °C) for various reaction times (10-48 h). After cooling the autoclave to room temperature, the collected precipitates were washed with water and ethanol for three times, afterward, dried at 60 °C for 8 h.
II. 3. Characterization of Bi2S3 photocatalysts X-ray diffraction patterns were collected by powder X-ray diffraction (BRUKER D8 advanced Cu target λ 1.54 A° 40kv 40mA. All measurements were recorded in a 2θ range between 10° and 80°. Field emission-secondary electron microscope (FE-SEM) images were taken with a FE scanning electron microanalyzer (JEOL-6300F, 5 kV) attached with energy dispersive spectroscopy (EDS). Transmission electron microscopy (TEM) was performed at 200 kV with a JEOL JEM-2100F-UHR field-emission instrument equipped with a Gatan GIF 2001 energy filter and a 1k-CCD camera in order to obtain EEL spectra. X-ray photoelectron spectra (XPS) were measured using a VG Escalab 200R electron spectrometer equipped with a MgKα (hν = 1253.6 eV, 1 eV =1.6302 x 10-19 J) X-ray source powered at 100 W. The binding energies (BE) were calibrated relative to adventitious carbon (C-C/C-H) using the
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ACCEPTED MANUSCRIPT C1s peak at 284.8 eV. The nitrogen adsorption/desorption isotherms at 77 K were measured using a Quantachrome Autosorb equipment after degassing at 200 °C overnight. The sorption data were analyzed using the Barrett-Joyner-Halenda (BJH) model with Halsey equation[29]. Raman spectra were
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recorded by Raman Microscope spectrometer equipped with a laser beam emitting at 532 nm with a 100 mW output power. Fourier transforms infrared spectrometer (FT-IR; Perkin Elmer) spectrum was
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recorded in KBr dispersion in the range of 400 to 4000 cm-1. The bandgap energy of the prepared
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photocatalysts were recorded at 200-800 nm wavelength using diffuse reflectance spectroscopy with a JASCO V-570 UV-vis spectrophotometer equipped with a Labsphere integrating sphere diffuse
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reflectance accessory[30]. The diffuse reflectance mode (R) were recorded and transformed to the Kubelka-Munk function F(R) to separate the extent of light absorption from scattering light. Furthermore, the bandgap energy was calculated from the plot of the modified Kubelka-Munk function
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(F(R)E)2) versus the energy of the absorbed light E, according to the formula as follows [31]. 2 F(R)E2 = (1 R) h
2R
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II. 4. Photocatalytic evaluation
The photocatalytic activity of Bi2S3 photocatalysts was examined by the photodegradation of MB under
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UV illumination. 0.1g of Bi2S3 photocatalyst was mixed with 200 ml MB [50 mg/L] aqueous solution in a quartz photoreactor with H2O circulation for photoreactor cooling. Prior to a photocatalytic reaction, the suspension was kept in dark with stirring at room temperature for 30 min to reach adsorption equilibrium. The above solution was photo-irradiated by using a 150 W medium pressure Hg lamp immersed into the photoreactor with the wavelength ranged between 280-360 nm. The MB concentration after equilibration was recorded to be the initial concentration (Co). It was withdrawn at regular intervals (Ct) from the upper part of the photoreactor. The photocatalyst was separated from the liquid phase by filtration through nylon syringe filters. The UV spectra of MB solution before and after illumination were measured using a JASCO V-570 UV-vis spectrophotometer. The photocatalytic activity of Bi2S3 photocatalysts for MB photodegradation was calculated as follows: % photocatalytic activity = (1- Ct)/ Co × 100 III. Results and discussion III. 1. Structural investigation of Bi2S3 Crystal structure and morphology of Bi2S3 synthesized by hydrothermal method without using either templates or capping agents at different Bi/S molar ratios, reaction times and temperature have been
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ACCEPTED MANUSCRIPT investigated. It was observed that the prepared samples were Bi2S3 nanorods. Additionally, the presence of other phases, owing to the thiourea contents added into Bi(NO3)3 solution, indicated that the Bi/S molar ratio is playing a vital role in determining the structure and morphology of the prepared Bi2S3.
