Insights into Promoted Adsorption Capability of Layered BiOCl

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Insights into Promoted Adsorption Capability of Layered BiOCl Nanostructures Decorated with TiO2 Nanoparticles Feifan Xu, Gang Cheng, Shuang Song, Yi Wei, and Rong Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01920 • Publication Date (Web): 06 Oct 2016 Downloaded from http://pubs.acs.org on October 8, 2016

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ACS Sustainable Chemistry & Engineering

Insights into Promoted Adsorption Capability of Layered BiOCl Nanostructures Decorated with TiO2 Nanoparticles Feifan Xu, Gang Cheng*, Shuang Song, Yi Wei, and Rong Chen* School of Chemistry and Environmental Engineering, Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Xiongchu Avenue, Wuhan, 430073, PR China. *Corresponding author: [email protected] (G. Cheng); [email protected] (R. Chen) KEYWORDS: BiOCl-TiO2 nanocomposites, adsorption, hydrophilic property, water treatment, Photocatalytic regeneration ABSTRACT Rational design of excellent adsorbents with remarkable adsorption capability and renewable property is of significant importance to practical application. Herein, the adsorbent of BiOCl nanoplates decorated with TiO2 nanoparticles was successfully prepared from porous BiOCl microspheres and tetrabutoxytitanium. Through TiO2 nanoparticles decoration, the tuned morphology, surface charge property, Brunauer-Emmett-Teller (BET) surface area, and hydrophilic property of the BiOCl microsphere were favorable to achieve optimization of adsorption capability towards Congo red removal with maximum adsorption amount of 254.7

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mg/g. A possible adsorption mechanism model involved of BET surface area, electrostatic interaction, and competitive adsorption between solvent and solute was proposed to account for the enhancement of adsorption capability of BiOCl-TiO2 nanocomposite compared to pure BiOCl and TiO2. Furthermore, the adsorbent of BiOCl-TiO2 nanocomposite can be simply regenerated by a photocatalysis technique and efficiently reuse to adsorption repeatedly. In addition, TiO2 nanoparticles-based decoration could be a general strategy to boost the adsorption capacity of other Bi-based nanostructures, which is confirmed by Bi2O2CO3-TiO2, Bi2S3-TiO2, Bi2MoO6-TiO2 and Bi2O3-TiO2 nanocomposites.

1. INTRODUCTION Rapid industrialization-derived water pollution has become a major issue on a global level. A large amount of dyes wastewater is discharged without effective treatment during the production and employs process, which brings about a serious pollution to the environment as well as does harm to human health.1,2 Adsorption is one of most convenient and popular methods for dyes removal from aqueous systems because of its simplicity, high efficiency, and low energy requirements.3-5 To tackle the issue of the environmental contamination, the exploration of new adsorbents exhibiting high adsorption capacities and rates, good separation, and desirable regeneration capability have been drawing considerable attention and still remain big challenge. Currently, nanostructured functional materials can be used to adsorb various contaminants to remedy the environment. Furthermore, the impacts of BET surface areas, morphology tailoring, surface charge property of nanostructured adsorbents on their adsorption capabilities have been widely investigated.6-10 As a matter of fact, in the dye solution system, when the solvent (eg. water) is easy to adsorb on the surface of the adsorbent, the adsorption capacity towards target


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dye pollutant (eg. Congo red) could be suppressed because the adsorption sites were mainly occupied by the solvent, insteading of dye molecules which should be removed by the adsorbents. In other words, in the aqueous solution adsorption system, engineering the hydrophilic property of adsorbents could have important influence on the competitive adsorption between the solvent and the pollutant solute, and finally affect the adsorption capability of the adsorbents. However, there are few reports relevant to investigate the surface wettability of adsorbent on the related adsorption property. Among numerous adsorbents, bismuth oxyhalides have been developed rapidly not only due to its good property and facile morphology-tailored fabrication via one-step co-precipitation, but also its excellent photocatalytic capability for adsorbent regeneration.11-17 Until now, although much effort has been made to improve the adsorption capacity of BiOCl and BiOI by tailoring the morphology, BET surface areas, and surface charge property, it is a big challenge to achieve optimization of adsorption capability as well as give an insight into the promoted adsorption capacity by exploiting a facile and designed way. In this work, TiO2 nanoparticles decorated BiOCl nanoplates were facile synthesized by a solvothermal method from BiOCl microspheres and tetrabutoxytitanium. It was found the decoration of TiO2 nanoparticles could engineer the BET surface area, surface charge property, and hydrophilic property of BiOCl nanoplates and accordingly promote its adsorption capability towards Congo red removal. Furthermore, the BiOCl-TiO2 nanocomposite adsorbent not only could be regenerated by photocatalysis technology but also still showed good adsorption property. In addition, this TiO2 nanoparticles decoration strategy could be a general approach to improve the adsorption capability of Bi-based adsorbents including Bi2O2CO3, Bi2S3, Bi2MoO6, and Bi2O3.


