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Decoration of MoO3 nanoparticles by MWCNTs driven visible light for the reduction of Cr(VI)
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Decoration of MoO3 nanoparticles by MWCNTs driven visible light for the reduction of Cr(VI) K.S. Al-Namshaha,∗, R.M. Mohamedb,c,∗∗ a
College of Science, Main Campus King Abdullah Rd, King Khalid University, Abha, Saudi Arabia Department of Chemistry, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia c Advanced Materials Department, Central Metallurgical R&D Institute, CMRDI, P.O. Box 87, Helwan, Cairo, Egypt b
A R T I C LE I N FO
A B S T R A C T
Keywords: Chromium (VI) MoO3 MWCNTs Photocatalyst Visible light
A simple one-step sol-gel method was used to prepare mesoporous molybdenum oxide using a F127 triblock copolymer structure-directing agent for the first time. MoO3@MWCNT nanocomposites with 1%, 2%, 3%, and 4% weight MWCNT were prepared using the impregnation method. MoO3 and MoO3@MWCNT nanocomposites were analyzed to identify their physical and chemical properties. The TEM results showed that MoO3 has a nanoparticle shape, and MoO3 nanoparticles were well dispersed on the surface of MWCNTs. The bandgap of the MoO3 sample can be adapted by adjusting the weight percent of MWCNTs. Additionally, the band gap of MoO3 was reduced by the addition of MWCNTs and improved their photocatalytic reduction of chromium (VI) under visible light. Finally, MoO3@MWCNT nanocomposites were used to reduce chromium (VI) beyond the loss of photocatalytic performance, which means they have high photocatalytic stability.
1. Introduction In recent years, purification of the environment has been a focus of scientific research. One of the most demanding tasks for scientists is finding photocatalysts that can completely degrade pollutants in the air or water [1–8]. Excellent photocatalytic materials should be stable, nontoxic and easy to fabricate, and they should have a high surface area and absorb in the visible region [4,5]. Many photocatalysts have been used for degradation of pollutants using UV and visible light, such as Bi2O3/Bi2O4-x, TiO2, NaNbO3, WO3 and Bi12TiO20 [9–13]. Titanium dioxide is considered the most predictable photocatalyst due to its high stability, nontoxicity and low cost. However, the widespread application of titanium dioxide in photocatalysts and photovoltaics is lacking due to its poor quantum yield and poor sunlight absorption ability as a result of its high electron-hole recombination and wide bandgap energy, respectively [14–16]. Consequently, it is essential to synthesize a photocatalyst that efficiently absorbs visible light and prevents electron-hole recombination. A variety of methods have been used to expand the absorption of photocatalysts from UV to the visible region, such as metal and nonmetal doping, coupling with semiconductors, or coupling with CNTS (carbon nanotubes) [10, 11, 17–20]. Therefore, one of the most demanding tasks for researchers is fabricating a photocatalyst with a specific structural arrangement to make it has high
∗
photocatalytic activity using natural sunlight and visible light irradiation. The lack of a specific structural arrangement may delay the proper trading of chemical and electron species between the different active ingredients. To construct powerful and viable photocatalysts using visible and sunlight radiation with high stability and reactivity, their bandgap energy must be narrow, or their value must be less than 3.0 eV [21–23]. MoO3 (molybdenum oxide) is a photocatalysts that meets these requirements and can be excited using visible or UV light [23]. Due to its high electron-hole recombination rate and its poor stability, the widespread application of molybdenum oxide as a visible photocatalyst is limited. Therefore, the reduction of the electron-hole recombination rate and improvement of the stability of molybdenum oxide are essential for developing molybdenum oxide photocatalytic activity. Thus, to construct powerful and viable photocatalysts using visible and sunlight radiation with high stability and reactivity, we established the synthesis and characterization of MoO3-MWCNT nanocomposites. The addition of MWCNTs to molybdenum oxide photocatalysts develops charge separation and enhances the stability of molybdenum oxide. To our knowledge, there are published studies about preparation of mesoporous MoO3-MWCNT nanocomposites. Here, we prepare, for the first time, mesoporous MoO3-MWCNT nanocomposites for photocatalytic reduction of Cr(VI) using visible light.
Corresponding author. College of Science, Main Campus King Abdullah Rd, King Khalid University, Abha, Saudi Arabia. Corresponding author. Department of Chemistry, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia. E-mail addresses: [email protected] (K.S. Al-Namshah), [email protected] (R.M. Mohamed).
∗∗
https://doi.org/10.1016/j.ceramint.2019.11.187 Received 12 November 2019; Received in revised form 19 November 2019; Accepted 20 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: K.S. Al-Namshah and R.M. Mohamed, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.187
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2. Experimental
horizontal cylinder rounded batch reactor was used for the photocatalytic reaction. First, molybdenum oxide or MoO3@MWCNT sample was scattered in an aqueous solution of 500 mL potassium chromite with 150 ppm chromium. The photocatalytic reaction was performed at room temperature. The concentration of chromium (VI) was determined using a V-570, JASCO (Japan) UV–Vis spectrophotometer, and its absorbance was measured at 540 nm according to the standard diphenylcarbazide method [24]. The photoreduction performance of chromium (VI) as a percentage was calculated by applying the equation:
2.1. Preparation of mesoporous MoO3 nanoparticles A simple one-step sol-gel method was used to prepare mesoporous molybdenum oxide using F127 triblock copolymer as a structure-directing agent. First, 7.0 mL of acetic acid, 2.5 g of F127, and 0.80 mL of hydrochloric acid were dissolved in 50 mL of ethyl alcohol. Then, molybdic acid (1.5 g) was added to the solution, and the resulting mixture was stirred at room temperature for 60 min and placed in a Petri dish to evaporate the ethyl alcohol for 12 h under 40% humidity at a temperature of 40 °C in an oven; then, the samples were dried at 65 °C for 24 h. Finally, the samples were calcined in air for 4 h at 450 °C by using 1 °C/min and 2 °C/min as the heating and cooling rates, respectively, to obtain mesoporous molybdenum oxide after removing the F127 surfactant.
