Methylene Blue Dye as Photosensitizer for Scavenger-Less Water Photo Splitting: New Insight in Green Hydrogen Technology

Abstract

In this study, hydrogen generation was performed by utilizing methylene blue dye as visible-light photosensitizer while the used catalyst is working as a transfer bridge for the electrons to H⁺/H₂ reaction. Silica NPs-incorporated TiO₂ nanofibers, which have a more significant band gap and longer electrons lifetime compared to pristine TiO₂, were used as a catalyst. The nanofibers were prepared by electrospinning of amorphous SiO₂ NPs/titanium isopropoxide/poly (vinyl acetate)/N, N-dimethylformamide colloid. Physicochemical characterizations confirmed the preparation of well morphology SiO₂-TiO₂ nanofibers with a bandgap energy of 3.265 eV. Under visible light radiation, hydrogen and oxygen were obtained in good stoichiometric rates (9.5 and 4.7 mL/min/gcat, respectively) without any considerable change in the dye concentration, which proves the successful exploitation of the dye as a photosensitizer. Under UV irradiation, SiO₂ NPs incorporation distinctly enhanced the dye photodegradation, as around 91 and 94% removal efficiency were obtained from TiO₂ nanofibers containing 4 and 6 wt% of the used dopant, respectively, within 60 min.

Keywords: electrospinning; hydrogen; photosensitizer; silica nanoparticles; water photo splitting.

Figure 1

Conceptual illustration for the electron transfer mechanisms in the heterogeneous catalytic reactions. (A). direct electron transfer mechanism, (B). Sensitizer-assisted electron transfer mechanism and (C). Photocatalyst-assisted electron transfer mechanism.

Figure 2

Schematic diagram for SiO₂ NPs–incorporated TiO₂ nanofibers synthesis process.

Figure 3

XRD pattern for the produced SiO₂ NPs-incorporated TiO₂ nanofibers.

Figure 4

EDX analysis for the produced SiO₂ NPs–incorporated TiO₂ nanofibers. The inset table displays the elemental analysis data, and the image represents the FE SEM result.

Figure 5

Normal; (A) and high resolution; (B) TEM images for the produced nanofibers. The inset represents the SAED result.

Figure 6

UV-vis. spectra for SiO₂ NPs, and pristine and SiO₂–incorporated TiO₂ nanofibers (A); and Tauc plots for determination the bandgap energies for the pristine and SiO₂–incorporated TiO₂ nanofibers ((B) and (C), respectively).

Figure 7

Natural methylene blue dye removal rate (UV only) and in the presence of the prepared nanofibers photocatalysts under U.V. irradiation; (A), and linear plot of Ln(C/C₀) vs. time for the photocatalytic degradation results using SiO₂–incorporated TiO₂ nanofibers; (B).

Figure 8

Hydrogen and oxygen production rates under visible light irradiation using SiO₂–incorporated TiO₂ nanofibers (6 wt% sample) and methylene blue as photosensitizer; (A), and dye photodegradation rate during the water splitting process; (B).

Figure 9

Conceptual illustration for the mechanism of water splitting under visible light radiation using methylene blue dye as a photosensitizer.

Figure 10

Conceptual illustration for the mechanism of methylene blue dye photo degradation under UV light irradiation using the prepared SiO₂–incoportaed TiO₂ nanofibers as photocatalyst.