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2. EXPERIMENTAl SECTION 2.1 Materials synthesis All chemicals were of analytical grade and used directly without further purification. BiOCl microspheres (Figure S1) were synthesized according to previously reported method using Bi(NO3)3·5H2O (1.455 g) and NaClO3 (0.318 g) as the precursor in the triethylene glycol (75 mL).15 In a typical fabrication of BiOCl-TiO2 composites, 1.06 mL Ti(OC4H9)4 (3 mmol) was dissolved in 10 mL ethanol with sonication for 3 min, followed with the addition of 0.26 g BiOCl (1 mmol). Then the mixed liquor containing 23 mL H2O and 2 mL C2H5OH was transferred to the above homogeneous suspension. The resulting suspension was maintained at 180 °C for 12 h in a 50 mL Teflon-lined autoclave. Finally, the products were washed thoroughly with deionized water and denoted as BiOCl-TiO2 (BiOCl-TiO2 composites with molar ratios of 1:3). For comparison, pure TiO2 nanoparticles were also synthesized using the same method without the addition of BiOCl microspheres. BiOCl-TiO2 composites with molar ratios of 1:1, 1:2, and 1:5 (BiOCl to TiO2) were also synthesized via the same process, which was denoted as BiOCl-TiO2 (1:1), BiOCl-TiO2 (1:2), and BiOCl-TiO2 (1:5), respectively. Other composites including Bi2O2CO3-TiO2, Bi2S3-TiO2, Bi2MoO6-TiO2, and Bi2O3-TiO2 were also fabricated through the similar synthesis strategy and the detailed synthesis process was given in supporting information. 2.2 Materials Characterization The composition and crystal phase of the obtained products were characterized by powder X-ray diffraction (XRD, Bruker axs D8 Discover) with Cu Kα radiation of 1.5406 Å. The morphology and structure of the obtained samples were characterized by scanning electron microscope (SEM, Hitachi S4800) operating at 5.0 kV and transmission electron microscope (TEM, Philips Tecnai


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G2 20) operating at 200 kV. BET specific surface area was measured by nitrogen adsorption on a Micromeritics ASAP 2020 nitrogen adsorption apparatus (USA). X-ray photoelectron spectra (XPS) were performed on a VG Multilab2000 spectrometer by using Al Kα (1486.6 eV) radiation as the source. All of the binding energy values were calibrated by the C 1s peak at 284.6 eV of the surface adventitious carbon. Raman spectra were recorded by using a Horiba Jobin-Yvon LabRam HR800 Raman microspectrometer with an excitation laser at 320 nm. Static contact angles (CAs) of the as-prepared tablets were measured by KRUSS DSA 100 (Germany) contact angle measuring instrument. A distilled water droplet (drop volume 4 µL) was used as the indicator in the experiment to characterize the wetting property of the as-prepared tablets. The digital photographs of water droplets on surfaces were obtained by a digital camera (Sony, from Japan). Zeta Potentials of different samples dispersed in deionized water through sonication were determined by measuring the supernatant with Zetasizer Nano (ZEN3600, Malvern). The Fourier transform infrared (FTIR) spectrum was measured on a Bruker Tensor 27 Spectrometric Analyzer using KBr pellets. UV-vis diffuse reflectance spectra (DRS) were recorded on a UVvis spectrometer (Shimadzu UV-2550) by using BaSO4 as a reference and were converted from reflection to absorbance by the Kubelka-Munk method. 2.3 Evaluation of adsorption capacity of adsorbents The adsorption capability of the as-prepared products were evaluated via a comparative CR adsorption experiments. The experiments were carried out in a 50 mL glass vial using 0.02 g adsorbent and 40 mL CR solution with a certain concentration that were placed on a shaker and stirred at 150 rpm. At several time intervals, 3 mL of the suspensions were collected and centrifuged to remove the adsorbents. The solution were analyzed by a Shimadzu UV2800