% photoreduction performance = (Co - Ct)/Co × 100 where Co is chromium (VI) concentration at the initial time and Ct is chromium (VI) concentration at t time.
2.2. Preparation of MoO3@MWCNT nanocomposite
3. Results & discussion
Mesoporous molybdenum oxide samples were dispersed in a solution of ethyl alcohol using an ultrasonic bath for 10 min to gather mesoporous molybdenum oxide on the surface of MWCNTs. To evaporate the ethyl alcohol, the resulting mixture was dried in air for 12 h at 60 °C to obtain mesoporous MoO3@MWCNT nanocomposites with the mesoporous molybdenum oxide well attached on the surface of MWCNTs. The relative amount of MWCNTs was changed to prepare different samples of MoO3@MWCNT nanocomposites with 1%, 2%, 3%, and 4% weight fractions of MWCNT.
3.1. Characterization of photocatalysts The XRD patterns of MoO3 and MoO3@MWCNTs samples are shown in Fig. 1. For the MWCNT sample, XRD results showed a peak at 2-theta equal to 25.8°, which verifies the presence of (002) of MWCNT, in keeping with JCPDS card no. 26-1079. For the MoO3 sample, XRD results showed molybdenum oxide phase and XRD diffraction peaks consistent with JCPDS card no. 05-0506. For MoO3@MWCNT samples, the results showed only peaks of MoO3 for all MoO3@MWCNT samples. Additionally, there are no peaks for MWCNTs for all MoO3@MWCNTs samples, likely due to the low content of MWCNTs in MoO3@MWCNT samples. In addition, the presence of MWCNTs in MoO3@MWCNT samples hinders the crystallization of MoO3 and hence reduces its crystallite size as shown in Fig. 1. Fig. 2 displays high-resolution XPS spectra of Mo3d, O1s, and C1s of MoO3-3% wt MWCNT samples. The binding energy for C1s at 283.4 eV indicates a MWCNT C–C bond in MoO3@MWCNT nanocomposites, as shown in Fig. 2 A. Additionally, there is a C–O band based on the peak at 285.5 eV. Fig. 2 B shows the high resolution spectra of Mo3d. The existence of two peaks for Mo3d5/2 and Mo3d3/2 at 232.6 and 235.7 eV, respectively, indicate the presence of Mo6+ in MoO3@MWCNT nanocomposites [25–28]. Fig. 2C displays high resolution spectra of O1s. The peaks at 530.4 and 532.3 eV for O1s verify the presence of Mo–O and C–O bonds, respectively [29-30]. These results confirm that the carbon atoms of MWCNTs are linked covalently to oxygen atoms of MoO3 nanoparticles [29-30].
2.3. Characterization techniques A Bruker axis D8 was employed to measure the X-ray diffraction patterns of mesoporous molybdenum oxide and MoO3@MWCNTs samples using Cu Kα radiation with λ equal to 1.540 Å. A Nova 2000 series Chromatech apparatus was used to measure the surface area and adsorption-desorption isotherm of molybdenum oxide and MoO3@ MWCNT samples. The specific surface area was calculated from N2adsorption measurements, which were obtained using a Nova 2000 series Chromatech apparatus at 77 K. Prior to the measurement, the mesoporous molybdenum oxide and MoO3@MWCNTs samples were evacuated for 2 h at 200 °C. A UV/Vis/NIR spectrophotometer model V570, JASCO, Japan was used to measure the UV–Vis spectra of molybdenum oxide and MoO3@MWCNT samples. The spectra were used to calculate the bandgap energy of molybdenum oxide and MoO3@ MWCNT samples. A JEOL-JEM-1230 microscope was used for transmission electron microscopy to determine the structural morphology of molybdenum oxide and MoO3@MWCNT samples. Previously, molybdenum oxide and MoO3@MWCNT samples were scattered in a solution of ethyl alcohol using an ultrasonic bath for 30 min. A small part of this suspension was put on a carbon-coated copper grid, and after drying the copper grid, it was put on a TEM device. A Shimadzu RF5301 fluorescence spectrophotometer was used to measure the photoluminescence emission spectra of molybdenum oxide and MoO3@ MWCNT samples. A Thermo Scientific K-ALPHA, XPS, England was used to measure the state of element of molybdenum oxide and MoO3@ MWCNT samples. A Zahner Zennium electrochemical workstation was used to measure the transient photocurrent of molybdenum oxide and MoO3@MWCNT samples. 2.4. Photocatalysis experiment The photocatalytic reduction of chromium (VI) under visible light was selected as a model to investigate the photocatalytic performance of molybdenum oxide and MoO3@MWCNT samples. A xenon lamp with 300 W light power with UV cut filter was used to produce visible light for irradiation of molybdenum oxide and MoO3@MWCNT samples. A
Fig. 1. XRD patterns of MoO3 and MoO3@MWCNT samples. 2
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Fig. 2. High-resolution XPS spectra of Mo3d(A), O1s(B), and C1s(C) of the MoO3-3wt % MWCNT sample.