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XRD patterns of all prepared samples at different conditions (Bi/S molar ratios, reaction times and temperature) were examined to determine the structure of Bi2S3 (Fig.1). Fig. 1a exhibits the XRD
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patterns of samples that have been synthesized at different Bi/S molar ratios; 1/0.5, 1/1, 1/1.5, 1/2 and
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1/5. A low sulfur content of Bi/S molar ratios (1/0.5), (1/1), (1/1.5), the cannonite phase impurity with the main Bi2S3 phase is detected. The diffraction peaks are indexed as a cannonite Bi2O(OH)2(SO4)
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monoclinic phase, [32] which is corresponding to the standard diffraction pattern (JCPDS card no. 761102). The impurity cannonite phase disappears with increasing the sulfur contents at Bi/S molar ratios
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1/2 and 1/5. The different peaks can be indexed as the orthorhombic Bi2S3 structure (JCPDS Card No. 00-017-0320) with the major peaks attributed to (020), (130), (211), and (221) planes with unit cell constants of a = 11.15 Å, b =11.30 Å and c = 3.97 Å. Fig. 1b shows the effect of reaction time; 10 h, 20
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h, 36 h, 48 h on the crystalline phase at constant Bi/S molar ratio of (1/2). The patterns of the
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synthesized product indicate that Bi2S3 phase existed at all reaction times. All of the diffraction peaks can be indexed to an orthorhombic phase of Bi2S3. No impurities are detected in this pattern, which
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indicates that pure Bi2S3 can be obtained under the current synthesis conditions. However, with the increase of reaction time from 10 to 36 h, the crystallinity was increased but from the economic point of view and saving energy, the optimum reaction time is taken as 20 h. The XRD patterns of Bi2S3 samples obtained at different temperatures; 100 °C, 150 °C, 180 °C and 250 °C are shown in Fig.1c. The XRD pattern of the prepared sample at 100 °C has some impurity phases besides the Bi2S3 main phase. Also, the XRD pattern prepared at 100 °C shows broad and diffused peaks centered at around 2θ = 24.8 indicating the small size and poor degree of crystallinity (Fig.1c and Table 1).[5,7,24] The impurity phase disappeared with the increase of reaction temperature above 100 °C. All the diffraction peaks correspond well with the standard diffraction data and display a greater degree of crystallinity and an increase of crystal size as indicated by sharper diffraction peaks (Table 1). The suggested procedure is that the formed nanodots is unstable so it assembled in a line, and with extended heating time, the recrystallization taken place, resulting in nanorods formation. However, under a low temperature of 180 °C, the energy is not enough for complete crystallization and thus results in amorphous fringes in nanorods.[33] In general, all peaks can be listed to orthorhombic Bi2S3 and no impurity is detected in this pattern, indicating that the products are high purity. The XRD pattern is refined using Rietveld
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ACCEPTED MANUSCRIPT method (using Fullprof software) [34] to calculate the accurate unit cell dimensions, applying the space group number 62 (Pnma). The results of this refinement give the following lattice parameters, atomic positions and thermal parameters that are shown in Table S1. The goodness of fitting is the acceptable
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range. It is quite clear that our results are in good accordance with those reported in literature [35,36]. The 3D unit cell of Bi2S3 drawn using Diamond software [37] gives the orthorhombic crystal symmetry,
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where the Bi atoms are white spheres and S atoms are blue spheres as shown in Fig S1.
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The crystallinity of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h was also examined by Raman spectroscopy (Figure 2a). The Raman spectrum exhibits four vibrational peaks
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associated with the Bi-S bonds at 236, 413, 887 and 1348 cm-1, in agreement with the literature [38-40]. The broad peak was assigned at 236 cm-1 owing to the vibration modes of the Bi-S bond, which is
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agreement with the reported Bi2S3 values[41,42]. It is well known that the different morphologies can shift the wavenumbers due to the difference in the size (quantum size effect) and surface phonon modes [43]. This finding further confirms that the prepared Bi2S3 is the orthorhombic structure and it is
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harmonious with the obtained XRD results. Fig. 2b displays the FT-IR of the prepared Bi2S3 at Bi/S
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molar ratio (1/2) at 180 °C for 20 h. A broad band at 3200-3400 cm-1 is attributed to the stretching vibration of the O-H bond, while the band centered at 1635 cm-1 corresponds to the bending vibrations
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of H2O. It can also be found that the absorption peak at 2336 cm-1 appeared, which indicated a bond formation between Bi and S [44].