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spectrophotometer. For comparison, similar experiments were performed by replacing CR solution with Rhodamine B (RhB), Methylene blue (MB), and Methyl orange (MO) solution. For adsorption isotherms experiments, 0.01 g adsorbents were added to 40 mL CR solutions with different concentrations (50, 80, 120, 150, 200, and 250 mg/L). After it reached adsorptiondesorption equilibrium, the suspensions were immediately separated by centrifugation and the supernatants were determined via a Shimadzu UV-vis spectrometer and the characteristic absorption of CR at 498 nm was used to monitor the adsorption process. The equilibrium adsorption was quantified according to the following equation: =

1

Where qe (mg/g) is the adsorption amount of the adsorbent at equilibrium time, C0 and Ce (mg/L) are the initial and equilibrium concentration of CR in the aqueous phase, V (L) is the volume of CR solution, and m (g) is the mass of the used adsorbent. All the adsorption experiments were carried out at room temperature. 2.4 Regeneration of adsorbents Regeneration of adsorbents was achieved via photocatalytic process. Take BiOCl-TiO2 adsorbent as an example, 0.02 g BiOCl-TiO2 composites were firstly dispersed in a high concentration of CR aqueous solution (2 g/L) to reach the adsorption saturation. Secondly, the CR-adsorbed BiOCl-TiO2 composites were collected through centrifugal separation, then dispersed in 40 mL deionized water and finally was irradiated under a 500 W xenon lamp for 5 h to remove the adsorbed CR on the surface of BiOCl-TiO2 composites through photocatalytic degradation. After photocatalytic treatment, the regenerated BiOCl-TiO2 composites were collected and further reused to adsorb CR (50 mg/L). The recyclability of the BiOCl-TiO2 composites was also


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examined by the repeated experiments based on above adsorption and photocatalysis process.16 The supernatant solutions were analyzed by UV-vis spectroscopy. 3. RESULTS AND DISCUSSION Figure 1a shows X-ray diffraction pattern (XRD) of BiOCl-TiO2 composite, where the diffraction peaks can be indexed as tetragonal phase of BiOCl (JCPDS No.6-249) and anatase TiO2 (JCPDS No.4-477). However, TiO2 in the composites displayed no obvious strong diffraction peaks, which could be due to the low crystallinity and uniform dispersion of TiO2 in the composite. On the other hand, some weak diffraction peaks of TiO2 could be covered by the strong peaks of BiOCl. Raman scattering spectroscopy is another powerful technique that can provide additional structural information on the nanomaterials. As shown in Figure 1b, typical Raman peaks of anatase located at 147, 397, 517, and 637 cm-1 were observed, assigning well with prominent Raman modes of Eg(1), B1g(1), A1g+B1g(2), and Eg(2), respectively.18,19 In addition, three characteristic bands at 59, 147, and 201 cm-1 can be assigned to the A1g external Bi-Cl, A1g internal Bi-Cl, and Eg internal Bi-Cl stretching modes, respectively.20,21 The above results indicated the composite was comprised of BiOCl and TiO2.


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Figure 1. (a) XRD pattern, (b) Raman spectra, (c) Ti 2p XPS spectra, (d) O 1s XPS spectra, (e) Bi 4f XPS spectra, and (f) Cl 2p XPS spectra of BiOCl-TiO2 composites. X-ray photoelectron spectroscopy (XPS) was employed to further investigate the intrinsic characteristic and chemical states of the compositional elements of BiOCl-TiO2 composite. Figure S2 depicts a wide survey scan XPS spectra of the BiOCl-TiO2 composite, which shows the presence of the C, Bi, O, Cl and Ti peaks, confirming the composition. As shown in Figure 1c, the two peaks at 464.6 eV and 458.5 eV are assigned to Ti 2p1/2 and Ti 2p3/2, respectively, corresponding to Ti4+.22,23 The O 1s region (Figure 1d) can be fitted into three peaks at 529.7, 530.9, and 532.3 eV. The peak at 530.9 eV is attributed to the Ti-O band in TiO2, and the peak at 529.7 eV belongs to O2- in a Bi-O bond in BiOCl14. The peak at 532.3 eV is assigned to the hydroxyl group. Two intensive peaks at 164.7 and 159.4 eV are assigned to Bi 4f5/2 and Bi 4f7/2, respectively, corresponding to Bi3+ in BiOCl (Figure 1e).24 In Figure 1f, the peaks with binding energies of 199.6 and 198.0 eV correspond to Cl 2p1/2 and Cl 2p3/2, respectively, which are characteristic of Cl- in BiOCl product.25 Hence, the XRD, Raman, and XPS measurements confirm the as-prepared products to be pure BiOCl-TiO2 composite.


