Photocatalytic Reduction of CO₂ in Aqueous Solution on Surface-Fluorinated Anatase TiO₂ Nanosheets with Exposed {001} Facets

Abstract

Photocatalytic reduction of carbon dioxide can activate chemically inert carbon dioxide by the use of renewable energy. In the present work, the main products of photocatalytic reduction of CO 2 in aqueous TiO 2 suspensions were found to be methane, methanol, formaldehyde, carbon monoxide, and H 2 . Anatase TiO 2 catalysts with various morphologies, such as nanoparticle, nanotube, and nanosheet, were synthesized through a hydrothermal method. The TiO 2 nanosheets were more active than the nanotubes or nanoparticles in the reduction of CO 2 in aqueous solution. This is because the photogenerated carriers prefer to flow to the specific facets. The TiO 2 sheet with high-energy exposed {001} facets facilitates the oxidative dissolution of H 2 O with photogenerated holes, leaving more photogenerated electrons available for the reduction of CO 2 on {101} facets. Moreover, surface fluorination promotes the formation of Ti 3+ species, which is helpful in the reduction of CO 2 to CO 2 − and in extending the lifetime of photogenerated electron−hole pairs. The optimum ratio of exposed {001} to {101} facets for surface-fluorinated TiO 2 nanosheets was found to be ∼72:28, which corresponds to an initial F/Ti ratio of 1. From our analysis of the effect of adding of known intermediates on the photocatalytic reduction of CO 2 , we propose that the photocatalytic reduction of CO 2 with H 2 O on surface-fluorinated TiO 2 nanosheets proceeds via a mechanism involving generation of hydrogen radicals and carbon radicals.

Light-Driven Heterogeneous Reduction of Carbon Dioxide: Photocatalysts and Photoelectrodes

“…160 The TNTs calcined at 400°C yielded 0.50 μmol g −1 h −1 of methane, whereas TNRs produced only 0.33 μmol g −1 h −1 ; the authors attributed this difference to the greater surface polarity and acidity present on the inner He et al and generated CO, formaldehyde, methanol, and methane in basic solution. 145 The quantum efficiencies of these TNTs were about the same as those of the nanoparticles, on a surface area basis. Wang et al modified anatase TNRs with rutile NPs and found an increase in activity from 1.29 to 2.36 μmol g −1 h −1 of methane under UV light.…”

“…Doping of TiO 2 with other elements has been employed to alter the band gap and increase catalytic activity for CO 2 reduction. These dopants, which include nitrogen, 140,143,144 fluorine, 145 iodine, 141 cerium, 142 and rhodium, 146 were incorporated into the structure of TiO 2 .…”

“…145 The fluorine increased the number of Ti 3+ defects present at the surface by adding electrons into the 3d orbitals of Ti 4+ . These Ti 3+ species, bonded to highly electronegative fluorine, served as electron trap states and promoted the photoreduction of CO 2 more readily than their hydroxylated counterparts.…”

Foamed single-crystalline anatase nanocrystals exhibiting enhanced photocatalytic activity

“…It is known, however, that the TiO 6 octahedra share edges and corners in rutile but only edges in anatase, and the probability of the latter can be much higher. 22,69,70 It is known that urea homogeneous precipitation is a common method to prepare monodisperse spherical particles. For the above reasons, anatase rather than rutile nanoparticles are directly crystallized in this study.…”

Low Temperature Synthesis of Nest-Like Microsphere with Exposed (001) Facets and Its Enhanced Photocatalytic Performance by NaOH Alkalization

“…Since then, studies on (001) faceted anatase TiO 2 have been completed [20][21][22][23][24][25][26][27]. However, most of the (001) faceted anatase TiO 2 crystals were prepared at high temperature through calcination or hydrothermal techniques [20][21][22][23][24][25][26][27].…”