The morphology of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h was investigated by the SEM and TEM (Fig. 3). Fig. 3a reveals the FESEM image of Bi2S3 sample and it is a flower-like built up from many nanorods with an average 30-50 nm in diameter and 0.5-1µm long. [45] The Bi2S3 nanorods were further investigated by the TEM (Fig. 2b,c); TEM image (Fig. 2b) revealed that the size of the nanorods is ~ 50 nm in diameter and it contains many nanorods, evidencing the above SEM investigations and also suggesting that the flower-like pattern obtained is obviously different from the urchin like morphologies.[46- 48] The index of the spots in the SAED pattern reveals that the Bi2S3 nanorod is a single crystal [49, 50]. Figure 3c displays the TEM image of one Bi2S3 nanorod situated in the flower-like structure. It demonstrates that the Bi2S3 nanorods grow nearly exclusively in the [001] direction with nanorods that are on ~50 nm in diameter and ~0.5 µm long. Energy dispersive X-ray spectra (EDS) were examined to measure the stoichiometric composition of the Bi2S3 nanorods, suggesting that the optimum obtained Bi2S3 nanorods consisted of pure Bi2S3. The Bi/S molar ratio of
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ACCEPTED MANUSCRIPT Bi2S3 obtained from the peak areas of the EDS graph is 39.76: 60.24 and thus consistent to the prospective 2:3 ratio of the prepared Bi2S3. Nitrogen adsorption/desorption isotherms of Bi2S3 nanorods prepared at 180 °C for 20 h and Bi/S molar
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ratios of 1/2 and 1/5 are shown in Figure 4. At relative pressures p/p0 between 0.45 and 0.9, the sharpness of the inflection resulting from capillary condensation is attributed to a characteristic for
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mesoporous structure of the prepared Bi2S3 nanorods [51]. A typical reversible type IV adsorption
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isotherms are manifested in particular for Bi/S molar ratio 1/2 sample, however the 1/5 molar ratio Bi/S sample has a mesoporous structure with disordered state. The mesoporous Bi2S3 nanorods possess
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surface areas of 28.15 m2g-1 and large pore volumes of 0.036 cm3g-1 of Bi/S molar ratio 1/2 sample, which reduced to 9.2 m2g-1 and 0.016 cm3g-1 of Bi2S3 nanorods at Bi/S molar ratio 1/5 (see Fig. 4,
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inset). The pore size distribution of sample prepared at Bi/S molar ratio 1/2 is homogeneous with average 7.1 nm, however the pore size of the prepared at Bi/S molar ratio 1/5 is inhomogeneous and has two pore size values at 3.5 nm and 10.6 nm (see Fig. 4, inset). In general, the hysteresis loop can be
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explained by resulting from the voids between non-ordered particles and all pores can be regarded as
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irregular voids between Bi2S3 nanorods, in particular Bi2S3 sample at Bi/S molar ratio 1/5.
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In order to evidence the oxidation state of Bi and S elements, XPS measurement was conducted for the Bi2S3 nanorods (Fig. 5a). The results exhibited two peaks assign at 158.5 and 163.6 eV, corresponding to the Bi 4f7/2 and Bi 4f 5/2 peaks of Bi3+ oxidation state (Fig. 5a). The peak centered at 161.2 is appeared to S 2p with S2- oxidation states.[24] Therefore, the valence of Bi atoms is +3, whereas for S atoms is -2, which is consistent with the charge neutrality calculated for the title compound. The XPS spectra also prove the stoichiometric composition of Bi2S3 that the atomic ratio of Bi/S is ~ 2:3 according to the calculation of the peak areas of Bi 4f and S 2s.
Ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectroscopy was recorded to measure the optical properties of the optimized prepared Bi2S3 nanorods as shown in Fig. 5b. The onset absorption wavelength of the sample is 100 nm, which is located in the infrared region, indicating a good absorption of visible light.[33] The band gap (Eg) of the optimized sample Bi2S3 is further explored by the extrapolation method based on Kubelka-Munk equation.[31] Fig. 5b (inset) plots the (αhν)2 against the photon energy curve, where α is the absorption coefficient and hν (h is Planck’s constant, ν is the frequency) represents the photon energy. The value of Eg was amounted to be 1.45 eV, which indicates
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ACCEPTED MANUSCRIPT that the obtained Bi2S3 photocatalyst is a relatively narrow band-gap semiconductor. Remarkably, the band gap is quite similar to the band gap of bulk Bi2S3 (1.3 eV).[52] In addition, the electron transition
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in bulk Bi2S3 occurs between the Bi-6s-S-3p and the Bi-6p states. The proposed mechanism for preparing Bi2S3 nanorods has been reported [15, 53]. In the present work,
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we suggest a crystal growth of Bi2S3 nanorods according the findings of SEM images (Fig. 6 and
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supporting information Fig. S2 and S3). Upon hydrothermal heating, the isomerization of thiourea may initially take place below 150 °C [Eq. (1)].[15] Then, the generated isothiourea has a strong nucleophilic
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substitution of the Bi3+ cation in Bi(NO3)3 solution, the formed S2- reacts with Bi3+ to obtain Bi2S3 nanorods. [Eq. (2)]. [15] It is well known that extend reaction time and high temperature are suitable for
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Bi2S3 nanorods formation as shown in Fig. 6 and supporting information S3. The formation of Bi2S3 nanorods boosted with the increase of reaction time to 48h. From Fig. 6, it is clearly seen that the precursors decompose into Bi2S3 crystallites when the temperature boosted to 180 °C. As the