Fig. 4. HRTEM image of the MoO3 -3wt % MWCNT sample. Fig. 3. TEM images of MWCNTs(A), MoO3(B) and MoO3@MWCNT (C) samples.
which proves the existence of mesoporous materials in MoO3@MWCNT nanocomposite. The BET surface area of MoO3 and MoO3@MWCNT samples are displayed in Table 1. The BET values are 130, 137, 140, 142 and 143 m2/g for MoO3 and MoO3-1 wt % MWCNTs, MoO3-2 wt % MWCNTs, MoO3-3 wt % MWCNTs and MoO3-4 wt % MWCNTs, respectively. The MoO3 has a high surface area (130 m2/g) due to the preparation of mesoporous MoO3 by our new method. In addition, MoO3@MWCNT nanocomposites have a BET surface area larger than that of MoO3 sample due to the addition of high surface area materials (MWCNT) to low surface area materials (MoO3). Fig. 6 shows UV–Vis spectra of MoO3 and MoO3@MWCNT samples. All samples absorbed in the visible region, and addition of MWCNT to MoO3 shifted absorption edge of MoO3 to higher wavelengths. The UV–Vis spectra of MoO3 and MoO3@MWCNT samples were used for the determination of the corresponding bandgap energy [31], shown in
Fig. 3 displays the TEM images of MWCNTs (A), MoO3 (B) and MoO3@MWCNT (C) samples. The results demonstrated that the MoO3 sample has a nanoparticle shape with 40 nm average particle size as shown in Fig. 3 A. Fig. 3 B shows the shape of MWCNTs with a 40 nm average particle size. Fig. 3C shows the shape of MoO3@MWCNT sample. The results show that MoO3 was scattered on the MWCNT surface (see Fig. 4). The HRTEM image of MoO3 3% wt MWCNT sample shows d-spacing for (200) at 0.370 nm, proving the presence of MoO3 in MoO3@ MWCNT nanocomposites. The existence of d-spacing for (002) at 0.330 nm proves the presence of MWCNTs in MoO3@MWCNT nanocomposites as shown in Fig. 4. Fig. 5 shows adsorption-desorption isotherm of MoO3 3% wt MWCNT samples. The isotherm of MoO3 3% wt MWCNTs is type IV, 3
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Fig. 5. Adsorption-desorption isotherm of the MoO3 -3wt % MWCNT sample. Fig. 7. PL spectra of MoO3 and the MoO3@MWCNT samples. Table 1 BET surface area of MoO3 and MoO3-MWCNT samples.
MoO3 MoO3-1wt MoO3-2wt MoO3-3wt MoO3-4wt
MoO3 was reduced by the addition of the MWCNTs, thus improving their photocatalytic performance in the reduction of chromium (VI) under visible light, as discussed in the photocatalytic section. Fig. 7 displays the PL spectra of MoO3 and MoO3@MWCNT samples. All samples have emission peaks in the visible region, and addition of MWCNT to MoO3 shifted the emission peak of MoO3 to a higher wavelength. The PL spectra of MoO3 and MoO3@MWCNT samples were used to determine the corresponding bandgap energy. The outcomes band gap values are 3.01, 2.31, 2.28, 2.21 and 2.19 eV for MoO3 and MoO3-1 wt % MWCNTs, MoO3-2 wt % MWCNTs, MoO3-3 wt % MWCNTs and MoO3-4 wt % MWCNTs, respectively. The values of bandgap energy estimated from PL spectra are very close to the values estimated from UV–Vis spectra. In addition, the band gap of the MoO3 sample can be adapted by adjusting the weight percent of MWCNTs. The band gap of MoO3 was reduced by addition of MWCNTs, improving their photocatalytic performance in the reduction of chromium (VI) under visible light, as discussed in the photocatalytic section. Fig. 8 displays the photocurrent response of MoO3 and MoO3@ MWCNT samples. The results reveal that MoO3@MWCNT samples have photocurrent response values greater than the MoO3 sample, which means MoO3@MWCNTs samples have a low electron-hole recombination rate. In other words, the electron-hole recombination rate of the MoO3 sample can be adapted by adjusting the weight percent of MWCNTs. Additionally, the electron-hole recombination rate of MoO3
BET surface area, m2/g
Sample
% % % %
MWCNT MWCNT MWCNT MWCNT
130.00 137.00 140.00 142.00 143.00
Fig. 6. UV–Vis spectra of MoO3 and the MoO3@MWCNT samples. Table 2 Band gap energy of MoO3 and MoO3 -MWCNT samples. Sample MoO3 MoO3-1wt MoO3-2wt MoO3-3wt MoO3-4wt
Band gap energy, eV
% % % %
MWCNT MWCNT MWCNT MWCNT
3.00 2.30 2.27 2.20 2.18
Table 2. The band gap values are 3.00, 2.30, 2.27, 2.20 and 2.18 eV for MoO3 and MoO3-1 wt % MWCNTs, MoO3-2 wt % MWCNTs, MoO3-3 wt % MWCNTs and MoO3-4 wt % MWCNTs, respectively. Therefore, the results show that the band gap of the MoO3 sample can be altered by changing the weight percent of MWCNTs. Additionally, the band gap of
Fig. 8. Photocurrent response of MoO3 and the MoO3@MWCNT samples. 4
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Fig. 9. Performance of MWCNTs, MoO3 and the MoO3@MWCNT nanocomposites in Cr(VI) reduction using visible light. Fig. 10. High-resolution XPS spectra Cr2p of the used MoO3-3wt % MWCNT sample.
was increased by the addition of MWCNTs, improving their photocatalytic performance for reduction of chromium (VI) under visible light, as discussed in the photocatalytic section.
