Page 103 - Multifunctional Photocatalytic Materials for Energy

92 Multifunctional Photocatalytic Materials for Energy

Several studies have emphasized the high performance of graphene-based photo-
catalyst composites for the CO 2 reduction to solar fuels. For example, our research
group [48] reported the effect of pH and copper oxide precursor salt on graphene
derivative-TiO 2 (GOT) composites for the photocatalytic reduction of CO 2 with water.
The pH was identified as the key variable determining the product distribution. Thus it
was observed that the prepared GOT composite photocatalyst exhibited superior pho-
−1 −1
tocatalytic activity for ethanol (EtOH) production (144.7 μmol g h ) at pH 11.0 and
for methanol (MeOH) production at pH 4.0. Moreover, the presence of both GO and
copper in the GOT composites extended the absorption to the visible spectral range,
enhancing the CO 2 photoreduction in the aqueous phase (Fig. 5.8A). The combination
of TiO 2 with GO generates a synergistic effect that potentially enhances the photo-
activity because of the increase of the adsorption capacity and efficient interfacial
electron transfer between the two constituent phases (Fig. 5.8B) [56]. Moreover, the
combined contribution of the lower band gap energy and the known quenching of pho-
toluminescence determined by Raman spectroscopy for the GOT catalyst are possible
explanations for the higher performance observed, as compared with P25.
The synthesis of Cu 2 O/rGO composites has been reported in regard to the photo-
catalytic reduction of CO 2 by using a 150 W Xe lamp as a light source [71]. Given
the good electron conductivity and large specific surface area of rGO, the composite
exhibited a low electron-hole recombination rate and high number of surface-active
sites. As a result, the prepared material showed a photocatalytic activity six times
greater than that of the optimized Cu 2 O for the CO 2 reduction. More interesting, how-
ever, is the fact that the photocorrosion problem of Cu 2 O can also be effectively mit-
igated by the rGO loading. Therefore the loading of graphene not only improved the
photocatalytic activity of the photocatalyst but also enhanced its photostability.
Graphene exhibited unique properties when it was hybridized with other materi-
als to function as a co-catalyst. The production of MeOH was described by the pho-
tocatalytic reduction of CO 2 under visible light illumination using a graphene and

Fig. 5.8 (A) CO 2 photoreduction over GOT, GOT-500, and Cu-loaded GOT catalysts at pH
11.0 and 180 min; (B) conceptual scheme of the CO 2 photoreduction catalyzed by Cu-loaded
GOT composites.
Reproduced with permission from L.M. Pastrana-Martínez, A.M.T. Silva, N.N.C. Fonseca,
J.R. Vaz, J.L. Figueiredo, J.L. Faria, Photocatalytic reduction of CO 2 with water into methanol
and ethanol using graphene derivative–TiO 2 composites: effect of pH and copper(I) oxide,
Top. Catal. 59 (15–16) (2016) 1279–1291. Copyright 2016, Springer.