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temperature boosted above 180°C, these crystallites of Bi2S3 nanorods self-assembled owing to reduce
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their surface energies [54, 55]. At 100 °C, the Bi2S3 crystallites are not completely formed because the thiourea is not decomposed resulting in a very low yield. At low concentration of thiourea, the
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formation of S2- anions is slow so a small quantity of Bi2S3 nuclei in the solution is produced (Fig. S2). At high thiourea content as in Bi/S molar ratio of 1/5 sample, the excess of adsorbed thiourea is not only attractive but also repulsive, which functioned together and are beneficial for the system to obtain the wanted balance among the nucleation, growth, and assembly.[53] S
S
NH2
C
Tautomerism
NH2
NH2
C
S
2 NH2
C
NH2
Isothiourea
+
Bi(NO3)3
(1)
NH2
S
Bi
C
NH + 2HNO3
Nucleophilic Substituation
2NH2
(2)
Bi2S3 nanorods growth
III. 2. Photocatalytic evaluation III. 2. 1. Impact of crystal structure and morphology of Bi2S3 on photocatalytic activity
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ACCEPTED MANUSCRIPT The photocatalytic performance of the synthesized Bi2S3 nanorods was evaluated for MB photodegradation. The degradation of the MB under UV illumination was followed by recording the absorption spectra using a UV/VIS spectrophotometer at varied time intervals (Fig. 7). Prior to the
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photocatalytic test runs, the photolysis of MB and the adsorption in dark was examined. The results indicate that the photolysis of MB was insignificant without Bi2S3 photocatalyst and its adsorption
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capacity was only 3%. MB can be degraded by either oxidative degradation of the MB molecule or by
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a two-electron reduction[56-58]. The photocatalytic evaluation of the optimized Bi2S3 photocatalyst is shown in Fig. 7. The absorption bands of MB centered at λ = 663 nm and λ = 291 nm steadily decrease
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upon boosting illumination time. Such an experimental finding indicates that the MB decoloration is notably completed under the UV-light illumination in the presence of Bi2S3 photocatalyst. The
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decoloration of MB dye is attributed to the oxidative degradation. It is clearly seen that the absorbance at 663 nm decreased from 8.41 to 0.16 after nearly 60 min illumination. The photocatalytic activity of Bi2S3 nanorods was 98%, in spite of the high concentration of MB [50 ppm].
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Impact of crystal structure and morphology of the synthesized Bi2S3 at different Bi/S molar ratios,
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reaction times and temperatures on the photocatalytic activity of MB photodegradation was studied by details ( Fig.8). The photodegradation of MB over Bi2S3 nanorods at different Bi/S molar ratios 1/0.5,
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1/1, 1/1.5, 1/2 and 1/5 was evaluated (Fig. 8a). The results revealed that the photocatalytic activities of Bi2S3 at Bi/S molar ratios 1/0.5, 1/1 and 1/1.5 for MB photodegradation is insignificant and the photocatalytic activity is reached to 40% at Bi/S molar ratio 1/1.5 (Fig. 8a). This is explained by the photoactive Bi2S3 phase is not well formed and also there is other phase (as shown in XRD results) which might be served as a recombination center for the charge carriers. With boosting the Bi/S molar ratio up to 1/2, the photocatalytic activity was strongly improved and reached to 98% and then the photocatalytic activity decreased to 51% with the increase of Bi/S molar ratio to 1/5. This may be attributed to that the Bi2S3 phase at a Bi/S molar ratio of 1/2 is clearly formed with high crystallinity, however the Bi2S3 phase formed at a Bi/S molar ratio 1/5 showed a weak crystallization as clearly seen in Fig.1a and Table 1[59]. The optimum Bi/S ratio is accordingly taken at 1/2, giving the highest photocatalytic activity of 98%. The effect of different surface areas may be considered as a second reason. The surface area of Bi2S3 phase (28.15 m2g-1 ) obtained at Bi/S molar ratio 1/2 is higher three times than Bi2S3 phase (9.2 m2g-1) prepared at Bi/S molar ratio 1/5. Furthermore, such outstanding photocatalytic activity of Bi2S3 at Bi/S molar ratio 1/2 as compared with Bi2S3 at Bi/S molar ratio 1/5 can be explained by several effects, such as depress light scattering and •OH accumulated onto the
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ACCEPTED MANUSCRIPT mesoporous structure, [59-62] or a speedy transfer of the MB molecule to the active sites of Bi2S3 nanorods owing to the ease diffusion of the MB through the porous structure. With the boost of S content at a Bi/S molar ratio 1/5, the diameter and length of Bi2S3 nanorods was reduced, even
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becoming non-uniform shaped nanocrystals.[24] Fig. 8b exhibits the effect of Bi2S3 photocatalysts prepared at different reaction times 10 h, 20 h, 36 h, 48 h on the MB photodegradation. Also, the
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relationship of reaction time-crystallinity and temperature-crystallinity of the prepared Bi2S3 nanorods
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on their photocatalytic activity are listed in Table 1. The findings revealed that the photocatalytic activity of Bi2S3 gradually boosted with the increase reaction time from 10 to 20 h to reach 98%