3.2. Photocatalytic performance Fig. 9 shows the photocatalytic performance of MWCNTs, MoO3 and MoO3@MWCNT nanocomposites for Cr(VI) reduction using visible light. The photocatalytic experiment was performed using K2Cr2O7 as a source of Cr(VI), 500 mL potassium chromate, 125 ppm Cr(VI) concentration, 60 min reaction time and 0.8 g/L dose of photocatalyst. The results showed that MWCNTs and MoO3 samples have 0.4 and 14% photocatalytic performance for reduction of Cr(VI), respectively. Therefore, MWCNTs have no photocatalytic activity, and MoO3 has minimal photocatalytic activity due to its high electron-hole recombination rate. However, MoO3@MWCNT nanocomposites have high photocatalytic ability to reduce Cr(VI). Increasing the weight percent MWCNTs from 1 to 3% wt improved the photocatalytic performance, reducing Cr(VI) from 55 to 98%, respectively. Addition of MWCNT to MoO3 decreases the bandgap energy of MoO3, reduces the electron-hole recombination rate and increases the surface area. The photocatalytic performance for reducing Cr(VI) is almost unchanged with further increase in the weight percent of MWCNT. To prove the reduction of Cr(VI) to Cr(III) by MoO3@MWCNT nanocomposites, the sample of MoO3 3% wt MWCNT after testing for reduction of Cr(VI) was investigated using a Thermo Scientific K-ALPHA, XPS (England) to quantify the Cr(VI) or Cr(III) in the sample. The outcomes are shown in Fig. 10 and indicate the presence of Cr(III) on the surface of MoO3 3% wt MWCNT sample through the presence of two peaks for Cr 2p1/2 and Cr 2p3/2. Fig. 11 displays the effect of dose of MoO3 3% wt MWCNT on the photocatalytic performance of MoO3 3% wt MWCNT for Cr(VI) reduction using visible light. The photocatalytic experiment was achieved using K2Cr2O7 as source of (VI), 500 mL potassium chromate, 125 ppm Cr(VI) concentration, 60 min reaction time and MoO3 3% wt MWCNT photocatalyst. The results demonstrated that photocatalytic performance for reducing Cr(VI) improved from 74 to 98% after 60 min by increasing the dose of the MoO3 3% wt MWCNT photocatalyst from 0.4 g/L to 0.8 g/L. In addition, increasing the MoO3 3% wt MWCNT photocatalyst from 1.2 g/L to 1.6 g/L decreased reaction time from 50 min to 30 min. This result is obtained because the active sites of MoO3@MWCNT nanocomposites, which are required for photocatalytic reduction of Cr(VI), are increased by increasing the amount of photocatalyst used. Additionally, the reaction time required for complete
Fig. 11. Effect of dose of MoO3-3 wt % MWCNT on photocatalytic performance in Cr(VI) reduction using visible light.
reduction of Cr(VI) increased from 30 min to 50 min with an increased dose of photocatalyst from 1.6 g/L to 2.0 g/L because the increased amount of photocatalyst reduces the ability of light to reach the surface of the photocatalyst, consequently hindering the photocatalytic process and increasing the reaction time. Fig. 12 displays the effect of reuse of MoO3 3% wt MWCNT on the photocatalytic performance of MoO3 3% wt MWCNT for Cr(VI) reduction using visible light. The photocatalytic experiment was performed using K2Cr2O7 as the source of Cr(VI), 500 mL potassium chromate, 125 ppm Cr(VI) concentration, 30 min reaction time, and 1.6 g/L MoO3 3% wt MWCNT photocatalyst. The MoO3 3% wt MWCNT photocatalyst can be used for the reduction of Cr(VI) five times without loss of photocatalytic activity. Therefore, MoO3 3% wt MWCNT photocatalysts have stable photoreduction activity on Cr(VI).
4. Conclusions A simple one-step sol-gel method was used to prepare mesoporous molybdenum oxide using a structure-directing agent of F127 triblock 5
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[3] [4]
[5] [6]
[7]
[8]
[9]
[10]
[11]
Fig. 12. Effect of reuse of MoO3-3 wt % MWCNT on photocatalytic performance in Cr(VI) reduction using visible light.
[12]
[13]
copolymer for the first time. MoO3@MWCNT nanocomposites with 1%, 2%, 3%, and 4% weight fractions of MWCNT were prepared using the impregnation method. The chemical and physical properties of MoO3 and MoO3@MWCNT samples were investigated using many techniques, such as TEM, XRD, photocurrent response, PL and UV–Vis. The TEM results showed that MoO3 has a nanoparticle shape, and MoO3 nanoparticles were well dispersed on the surface of MWCNTs. The bandgap of the MoO3 sample can be altered by adjusting the weight percent of MWCNTs. Additionally, the bandgap of MoO3 was reduced by the addition of MWCNTs, improving their photocatalytic performance in reduction of chromium (VI) under visible light. Finally, MoO3@MWCNT nanocomposites were used multiple times for reduction of chromium (VI) before loss of photocatalytic performance, which means they have high photocatalytic stability. The MoO3 3% wt MWCNT photocatalyst can reduce 100% of Cr(VI) within 30 min at 1.6 g/L.
[14]
[15] [16] [17] [18] [19]
[20] [21] [22]
Declaration of competing interest [23]
On behalf of all authors, the corresponding author states that there is no conflict of interest.
[24]
Acknowledgments
[25]
The authors would like to express their gratitude to King Khalid University, Saudi Arabia, for providing administrative and technical support.
