Developing visible light active Z-scheme WO3 / In2O3 photocatalysts for carbon dioxide reduction

This study focuses on the possibility of utilizing In2O3 and WO3 as a composite heterogeneous photocatalyst for CO2 reduction. The research aimed to enhance the visible light absorption properties of the composite photocatalyst by studying the effects of synthesis temperature, reaction time, and the order of the synthesis procedure on the characteristic properties. Finally, to evaluate the photocatalytic activity of the WO3/In2O3 composite samples for CO2 reduction. The composite catalysts were synthesized using hydrothermal/solvothermal methods. The hydrothermal/solvothermal reaction temperature and time were varied from 180°C to 190°C and 24 hours to 48 hours, respectively. In addition, the importance of the order of the synthesis procedure of the composite catalysts were also studied. The prepared catalysts were characterized by using scanning electron microscopy (SEM) in conjunction with energy- dispersive x-ray spectrometry (EDX), x-ray diffraction (XRD) and diffuse reflection measurements (DRS). According to the XRD results, under the studied conditions, prepared pristine In2O3 and WO3 samples depicted no significant effects from variations in hydrothermal/solvothermal temperature and synthesis time. However, during the preparation of the composite samples, these crystal structures were affected due to the order of the synthesis procedure and resulted with different tungsten oxide and indium oxide incorporated complex composite crystal phases. The In2O3/WO3 composite samples exhibited peaks relevant to cubic In2O3, hexagonal In2O3, and monoclinic NaIn(WO4)2 phases. And the WO3/In2O3 composite samples reported diffraction peaks for monoclinic WO3, cubic In2O3 and orthorhombic-In2(WO4)3. On the other hand, in the In2O3/WO3 composite samples, extended synthesis time resulted in a slight increase in NaIn(WO4)2 peak intensities and increased synthesis temperature had no effect on the crystal structures. And in the WO3/In2O3 composite samples, at 180oC, extended synthesis time resulted in disappearance of the monoclinic WO3 phase from the composite sample, and increased synthesis temperature led to the disappearance of some of the In2(WO4)3 peaks. And extended synthesis time at 190oC, increased the monoclinic WO3 peak intensities. These results suggested that hydrothermal/solvothermal synthesis temperature and time affected crystal phases in the WO3/In2O3 samples. XRD results further revealed that the pristine In2O3, pristine WO3 and In2O3/WO3 composite samples presented high crystallinity in the samples, while WO3/In2O3 samples showed a reduced crystallinity. Additionally, sodium contamination was observed in pristine WO3 and In2O3/WO3 composite samples. SEM-EDX analysis showed morphologies of planar plates and nanoparticles for the as- prepared pure WO3 and In2O3, respectively. And the morphology of the pristine WO3 was affected by the changes in the synthesis temperature and time. Surprisingly, the morphology of pristine In2O3 nanostructures remained unaffected under the studied conditions. Compared to the pristine samples, distinct morphologies were reported in the composite samples, and they were affected by the order of the synthesis route. In the WO3/In2O3 composites, a morphology of nanoparticle deposition on planar plates could be observed, except in the WO3/In2O3–180- 48 sample. The WO3/In2O3–180–48 sample exhibited a homogeneous dispersion of uniform rectangular nanoflakes. Conversely, in the In2O3/WO3 samples, the morphology transformed into a mixture of nanoparticles and plates. Furthermore, varying the synthesis temperature and time resulted in different morphologies in the WO3/In2O3 samples, and with increased temperature agglomeration was induced in the sample. Conversely, the In2O3/WO3 samples remained unaffected during the changes in synthesis temperature and time, except for In2O3/WO3–190–24 sample. Additionally, EDX results revealed that considerable sodium contamination could be observed in pristine WO3 and In2O3/WO3 composite samples. DRS results revealed that the band gap energies and the absorption edges of the prepared WO3 and In2O3 were reported to be about 2.68 eV and 3.28 eV, and 465 nm and at 390 nm, respectively. Furthermore, all the composite samples showed narrowed bandgap energies compared to the pristine In2O3. Compared to pristine In2O3, all the WO3/In2O3 samples exhibited increased absorption in both UV and visible regions, while the In2O3/WO3 samples exhibited a red-shifted absorption only in the wavelength region of 300 nm to 390 nm, implying that the order of the synthesis route had a affected the optical properties, and the synthesis order of WO3/In2O3 seemed to provide a more favorable structure for visible light absorption. Moreover, in the WO3/In2O3 samples, when the hydrothermal preparation time was increased, improved optical properties were observed, while a change in hydrothermal temperature had only a slight effect. On the other hand, changes in synthesis temperature and time had no effect on the optical properties of In2O3/WO3 samples. The transient absorption studies revealed that the In2O3 semiconductor photocatalyst material exhibited a considerable charge carrier generation around 400 nm wavelength. However, the rapid recombination of generated electrons and holes resulted in a complete recombination within 100 ns. Consequently, these results highlighted the necessity of modifying the In2O3 photocatalyst, or the inclusion of an appropriate hole scavenger to mitigate rapid recombination and enhance the photocatalytic efficiency of In2O3 for effective CO2 reduction. The methylene blue dye degradation experimental results showed that all the composite photocatalysts actively degraded methylene blue under UV light and exhibited high photocatalytic activity compared to pristine WO3 and pristine In2O3 samples. Moreover, the WO3/In2O3 composite samples generally outperformed the In2O3/WO3 samples, except for the composites prepared at 190oC for 24 hours. Due to the reported distinct crystal structure, morphology, and absorption properties, the WO3/In2O3 -180-48 sample resulted with the highest activity for methylene blue dye degradation. Conversely, the lowest activity was reported for the In2O3/WO3-190-48 composite sample. On the other hand, during the photocatalytic CO2 reduction experiments, in the presence of Na2SO3 hole scavenger, In2O3 sample achieved a CO2 reduction of 1.2%, while WO3 showed no capacity to reduce CO2. Moreover, the In2O3/WO3 sample displayed a modest CO2 reduction of 1.6%, and the WO3/In2O3 sample displayed a CO2 reduction of 3.5%, which was more than twice that of the In2O3/WO3 sample and almost threefold as of pristine In2O3. The above improved CO2 reductions in the composite samples confirmed the successful formation of a Z- scheme heterojunction between WO3 and In2O3 compounds. Furthermore, in the presence of Na2SO3, the WO3/In2O3 and In2O3/WO3 photocatalysts demonstrated CO production rates of 174.6 μmol/g/h and 86.1 μmol/g/h, respectively, whereas In2O3 alone yielded only 65.4 μmol/g/h of CO. Moreover, in addition to CO, a generation of HCOOH was observed in the system at low concentrations. These results indicated that, under the photocatalytic reduction experimental conditions employed, both composite catalysts and In2O3 exhibited favorable activity in reducing CO2 into CO. From the dark experiment it was confirmed that the photocatalysts were activated solely by light irradiation and the CO2 reductions were indeed facilitated by the photocatalytic activation of the respective catalysts. From above results confirmed that the formation of Z-scheme heterojunctions between WO3 and In2O3 in composite samples have improved the CO2 reduction ability of In2O3. Moreover, the hydrothermal/solvothermal synthesis conditions used for the WO3/In2O3 composite sample synthesis has formed a more favorable composite for CO2 reduction. Overall, the introduction of Na2SO3 as a hole scavenger and the heterojunction formation between WO3 and In2O3, exhibited a favorable activity in reducing CO2 into CO.