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photocatalytic activity. When the reaction time was prolonged to 36 h and 48h, the photocatalytic activity of Bi2S3 decreased to 69% and 63% , respectively. Longer reaction time is not preferred and has
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an opposite effect on the photocatalytic activity [63]. This is explained by the Bi2S3 particle sizes are playing an important role for the photocatalytic activity. This is attributed to the fact that the crystalitte size of Bi2S3 decreased from 68 nm to 50 nm with the increase of reaction time from 10 to 20 h and then
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it boosted again to reach 68 nm with the increase of reaction time to 48 h. Fig. 8c and Table 1 show the
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effect of Bi2S3 obtained at different temperatures 100-250 °C on the MB photodegradation. The photocatalytic activity of the prepared Bi2S3 at 100 °C was 32% due to the Bi2S3 phase has a poor
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degree of crystallinity and cannonite phase impurity as clearly shown in XRD data. [5,7,24] The photogenerated electrons will be easily trapped at the surface because of crystal defects providing nonradiative pathways for recombination.[24] With increasing reaction temperature to reach 180 °C, the photocatalytic activity increased to 98% and then decreased again to 90% with increasing reaction temperature to 250 °C. The reason for such a phenomenon is the crystallite sizes of the prepared photocatalysts. The crystallite size of the sample prepared at 180 °C is 50 nm, which increased to 103 nm at 250 °C reaction temperature. Wang et al. reported that at the reaction temperature of 250 °C, the Bi2S3 nanorods are still remained, however the shape and size of Bi2S3 nanorods are completely changed and highly dispersed[26]. It is well known that too large Bi2S3 particles could also serve as recombination centers, detrimental the photocatalytic activity by prohibiting the active photogenerated charge transfer to the reactant species at the Bi2S3 surface. The second reason, the photocatalytic activity of the newly prepared photocatalyst at 180 ºC is slightly increased compared to the prepared sample at 250 ºC owing to the sample prepared at 180 ºC has large surface area 28 m2g-1, which reduced to 4 m2g-1 of the Bi2S3 nanorods prepared at 250 ºC. III. 2. 2. Performance of the optimized Bi2S3 nanorods on MB photodegradation
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ACCEPTED MANUSCRIPT Generally, the performance of the optimized Bi2S3 nanorods on MB photodegradation was crucially affected by the pH values, the amount of photocatalyst loading and the illumination time. Herein, we performed a series of experiments to systematically determine the pH value, photocatalyst loading and
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illumination time (Fig. 9). MB photodegradation at different pH values was conducted and the findings are shown in Fig. 9a. The results revealed that the photocatalytic activity was boosted with the increase
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of the pH value from 3 to 7 and there was no change in the photocatalytic activity at higher pH value
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above 7. It is clearly seen that MB dye adsorption capacity is influenced by the pH variation. In acidic media, Bi2S3 surface has positively charged and negatively charged in alkaline media, with a point of
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zero charge at pH 6.5-7.[64]. This can be ascribed by considering the electrostatic attraction that occurs between the negatively charged surface of Bi2S3 and MB, a cationic dye. At acidic pH, the adsorption of
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MB was reduced as a result of the presence of excess H+ ions contending with the MB dye cation's for adsorption sites. At high pH value, the positively charged sites were reduced and the negatively charged sites were boosted which favored the MB cationic dye removal[59, 65]. The optimum pH value is ~7 for
D
the MB photodegradation which the photocatalytic efficiency is being maximum ~ 98%. The optimal
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amount of photocatalysts was determined by carrying out systematic experiments at pH 7 and varying of Bi2S3 photocatalyst loading 0.1 - 0.9 g/l (Fig. 9b). As revealed from Fig. 9b, the optimum
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photocatalyst loading for MB photodegradation is 0.5 g/l. It is clearly seen that the photocatalytic activity increases with the increment of Bi2S3 photocatalyst loading from 0.1 to 0.5 g/l due to an increase in the active sites of the Bi2S3 nanorods, which leads to increase the MB molecules adsorbed and the number of photons absorbed. The photocatalytic activity decreased behind the optimum amount 0.5 g/l. This phenomenon is likely related to the accumulation of Bi2S3 nanorods at a high photocatalyst loading causing a decrease in surface active sites and increase in light scattering and opacity of Bi2S3 nanorods, which led to a reduce in the passage of irradiation light through the Bi2S3 photocatalyst [66]. The effect of illumination time in the range 5-90 min was carried out at constant MB concentration and pH = 7.0 (Fig. 9c). The photocatalytic activity was found to increase with the increment of illumination time from 5 to 60 min. The results indicated that the photocatalytic activity reached a steady state at 60 min. Beyond 60 min illumination time, there was no significant observation for MB photodegration. In general, the MB was completely degraded within 60 min. Such data confirms the outstanding photocatalytic activity of the prepared Bi2S3 nanorods for accomplishing MB degradation at high concentration [50 ppm] and short illumination time.