[26]
Appendix A. Supplementary data
[28]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.11.187.
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[27]
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Decoration of MoO3 nanoparticles by MWCNTs driven visible light for the reduction of Cr(VI) K.S. Al-Namshaha,∗, R.M. Mohamedb,c,∗∗ a
College of Science, Main Campus King Abdullah Rd, King Khalid University, Abha, Saudi Arabia Department of Chemistry, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia c Advanced Materials Department, Central Metallurgical R&D Institute, CMRDI, P.O. Box 87, Helwan, Cairo, Egypt b
A R T I C LE I N FO
A B S T R A C T
Keywords: Chromium (VI) MoO3 MWCNTs Photocatalyst Visible light
A simple one-step sol-gel method was used to prepare mesoporous molybdenum oxide using a F127 triblock copolymer structure-directing agent for the first time. MoO3@MWCNT nanocomposites with 1%, 2%, 3%, and 4% weight MWCNT were prepared using the impregnation method. MoO3 and MoO3@MWCNT nanocomposites were analyzed to identify their physical and chemical properties. The TEM results showed that MoO3 has a nanoparticle shape, and MoO3 nanoparticles were well dispersed on the surface of MWCNTs. The bandgap of the MoO3 sample can be adapted by adjusting the weight percent of MWCNTs. Additionally, the band gap of MoO3 was reduced by the addition of MWCNTs and improved their photocatalytic reduction of chromium (VI) under visible light. Finally, MoO3@MWCNT nanocomposites were used to reduce chromium (VI) beyond the loss of photocatalytic performance, which means they have high photocatalytic stability.
1. Introduction In recent years, purification of the environment has been a focus of scientific research. One of the most demanding tasks for scientists is finding photocatalysts that can completely degrade pollutants in the air or water [1–8]. Excellent photocatalytic materials should be stable, nontoxic and easy to fabricate, and they should have a high surface area and absorb in the visible region [4,5]. Many photocatalysts have been used for degradation of pollutants using UV and visible light, such as Bi2O3/Bi2O4-x, TiO2, NaNbO3, WO3 and Bi12TiO20 [9–13]. Titanium dioxide is considered the most predictable photocatalyst due to its high stability, nontoxicity and low cost. However, the widespread application of titanium dioxide in photocatalysts and photovoltaics is lacking due to its poor quantum yield and poor sunlight absorption ability as a result of its high electron-hole recombination and wide bandgap energy, respectively [14–16]. Consequently, it is essential to synthesize a photocatalyst that efficiently absorbs visible light and prevents electron-hole recombination. A variety of methods have been used to expand the absorption of photocatalysts from UV to the visible region, such as metal and nonmetal doping, coupling with semiconductors, or coupling with CNTS (carbon nanotubes) [10, 11, 17–20]. Therefore, one of the most demanding tasks for researchers is fabricating a photocatalyst with a specific structural arrangement to make it has high
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photocatalytic activity using natural sunlight and visible light irradiation. The lack of a specific structural arrangement may delay the proper trading of chemical and electron species between the different active ingredients. To construct powerful and viable photocatalysts using visible and sunlight radiation with high stability and reactivity, their bandgap energy must be narrow, or their value must be less than 3.0 eV [21–23]. MoO3 (molybdenum oxide) is a photocatalysts that meets these requirements and can be excited using visible or UV light [23]. Due to its high electron-hole recombination rate and its poor stability, the widespread application of molybdenum oxide as a visible photocatalyst is limited. Therefore, the reduction of the electron-hole recombination rate and improvement of the stability of molybdenum oxide are essential for developing molybdenum oxide photocatalytic activity. Thus, to construct powerful and viable photocatalysts using visible and sunlight radiation with high stability and reactivity, we established the synthesis and characterization of MoO3-MWCNT nanocomposites. The addition of MWCNTs to molybdenum oxide photocatalysts develops charge separation and enhances the stability of molybdenum oxide. To our knowledge, there are published studies about preparation of mesoporous MoO3-MWCNT nanocomposites. Here, we prepare, for the first time, mesoporous MoO3-MWCNT nanocomposites for photocatalytic reduction of Cr(VI) using visible light.
Corresponding author. College of Science, Main Campus King Abdullah Rd, King Khalid University, Abha, Saudi Arabia. Corresponding author. Department of Chemistry, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia. E-mail addresses: [email protected] (K.S. Al-Namshah), [email protected] (R.M. Mohamed).
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https://doi.org/10.1016/j.ceramint.2019.11.187 Received 12 November 2019; Received in revised form 19 November 2019; Accepted 20 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: K.S. Al-Namshah and R.M. Mohamed, Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.187
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2. Experimental
horizontal cylinder rounded batch reactor was used for the photocatalytic reaction. First, molybdenum oxide or MoO3@MWCNT sample was scattered in an aqueous solution of 500 mL potassium chromite with 150 ppm chromium. The photocatalytic reaction was performed at room temperature. The concentration of chromium (VI) was determined using a V-570, JASCO (Japan) UV–Vis spectrophotometer, and its absorbance was measured at 540 nm according to the standard diphenylcarbazide method [24]. The photoreduction performance of chromium (VI) as a percentage was calculated by applying the equation:
2.1. Preparation of mesoporous MoO3 nanoparticles A simple one-step sol-gel method was used to prepare mesoporous molybdenum oxide using F127 triblock copolymer as a structure-directing agent. First, 7.0 mL of acetic acid, 2.5 g of F127, and 0.80 mL of hydrochloric acid were dissolved in 50 mL of ethyl alcohol. Then, molybdic acid (1.5 g) was added to the solution, and the resulting mixture was stirred at room temperature for 60 min and placed in a Petri dish to evaporate the ethyl alcohol for 12 h under 40% humidity at a temperature of 40 °C in an oven; then, the samples were dried at 65 °C for 24 h. Finally, the samples were calcined in air for 4 h at 450 °C by using 1 °C/min and 2 °C/min as the heating and cooling rates, respectively, to obtain mesoporous molybdenum oxide after removing the F127 surfactant.