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Figure 2. (a) SEM, (b) TEM, (c) HRTEM image, and (d) schematic illustration of possible formation process of BiOCl-TiO2 nanocomposites. The morphology of the as-synthesized BiOCl-TiO2 composite was observed by SEM and TEM images. Figure 2a and 2b show the SEM and TEM image of typical BiOCl-TiO2 product. It was observed that numerous TiO2 nanoparticles deposited on the surface of irregular BiOCl nanoplates. The corresponding HRTEM image (Figure 2c) show the clear lattice fringes with a d-spacing of 0.350 and 0.275 nm, corresponding to the (101) plane of anatase TiO2 and the (110) plane of tetragonal BiOCl, respectively, which further demonstrated that the composite of BiOCl nanoplates decorated with TiO2 nanoparticles has been successfully prepared by such a solvothermal protocol. Interestingly, when tetrabutoxytitanium (TBT) was not involved into the present reaction system, pure BiOCl nanoplates (Figure S3) evolved from three dimensional microspheres (Figure S1) were obtained, while pure TiO2 nanoparticles (Figure S4) were


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prepared in the absence of BiOCl microspheres. Based on the above results, a possible formation process of BiOCl-TiO2 nanocomposite was proposed in Figure 2d. In other word, the formation of BiOCl-TiO2 involved the same process of BiOCl nanoplates and TiO2 nanoparticles, in which TBT could attach to the surface of BiOCl microspheres and subsequently TiO2 nanoparticles formed on the BiOCl nanoplates transformed from such a microsphere via the solvothermal treatment.

Figure 3. (a) Adsorption performance of different BiOCl-TiO2 samples (0.01 g) towards CR solution (40 mL, 40 mg/L); (b) Adsorption isotherm curves of BiOCl-TiO2 composites at 25 °C for CR solution; (c) Adsorption kinetics of BiOCl-TiO2 composites (0.01 g) towards CR solution (40 mL, 70 mg/L). The adsorption performance of the as-obtained BiOCl-TiO2 nanocomposites was evaluated towards removal of CR solution. For comparison, pristine BiOCl microspheres, TiO2 nanoparticles, and BiOCl-TiO2 nanocomposites with different Bi/Ti molar ratio (Bi:Ti=1:1, 1:2, 1:3, and 1:5) were investigated for the adsorption treatment of CR. Figure 3a displays time dependent adsorption efficiency of CR on different samples with initial CR concentration of 40 mg/L and adsorbents dosage of 0.01 g. In comparison with pure BiOCl and TiO2, BiOCl-TiO2 nanocomposite (Bi:Ti=1:3, molar ratio) exhibited excellent adsorption capacity. Furthermore, the adsorption isotherm experiment was conducted to determine the maximum adsorption capacity.


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In Figure 3b, the Langmuir and Freundlich models are employed to fit the data of adsorption isotherms, which are presented as follows: =

2 1 + /

= 3 Where qe (mg/g) is the CR adsorption amount at equilibrium, qm (mg/g) is the capacity representing a full monolayer coverage of adsorbent surface according to Langmuir isotherm, Ce is the concentration of CR at equilibrium, b is a measure of the energy of adsorption, Kf is adsorption constant of Freundlich isotherm, and n is the Freundlich linearity index. It can be seen that the Langmuir model gives a somewhat better fit than the Freundlich model as shown in Table 1. The maximum adsorption capacities of BiOCl-TiO2 composite calculated from Langmuir model at 25 °C is 254.7 mg/g, which is significantly higher than pure BiOCl (162.1 mg/g) and TiO2 (106.7 mg/g, Figure S5). Table 2 shows the adsorption capacities of BiOCl-TiO2 composite and some materials reported previously. It appears that the adsorption capacity of CR by the BiOCl-TiO2 composite is significantly higher than that of most adsorbents reported previously.16,26-39 Table 1. Adsorption isotherm parameters of as-prepared BiOCl, TiO2, and BiOCl-TiO2 adsorbents. Samples

Langmuir model qm (mg/g)

b (L/mg)

BiOCl-TiO2

254.7

0.43

BiOCl

162.1

TiO2

106.7

Freundlich model R2

R2

Kf

n

0.969

167.46

11.86

0.746

0.12

0.946

55.70

4.69

0.728

0.31

0.826

51.44

6.25

0.565


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The kinetic curves in Figure 3c shows that the adsorption of CR was quite rapidly at the initial (

Insights into Promoted Adsorption Capability of Layered BiOCl

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