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ACCEPTED MANUSCRIPT The kinetics of MB degradation using Bi2S3 nanorods can be expressed by the first-order equation [13, 67]. This rate constant is employed to calculate the MB photodegradation rates by the formula as follows: ln (Co/C) = kt, where Co and C are the MB concentration at time 0 and t min, respectively, k is
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the first-order reaction rate constant. Fig. 10a presented the photodegradation kinetics of MB vs. the illumination time with employing Bi2S3 nanorods obtained at different Bi/S molar ratios 1/0.5, 1/1,
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1/1.5, 1/2 and 1/5. The k values estimated from the slopes are summarized in the inset of Fig. 10a. The
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results indicate a rather perfect correlation to the pseudo-first order reaction kinetic for all prepared photocatalysts. It has been observed that the MB photodegradation using Bi2S3 nanorods prepared at
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Bi/S molar ratio 1/2, proceeds much more rapidly than all obtained photocatalysts. It is also found that the determined k value for Bi2S3 nanorods prepared at the optimum Bi/S molar ratio 1/2 was higher 40
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and 5.5 times than that prepared at low and high molar ratios 1/0.5 and 1/5, respectively. It is clearly seen that the photocatalytic activity of the Bi2S3 nanorods prepared at Bi/S molar ratio 1/2 was superior to that of all synthesized samples and it exhibited the highest photocatalytic performance. The suggested
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mechanism to explain the superior photocatalytic activity of Bi2S3 nanorods is shown in scheme 1. When
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photons with higher energies than 1.45 eV are attacked the surface of Bi2S3 nanorods; the electrons in the valence band are excited to the conduction band leaving hole behind (equations 1&2 and Scheme 1).
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The thus generated charge carriers (photoelectrons and holes) take part in the Bi2S3 nanorods/H2O interface redox reactions. The photoholes transfer from the bulk to the surface of the Bi2S3 nanorods where they react with adsorbed H2O or
-
OH, creating adsorbed •OH radicals. Simultaneously, the
photogenerated electrons reduced the adsorbed O2 to form O2•¯ radicals (equation 3) [68-71]. The produced O2•¯ radicals onto the surface of Bi2S3 nanorods might be reacted with 2H+ to give H2O2 and hence •OH and ¯OH were generated (see equations 4&5). The adsorbed MB molecules can be easily degraded to CO2 and O2, when subjected to oxidizing agent species O2•¯ and •OH radicals. (equations 6&7)[72]. Bi2S3 (ecb- + hvb+)
Bi2S3 + light hvb+ + ¯OHs
•
OHs
-
Bi2S3 (e ) + O2 •¯
+
O2 + 2H H2O2 + e-
MB + •OHs
(2)
Bi2S3 + O2
•¯
(3)
H2O2
eq. •
OH
(4)
+ ¯OH
(5)
MB• + H2O
MB + Bi2S3)h) + •OHs
(1)
(6)
degradation products
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(7)
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To find out the advantage of Bi2S3 nanorods as efficient photocatalysts and their viability, re-use of the Bi2S3 nanorods was evaluated for the MB photodegradation for five cycles (Fig. 10b). The recycling
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experiments demonstarted that the optimized Bi2S3 photocatalyst was fully balanced during that solidliquid heterogeneous photocatalysis since no significant decrease in photocatalytic activity in the five
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cycles was noticed. However, the photocatalytic efficiency was slightly decreased from 98% to 95%
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after 5 cycles. The results exhibit a good potential applications owing to the good recyclability of the Bi2S3 nanorods based photocatalysts. A more detailed conception of the photocatalytic activity of Bi2S3
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nanorods under visible light and their doping with Fe2O3 and Cr2O3 is being under way and will be reported elsewhere.
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IV. Conclusions
In summary, by controlling the synthetic parameters of the hydrothermal technique, Bi2S3 flower-like structures consisting of nanorods are obtained. The photocatalytic performance of Bi2S3 nanorods was
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optimized by controlling the affecting preparation parameters such as Bi/S molar ratio, reaction time and
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reaction temperature. The highest catalytic activity towards MB photodegradation was achieved at reaction temperature of 180 °C for 20 h at Bi/S molar ratio 1/2. The photocatalytic properties
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demonstrated that the as-prepared Bi2S3 nanorods exhibited a remarkable photocatalytic activity ~ 98% at pH ~ 7 for 60 min illumination time in the presence of 0.5 g/l photocatalyst loading.
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References
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H2O
OH
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CO +
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Scheme 1. Schematic of the suggested mechanism to explain the outstanding photocatalytic activity of Bi2S3 nanorods for photodegradation of MB.
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H2O/ OH O 2 H +
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CO 2
OH
+ Bi2S3
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Figure captions Fig. 1. (a) XRD patterns obtained from Bi2S3 nanorods prepared at different Bi/S molar ratios (1/0.5), (1/1), (1/1.5), (1/2) and (1/5) for 20 h. reaction time at 180 ºC reaction temperature, (b) Effect of reaction time 10h, 20h, 36h and 48h on Bi2S3 prepared at Bi/S molar ratio (1/2) and 180 ºC reaction temperature; (c) Effect of reaction temperature of Bi2S3 nanorods prepared at 100 ºC, 150 ºC, 180 ºC and 250 ºC on its structure phase at Bi/S molar ratio (1/2) for 20 h reaction time.
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Fig. 2. (a) Raman spectroscopy of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h; (b) FT-IR of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h.