% photoreduction performance = (Co - Ct)/Co × 100 where Co is chromium (VI) concentration at the initial time and Ct is chromium (VI) concentration at t time.
2.2. Preparation of MoO3@MWCNT nanocomposite
3. Results & discussion
Mesoporous molybdenum oxide samples were dispersed in a solution of ethyl alcohol using an ultrasonic bath for 10 min to gather mesoporous molybdenum oxide on the surface of MWCNTs. To evaporate the ethyl alcohol, the resulting mixture was dried in air for 12 h at 60 °C to obtain mesoporous MoO3@MWCNT nanocomposites with the mesoporous molybdenum oxide well attached on the surface of MWCNTs. The relative amount of MWCNTs was changed to prepare different samples of MoO3@MWCNT nanocomposites with 1%, 2%, 3%, and 4% weight fractions of MWCNT.
3.1. Characterization of photocatalysts The XRD patterns of MoO3 and MoO3@MWCNTs samples are shown in Fig. 1. For the MWCNT sample, XRD results showed a peak at 2-theta equal to 25.8°, which verifies the presence of (002) of MWCNT, in keeping with JCPDS card no. 26-1079. For the MoO3 sample, XRD results showed molybdenum oxide phase and XRD diffraction peaks consistent with JCPDS card no. 05-0506. For MoO3@MWCNT samples, the results showed only peaks of MoO3 for all MoO3@MWCNT samples. Additionally, there are no peaks for MWCNTs for all MoO3@MWCNTs samples, likely due to the low content of MWCNTs in MoO3@MWCNT samples. In addition, the presence of MWCNTs in MoO3@MWCNT samples hinders the crystallization of MoO3 and hence reduces its crystallite size as shown in Fig. 1. Fig. 2 displays high-resolution XPS spectra of Mo3d, O1s, and C1s of MoO3-3% wt MWCNT samples. The binding energy for C1s at 283.4 eV indicates a MWCNT C–C bond in MoO3@MWCNT nanocomposites, as shown in Fig. 2 A. Additionally, there is a C–O band based on the peak at 285.5 eV. Fig. 2 B shows the high resolution spectra of Mo3d. The existence of two peaks for Mo3d5/2 and Mo3d3/2 at 232.6 and 235.7 eV, respectively, indicate the presence of Mo6+ in MoO3@MWCNT nanocomposites [25–28]. Fig. 2C displays high resolution spectra of O1s. The peaks at 530.4 and 532.3 eV for O1s verify the presence of Mo–O and C–O bonds, respectively [29-30]. These results confirm that the carbon atoms of MWCNTs are linked covalently to oxygen atoms of MoO3 nanoparticles [29-30].
2.3. Characterization techniques A Bruker axis D8 was employed to measure the X-ray diffraction patterns of mesoporous molybdenum oxide and MoO3@MWCNTs samples using Cu Kα radiation with λ equal to 1.540 Å. A Nova 2000 series Chromatech apparatus was used to measure the surface area and adsorption-desorption isotherm of molybdenum oxide and MoO3@ MWCNT samples. The specific surface area was calculated from N2adsorption measurements, which were obtained using a Nova 2000 series Chromatech apparatus at 77 K. Prior to the measurement, the mesoporous molybdenum oxide and MoO3@MWCNTs samples were evacuated for 2 h at 200 °C. A UV/Vis/NIR spectrophotometer model V570, JASCO, Japan was used to measure the UV–Vis spectra of molybdenum oxide and MoO3@MWCNT samples. The spectra were used to calculate the bandgap energy of molybdenum oxide and MoO3@ MWCNT samples. A JEOL-JEM-1230 microscope was used for transmission electron microscopy to determine the structural morphology of molybdenum oxide and MoO3@MWCNT samples. Previously, molybdenum oxide and MoO3@MWCNT samples were scattered in a solution of ethyl alcohol using an ultrasonic bath for 30 min. A small part of this suspension was put on a carbon-coated copper grid, and after drying the copper grid, it was put on a TEM device. A Shimadzu RF5301 fluorescence spectrophotometer was used to measure the photoluminescence emission spectra of molybdenum oxide and MoO3@ MWCNT samples. A Thermo Scientific K-ALPHA, XPS, England was used to measure the state of element of molybdenum oxide and MoO3@ MWCNT samples. A Zahner Zennium electrochemical workstation was used to measure the transient photocurrent of molybdenum oxide and MoO3@MWCNT samples. 2.4. Photocatalysis experiment The photocatalytic reduction of chromium (VI) under visible light was selected as a model to investigate the photocatalytic performance of molybdenum oxide and MoO3@MWCNT samples. A xenon lamp with 300 W light power with UV cut filter was used to produce visible light for irradiation of molybdenum oxide and MoO3@MWCNT samples. A
Fig. 1. XRD patterns of MoO3 and MoO3@MWCNT samples. 2
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Fig. 2. High-resolution XPS spectra of Mo3d(A), O1s(B), and C1s(C) of the MoO3-3wt % MWCNT sample.
