RETRACTED: Heterogeneous photo-enhanced conversion of carbon dioxide to formic acid with copper- and gallium-doped titania nanocomposites

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Copper- and gallium-doped titania photocatalysts prepared by means of sol–gel technique were comparatively evaluated with commercial TiO_2 (P25) for the photo-reduction of carbon dioxide to formic acid. The laboratory-made Cu_x–Ga_(1−x)/TiO_2 nanocomposites have been thoroughly characterized in crystallographic, structural, morphological, and elemental composition analyses. XRD revealed photocatalysts owning the specific crystalline phases of anatase, β-Ga_2O_3 and Cu_2O, which allowed inferring on the doping phenomena of both transition and post-transition metals. The quasi-homogeneous deposition of a Ga and Cu layer has been identified from the TEM morphological characterization and the Brunauer–Emmett–Teller and Barrett–Joyner–Halenda techniques unveiled quantitative differences in textural properties among the mesoporous Ga- and Cu-doped titania photocatalysts by underlining a decrease of surface area when augmenting the gallium dose. The laboratory-made photocatalysts presented bandgaps higher than 3 eV and the DRS spectra underlined the optical absorption edge of the nanocomposites with a considerable shift to the visible light region. The elemental composition quantified by means of XPS reproduced the binding energies relative of Ti, Cu and Ga (2p_(3/2), 2p_(1/2)), and the K-edge XANES characterization confirmed the effective doping and modulation of the electronic properties of the laboratory-made photocatalysts. Several experimental runs have been carried out with Cu_(0.78)–Ga_(0.22)/TiO_2 exhibiting the highest formic acid yields (394 μmol/g_(cat)) as well as superior quantum efficiency (49%) and selectivity (0.84). Accordingly, the photo-reduction of CO_2 was considerably promoted by doping Ga and Cu into the titania substrate, which ultimately avoided the surface recombination of electron–hole pairs, thereby enhancing the photo-activity of Cu_x–Ga_(1−x)/TiO_2 nanocomposites.