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Fig. 3. (a) SEM image of Bi2S3 nanorods, (b) TEM image of a typical Bi2S3nanorod; Inset: the index of the spots in the SAED pattern, (c) TEM image of a typical selected one Bi2S3 nanorod and (d) Energy dispersive X-ray spectra of the Bi2S3 nanorods. Fig. 4. Nitrogen adsorption/desorption isotherms and (inset) BJH pore size distribution plot for Bi2S3 nanorods at molar ratios of Bi/S (1/2) and (1/5).
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Fig. 5. (a) XPS spectra of Bi2S3 nanorods of Bi4f and S2p regions. (b) Ultraviolet−visible−near-infrared (UV-vis-NIR) absorption spectrum of Bi2S3 nanorods. (Inset) Plot of (αhν)2 vs. hν for the absorption spectrum.
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Fig. 6. Typical SEM images of Bi2S3 nanorods prepared at different reaction temperatures 100 ºC (a) , 150 ºC (b), 180 ºC (c) and 250 ºC (d). Fig. 7. Absorbance vs. wavelength as a function of illumination time for the photodegradation of MB on Bi2S3 nanorods hydrothermally prepared at optimum conditions: (Bi/S molar ratio 1/2 at 180 °C for 20 h) and the photocatalytic conditions of: (MB concentration [50 ppm], volume of MB = 200 ml, photocatalyst loading = 0.5 g/l). Fig. 8. Change in concentration vs. illumination time in the presence of Bi2S3 nanorods photocatalysts for MB photodegradation (a) at different Bi/S molar ratios, (b) reaction times and (c) reaction temperatures. Fig. 9. Change in concentration vs. illumination time in the presence of Bi2S3 nanorods photocatalysts for MB photodegradation at different pH values (a), the amount of photocatalyst loadings (b) and illumination times (c). Fig. 10. (a) The variation of ln (Co/C) vs. illumination time for MB photodegradation using Bi2S3 nanorods photocatalysts prepared at Bi/S molar ratio 1/2 and 180 °C for 20 h). (b) Repeated cycles up to 5 times of MB photodegradation over the optimized Bi2S3 nanorods.
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Figure captions Fig. 1. (a) XRD patterns obtained from Bi2S3 nanorods prepared at different Bi/S molar ratios (1/0.5), (1/1), (1/1.5), (1/2) and (1/5) for 20 h. reaction time at 180 ºC reaction temperature, (b) Effect of reaction time 10h, 20h, 36h and 48h on Bi2S3 prepared at Bi/S molar ratio (1/2) and 180 ºC reaction temperature; (c) Effect of reaction temperature of Bi2S3 nanorods prepared at 100 ºC, 150 ºC, 180 ºC and 250 ºC on its structure phase at Bi/S molar ratio (1/2) for 20 h reaction time.
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Fig. 2. (a) Raman spectroscopy of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h; (b) FT-IR of the optimized Bi2S3 sample obtained at Bi/S molar ratio (1/2) at 180 °C for 20 h.
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Fig. 3. (a) SEM image of Bi2S3 nanorods, (b) TEM image of a typical Bi2S3nanorod; Inset: the index of the spots in the SAED pattern, (c) TEM image of a typical selected one Bi2S3 nanorod and (d) Energy dispersive X-ray spectra of the Bi2S3 nanorods. Fig. 4. Nitrogen adsorption/desorption isotherms and (inset) BJH pore size distribution plot for Bi2S3 nanorods at molar ratios of Bi/S (1/2) and (1/5).
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Fig. 5. (a) XPS spectra of Bi2S3 nanorods of Bi4f and S2p regions. (b) Ultraviolet−visible−near-infrared (UV-vis-NIR) absorption spectrum of Bi2S3 nanorods. (Inset) Plot of (αhν)2 vs. hν for the absorption spectrum.
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Fig. 6. Typical SEM images of Bi2S3 nanorods prepared at different reaction temperatures 100 ºC (a) , 150 ºC (b), 180 ºC (c) and 250 ºC (d). Fig. 7. Absorbance vs. wavelength as a function of illumination time for the photodegradation of MB on Bi2S3 nanorods hydrothermally prepared at optimum conditions: (Bi/S molar ratio 1/2 at 180 °C for 20 h) and the photocatalytic conditions of: (MB concentration [50 ppm], volume of MB = 200 ml, photocatalyst loading = 0.5 g/l). Fig. 8. Change in concentration vs. illumination time in the presence of Bi2S3 nanorods photocatalysts for MB photodegradation (a) at different Bi/S molar ratios, (b) reaction times and (c) reaction temperatures. Fig. 9. Change in concentration vs. illumination time in the presence of Bi2S3 nanorods photocatalysts for MB photodegradation at different pH values (a), the amount of photocatalyst loadings (b) and illumination times (c). Fig. 10. (a) The variation of ln (Co/C) vs. illumination time for MB photodegradation using Bi2S3 nanorods photocatalysts prepared at Bi/S molar ratio 1/2 and 180 °C for 20 h). (b) Repeated cycles up to 5 times of MB photodegradation over the optimized Bi2S3 nanorods.