Fig. 4. HRTEM image of the MoO3 -3wt % MWCNT sample. Fig. 3. TEM images of MWCNTs(A), MoO3(B) and MoO3@MWCNT (C) samples.
which proves the existence of mesoporous materials in MoO3@MWCNT nanocomposite. The BET surface area of MoO3 and MoO3@MWCNT samples are displayed in Table 1. The BET values are 130, 137, 140, 142 and 143 m2/g for MoO3 and MoO3-1 wt % MWCNTs, MoO3-2 wt % MWCNTs, MoO3-3 wt % MWCNTs and MoO3-4 wt % MWCNTs, respectively. The MoO3 has a high surface area (130 m2/g) due to the preparation of mesoporous MoO3 by our new method. In addition, MoO3@MWCNT nanocomposites have a BET surface area larger than that of MoO3 sample due to the addition of high surface area materials (MWCNT) to low surface area materials (MoO3). Fig. 6 shows UV–Vis spectra of MoO3 and MoO3@MWCNT samples. All samples absorbed in the visible region, and addition of MWCNT to MoO3 shifted absorption edge of MoO3 to higher wavelengths. The UV–Vis spectra of MoO3 and MoO3@MWCNT samples were used for the determination of the corresponding bandgap energy [31], shown in
Fig. 3 displays the TEM images of MWCNTs (A), MoO3 (B) and MoO3@MWCNT (C) samples. The results demonstrated that the MoO3 sample has a nanoparticle shape with 40 nm average particle size as shown in Fig. 3 A. Fig. 3 B shows the shape of MWCNTs with a 40 nm average particle size. Fig. 3C shows the shape of MoO3@MWCNT sample. The results show that MoO3 was scattered on the MWCNT surface (see Fig. 4). The HRTEM image of MoO3 3% wt MWCNT sample shows d-spacing for (200) at 0.370 nm, proving the presence of MoO3 in MoO3@ MWCNT nanocomposites. The existence of d-spacing for (002) at 0.330 nm proves the presence of MWCNTs in MoO3@MWCNT nanocomposites as shown in Fig. 4. Fig. 5 shows adsorption-desorption isotherm of MoO3 3% wt MWCNT samples. The isotherm of MoO3 3% wt MWCNTs is type IV, 3
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Fig. 5. Adsorption-desorption isotherm of the MoO3 -3wt % MWCNT sample. Fig. 7. PL spectra of MoO3 and the MoO3@MWCNT samples. Table 1 BET surface area of MoO3 and MoO3-MWCNT samples.
MoO3 MoO3-1wt MoO3-2wt MoO3-3wt MoO3-4wt
MoO3 was reduced by the addition of the MWCNTs, thus improving their photocatalytic performance in the reduction of chromium (VI) under visible light, as discussed in the photocatalytic section. Fig. 7 displays the PL spectra of MoO3 and MoO3@MWCNT samples. All samples have emission peaks in the visible region, and addition of MWCNT to MoO3 shifted the emission peak of MoO3 to a higher wavelength. The PL spectra of MoO3 and MoO3@MWCNT samples were used to determine the corresponding bandgap energy. The outcomes band gap values are 3.01, 2.31, 2.28, 2.21 and 2.19 eV for MoO3 and MoO3-1 wt % MWCNTs, MoO3-2 wt % MWCNTs, MoO3-3 wt % MWCNTs and MoO3-4 wt % MWCNTs, respectively. The values of bandgap energy estimated from PL spectra are very close to the values estimated from UV–Vis spectra. In addition, the band gap of the MoO3 sample can be adapted by adjusting the weight percent of MWCNTs. The band gap of MoO3 was reduced by addition of MWCNTs, improving their photocatalytic performance in the reduction of chromium (VI) under visible light, as discussed in the photocatalytic section. Fig. 8 displays the photocurrent response of MoO3 and MoO3@ MWCNT samples. The results reveal that MoO3@MWCNT samples have photocurrent response values greater than the MoO3 sample, which means MoO3@MWCNTs samples have a low electron-hole recombination rate. In other words, the electron-hole recombination rate of the MoO3 sample can be adapted by adjusting the weight percent of MWCNTs. Additionally, the electron-hole recombination rate of MoO3
BET surface area, m2/g
Sample
% % % %
MWCNT MWCNT MWCNT MWCNT
130.00 137.00 140.00 142.00 143.00
Fig. 6. UV–Vis spectra of MoO3 and the MoO3@MWCNT samples. Table 2 Band gap energy of MoO3 and MoO3 -MWCNT samples. Sample MoO3 MoO3-1wt MoO3-2wt MoO3-3wt MoO3-4wt
Band gap energy, eV
% % % %
MWCNT MWCNT MWCNT MWCNT
3.00 2.30 2.27 2.20 2.18
Table 2. The band gap values are 3.00, 2.30, 2.27, 2.20 and 2.18 eV for MoO3 and MoO3-1 wt % MWCNTs, MoO3-2 wt % MWCNTs, MoO3-3 wt % MWCNTs and MoO3-4 wt % MWCNTs, respectively. Therefore, the results show that the band gap of the MoO3 sample can be altered by changing the weight percent of MWCNTs. Additionally, the band gap of
Fig. 8. Photocurrent response of MoO3 and the MoO3@MWCNT samples. 4
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Fig. 9. Performance of MWCNTs, MoO3 and the MoO3@MWCNT nanocomposites in Cr(VI) reduction using visible light. Fig. 10. High-resolution XPS spectra Cr2p of the used MoO3-3wt % MWCNT sample.
was increased by the addition of MWCNTs, improving their photocatalytic performance for reduction of chromium (VI) under visible light, as discussed in the photocatalytic section.
