Yang Wanga,
Jiaxu Liua,
Ye Wanga and
Mingyi Zhang*ab
aKey Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, PR China. E-mail: zhangmingyi@hrbnu.edu.cn; mysci@foxmail.com
bSchool of Materials Science and Engineering, Zhengzhou University, Zhengzhou, 45001, PR China
First published on 28th February 2020
In this work, Bi2W0.5Mo0.5O6 solid solution nanotubes have been synthesized through a structure-directing hard template approach, which demonstrated greatly enhanced CO2 photoreduction to CO/CH4. The crystalline phase, components and morphologies of the as-prepared composites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The present design of Bi2W0.5Mo0.5O6 solid solution nanotubes leads to remarkably enhanced photocatalytic activities with a peak CO/CH4 production rate of 6.55/3.75 mmol g−1 h−1 under visible light irradiation at room temperature, which was about 7 times that on pure Bi2WO6 and Bi2MoO6 nanotubes, respectively. Hollow nanotubular structures and synergistic electronic effects of various elements contribute to the enhanced visible light photocatalytic activity of Bi2W0.5Mo0.5O6 solid solution nanotubes.
Oxide semiconductors with Aurivillius structures are of immense importance due to their layered structures and unique properties.5–16 Among these semiconductors, Bi2WO6 is significant because of its excellent intrinsic physico-chemical properties. In order to further improve the utilization of visible light, Bi2WO6-based heterojunction composites have been developed.17–23 Among the strategies to improve the photocatalytic efficiency of Bi2WO6, it is an effective method to construct the solid solution photocatalyst. Based on the above consideration, the construction of Bi2MoxW1−xO6 solid solutions attracts our interest in particular because of their structural analogy. That is, (1) Bi2MoO6 has a similar Aurivillius layered structure to that of Bi2WO6. Compared with Bi2WO6, Bi2MoO6 with a narrower band gap has the ability to harness more sunlight. (2) The W component can endow the material with high photocatalytic activity, whereas the Mo constituent contributes to the narrow band gap, it is reasonable to postulate that the Bi2MoxW1−xO6 solid solutions may provide a suitable option.
In addition, many works show that the hollow structure has many useful functions: (1) the hollow structure is conducive to the rapid transfer of substances in its internal space; (2) to make its high surface volume ratio to absorb a large number of chemical substances; (3) the unique structure can make light in its internal space for multiple reflections, thus improving the utilization rate of light. To the best of our knowledge, the fabrication and photocatalytic property of tubes-structured Bi2MoxW1−xO6 solid solutions has not yet been reported.
In this paper, we report the synthesis of the Bi2Mo0.5W0.5O6 solid solution nanotubes using PAN nanofibers as the hard template through solvothermal method. Photocatalytic experiments showed that, compared with Bi2MoO6 and Bi2WO6 nanotubes, the Bi2Mo0.5W0.5O6 solid solution nanotubes exhibited excellent photocatalytic activity, which significantly improved the catalytic activity of CO2 photoreduction.
Fig. 1 (a) SEM image of PAN nanofibers. (b) SEM image of the PAN/BWMO nanofibers. (c) SEM image of the BWMO NTs. (d) TEM image of the BWMO NTs. |
In order to further clarify the composition distribution and structural characteristics of the product, TEM-EDX scanning along the BWMO NTs radial direction (red line in the inset of Fig. 2) was used to obtain the spatial distribution of the components of the nanotube structure. Three signal peaks for Bi, W and Mo is found in the wall region. This is consistent with the BWMO NTs configuration observed in the SEM and TEM image. In addition, EDX analysis indicates that the molar ratio of W and Mo is about 1:1.05 for the BWMO NTs.
From the XRD patterns (Fig. 3), we can see that the PAN nanofibers are poorly crystallized. As aforementioned, Bi2MoO6 and Bi2WO6 are Aurivillius structures so they have same crystal structure in the normal conditions. Therefore, XRD spectra cannot be utilized to distinguish between these two Aurivillius structures since their peaks are obtained at almost the same diffraction angles. As for the pattern of PAN/BWMO hybrid nanofibers, some strong diffraction peaks can be perfectly indexed to Bi2MoO6 or Bi2WO6 (JCPDS 76-2388), indicate the formation of Bi2W0.5Mo0.5O6 solid solution. The XRD pattern of the BWMO NTs shows higher intensity and narrower diffraction peaks, implying the leading role of calcination in the enhancement of crystallization.
Fig. 4 displays the UV-Vis diffuse reflectance (DR) spectroscopy of the PAN nanofibers, PAN-BWMO and the BWMO-NTs. As observed in Fig. 4 there was no peak of PAN nanofibers, while PAN-BWMO and the BWMO-NTs shows a major absorption band between 370 nm and 510 nm. The steep shape of the absorption edge indicates a band-gap transition rather than the transition from the impurity level. For a crystalline semiconductor, the optical absorption near the band edge follows the equation αhv = A(hv − Eg)n/2, where, α, v, Eg and A are the absorption coefficient, the light frequency, the band gap and a constant, respectively. Among them, n decides the characteristics of the transition in a semiconductor. In this paper, the band gap of BWMO-NTs is calculated as approximately 2.59 eV starting from the absorption edge (inset of Fig. 4), indicating that BWMO-NTs has a suitable band gap for photocatalytic reactions under visible-light irradiation.
Fig. 4 UV-Vis diffuse reflectance spectra of PAN nanofibers (curve a), PAN-BWMO (curve b) and BWMO-NTs (curve c). |
As discussed above, the as-prepared BWMO-NTs exhibit one dimensional hollow structure with large surface area, rendering it an excellent candidate for photocatalytic CO2 reduction. The photocatalytic performance of BWMO-NTs photocatalysts was evaluated and compared to that of pure BWO NTs and pure BMO NTs under visible light irradiation. Control experiments showed no CH4 or CO production in the absence of either catalyst or irradiation, implying that both catalyst and irradiation are necessary for the present gaseous CO2 photoreduction system (no show). The primary results showed CO2 can be reduced to CO and CH4 in the presence of H2O vapor and the BWMO NTs, and no other reduced product such as CH3OH, HCHO or HCOOH is detected. In addition, when N2 is used to replace CO2 in the photoreaction system, neither CO/CH4 nor other carbon-based organic compounds can be detected, indicating that the formation of CO/CH4 is caused by the CO2 photoreduction process on the catalyst.
As shown in Fig. 5a–c, under visible light irradiation, CO2 can be reduced to CH4 and CO in the presence of water vapor by using the Bi2W0.5Mo0.5O6 solid solution as the photocatalysts. The pure BWO NTs show poor CO and CH4 production (0.93 μmol g−1 h−1 and 0.13 μmol g−1 h−1), meanwhile, the pure BMO NTs also show poor CO and CH4 production (0.80 μmol g−1 h−1 and 0.12 μmol g−1 h−1) after irradiation for 4 h. In comparison, the BWMO NTs display more than 7 times higher activity for CO production (6.55 μmol g−1 h−1) and CH4 production (3.75 μmol g−1 h−1). The highly enhanced visible-light photocatalytic activity of BWMO NTs can be ascribed to tube structure and the modified band structure, which adapt the balance between adequate redox potentials and effective visible light adsorption. Furthermore, we have tested the stability and reusability of BWMO NTs for the photocatalytic CO2 reduction through a cycling test. As shown in Fig. 5d, the result indicates no obvious decrease of the photocatalytic activity after the three cycles, demonstrating the good stability of the BWMO NTs.
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