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Volume adsorbed (cc/g)
20
Pore volume / ( cc/g)
0.0010
0.2
0.4
0.6
Relative pressure P/Po
23
0.8
ACCEPTED MANUSCRIPT
Fig. 5
156
MA
NU
S 2p
SC
RI
Counts per second (au)
Bi 4f 5/2
PT
(a)
Bi 4f 7/2
158
160
162
164
166
168
TE
D
Binding energy (eV)
0.9
(b)
AC CE P
0.8
5
0.6
4
[F(R)h ]2
Absorbance
0.7
0.5
3
2
0.4
1
0.3 0 1
2
3
4
5
6
h(eV)
0.2
200
400
600
800
1000
Wavelength/ nm
24
1200
1400
ACCEPTED MANUSCRIPT
Fig. 6
(b)
SC
RI
PT
(a)
NU
1 µm
5 µm
(d)
5 µm
AC CE P
1 µm
TE
D
MA
(c)
25
ACCEPTED MANUSCRIPT
PT
Fig.7
SC
NU
4
0
400
500
Wavelength/ nm
AC CE P
TE
300
D
2
200
RI
0 Ads 5 min 10 min 20 min 30 min 40 min 50 min 60 min
6
MA
Absorbance
8
26
600
700
ACCEPTED MANUSCRIPT
Fig. 8 100
PT
1/ 0.5 1/ 1.0 1/ 1.5 1/ 2.0 1/ 5.0
80
RI
60
SC
Photocatalytic activity, %
(a)
40
NU
20
0
MA
0 10
20
30
40
50
60
Illumination time / min 100
D
10 h 20 h 36 h 48 h
TE
80
60
AC CE P
Photocatalytic activity, %
(b)
40
20
0
0
10
20
30
Illumination time / min
27
40
50
60
ACCEPTED MANUSCRIPT 100
80 o
PT
100 C o 150 C o 180 C o 250 C
RI
60
40
SC
Photocatalytic activity, %
(c)
0 0
10
20
NU
20
30
AC CE P
TE
D
MA
Illumination time / min
28
40
50
60
ACCEPTED MANUSCRIPT Fig. 9 100
80
PT
pH= 3 pH= 5 pH= 7 pH= 9 pH= 11
RI
60
40
SC
Photocatalytic activity, %
(a)
0 0
10
20
NU
20
30
40
50
60
MA
Illumination time / min
100
TE
D
80
60
40
AC CE P
Photocatalytic activity, %
(b)
0.1 g/l 0.3 g/l 0.5 g/l 0.7 g/l 1 g/l
20
0
0
10
20
30
Illumination time / min
29
40
50
60
ACCEPTED MANUSCRIPT 100
PT
80
RI
60
40
SC
Photocatalytic activity, %
(c)
0 0
10
20
30
40
NU
20
50
AC CE P
TE
D
MA
Illumination time / min
30
60
70
80
90
ACCEPTED MANUSCRIPT Fig. 10 4.0
1: 0.5 1 :1.0 1 :1.5 1 : 2.0 1 : 5.0 -1 k = 0.0016 min -1 k = 0.0044 min -1 k = 0.0084 min -1 k = 0.0642 min -1 k = 0.0117 min
2.0 1.5
PT
2.5
RI
ln (C/C)
3.0
SC
3.5
(a)
NU
1.0 0.5
0
10
MA
0.0
20
30
40
50
60
D
Illumination time / min
TE
(b)
80
AC CE P
Photocatalytic activity, %
100
60
40
20
0
1
2
3 Number of cycles
31
4
5
ACCEPTED MANUSCRIPT
67.0
20
80.6
36
91.2
48
100
72.3
NU
10
RI
%
Photocatalytic activity, %
SC
Crystallinity,
Reaction time, h
PT
Table 1. The relationship of reaction time-crystallinity and temperature-crystallinity of the prepared Bi2S3 nanorods on their photocatalytic activity.
68.9 63.3
100 °C
MA
Reaction
97.9
46
32
150 °C
73.0
87.4
80.6
97.9
98.8
90.9
TE
180 °C
D
Temperature
AC CE P
250 °C
32
ACCEPTED MANUSCRIPT
NU
2
SC
CO +
RI
e-
H O/ OH
MA
2
O 2 H +
+ Bi2S3 MB
D
CO 2
TE
OH
AC CE P
Graphical Abstract
PT
H2O
OH
33
ACCEPTED MANUSCRIPT
PT
Research highlight Bi2S3 nanorods with orthorhombic structure were successfully synthesized.
RI
Bi2S3 is nanorods with average 30-50 nm in diameter and 0.5-1µm length.
SC
The conditions for synthesis Bi2S3 were Bi/S molar ratio 1/2 for 20 h at 180 °C.
NU
Bi2S3 nanorods exhibit photoactivity ~ 98% at pH ~ 7 for 60 min illumination
MA
time.
k values for Bi2S3 nanorods was higher 40 and 5.5 time than other prepared
AC CE P
TE
D
samples.
34