3.2. Photocatalytic performance Fig. 9 shows the photocatalytic performance of MWCNTs, MoO3 and MoO3@MWCNT nanocomposites for Cr(VI) reduction using visible light. The photocatalytic experiment was performed using K2Cr2O7 as a source of Cr(VI), 500 mL potassium chromate, 125 ppm Cr(VI) concentration, 60 min reaction time and 0.8 g/L dose of photocatalyst. The results showed that MWCNTs and MoO3 samples have 0.4 and 14% photocatalytic performance for reduction of Cr(VI), respectively. Therefore, MWCNTs have no photocatalytic activity, and MoO3 has minimal photocatalytic activity due to its high electron-hole recombination rate. However, MoO3@MWCNT nanocomposites have high photocatalytic ability to reduce Cr(VI). Increasing the weight percent MWCNTs from 1 to 3% wt improved the photocatalytic performance, reducing Cr(VI) from 55 to 98%, respectively. Addition of MWCNT to MoO3 decreases the bandgap energy of MoO3, reduces the electron-hole recombination rate and increases the surface area. The photocatalytic performance for reducing Cr(VI) is almost unchanged with further increase in the weight percent of MWCNT. To prove the reduction of Cr(VI) to Cr(III) by MoO3@MWCNT nanocomposites, the sample of MoO3 3% wt MWCNT after testing for reduction of Cr(VI) was investigated using a Thermo Scientific K-ALPHA, XPS (England) to quantify the Cr(VI) or Cr(III) in the sample. The outcomes are shown in Fig. 10 and indicate the presence of Cr(III) on the surface of MoO3 3% wt MWCNT sample through the presence of two peaks for Cr 2p1/2 and Cr 2p3/2. Fig. 11 displays the effect of dose of MoO3 3% wt MWCNT on the photocatalytic performance of MoO3 3% wt MWCNT for Cr(VI) reduction using visible light. The photocatalytic experiment was achieved using K2Cr2O7 as source of (VI), 500 mL potassium chromate, 125 ppm Cr(VI) concentration, 60 min reaction time and MoO3 3% wt MWCNT photocatalyst. The results demonstrated that photocatalytic performance for reducing Cr(VI) improved from 74 to 98% after 60 min by increasing the dose of the MoO3 3% wt MWCNT photocatalyst from 0.4 g/L to 0.8 g/L. In addition, increasing the MoO3 3% wt MWCNT photocatalyst from 1.2 g/L to 1.6 g/L decreased reaction time from 50 min to 30 min. This result is obtained because the active sites of MoO3@MWCNT nanocomposites, which are required for photocatalytic reduction of Cr(VI), are increased by increasing the amount of photocatalyst used. Additionally, the reaction time required for complete
Fig. 11. Effect of dose of MoO3-3 wt % MWCNT on photocatalytic performance in Cr(VI) reduction using visible light.
reduction of Cr(VI) increased from 30 min to 50 min with an increased dose of photocatalyst from 1.6 g/L to 2.0 g/L because the increased amount of photocatalyst reduces the ability of light to reach the surface of the photocatalyst, consequently hindering the photocatalytic process and increasing the reaction time. Fig. 12 displays the effect of reuse of MoO3 3% wt MWCNT on the photocatalytic performance of MoO3 3% wt MWCNT for Cr(VI) reduction using visible light. The photocatalytic experiment was performed using K2Cr2O7 as the source of Cr(VI), 500 mL potassium chromate, 125 ppm Cr(VI) concentration, 30 min reaction time, and 1.6 g/L MoO3 3% wt MWCNT photocatalyst. The MoO3 3% wt MWCNT photocatalyst can be used for the reduction of Cr(VI) five times without loss of photocatalytic activity. Therefore, MoO3 3% wt MWCNT photocatalysts have stable photoreduction activity on Cr(VI).
4. Conclusions A simple one-step sol-gel method was used to prepare mesoporous molybdenum oxide using a structure-directing agent of F127 triblock 5
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[3] [4]
[5] [6]
[7]
[8]
[9]
[10]
[11]
Fig. 12. Effect of reuse of MoO3-3 wt % MWCNT on photocatalytic performance in Cr(VI) reduction using visible light.
[12]
[13]
copolymer for the first time. MoO3@MWCNT nanocomposites with 1%, 2%, 3%, and 4% weight fractions of MWCNT were prepared using the impregnation method. The chemical and physical properties of MoO3 and MoO3@MWCNT samples were investigated using many techniques, such as TEM, XRD, photocurrent response, PL and UV–Vis. The TEM results showed that MoO3 has a nanoparticle shape, and MoO3 nanoparticles were well dispersed on the surface of MWCNTs. The bandgap of the MoO3 sample can be altered by adjusting the weight percent of MWCNTs. Additionally, the bandgap of MoO3 was reduced by the addition of MWCNTs, improving their photocatalytic performance in reduction of chromium (VI) under visible light. Finally, MoO3@MWCNT nanocomposites were used multiple times for reduction of chromium (VI) before loss of photocatalytic performance, which means they have high photocatalytic stability. The MoO3 3% wt MWCNT photocatalyst can reduce 100% of Cr(VI) within 30 min at 1.6 g/L.
[14]
[15] [16] [17] [18] [19]
[20] [21] [22]
Declaration of competing interest [23]
On behalf of all authors, the corresponding author states that there is no conflict of interest.
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Acknowledgments
[25]
The authors would like to express their gratitude to King Khalid University, Saudi Arabia, for providing administrative and technical support.
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ceramint.2019.11.187.
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References [31]
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