Hierarchical hyper-branched titania nanorods with tuneable selectivity for CO2 photoreduction

Abstract

Utilising captured CO₂ and converting it into solar fuels can be extremely beneficial in reducing the constantly rising CO₂ concentration in the atmosphere while simultaneously addressing energy crisis issues. Hence, many researchers have focused their work on the CO₂ photoreduction reaction for the last 4 decades. Herein, the titania hyper-branched nanorod (HBN) thin films, with a novel hierarchical dendritic morphology, revealed enhanced CO₂ photoreduction performance. The HBNs exhibited enhanced photogenerated charge production (66%), in comparison with P25 (39%), due to the unique hyper-branched morphology. Furthermore, the proposed HBN thin films exhibited a high degree of control over the product selectivity, by undergoing a facile phase-altering treatment. The selectivity was shifted from 91% towards CO, to 67% towards CH₄. Additionally, the HBN samples showed the potential to surpass the conversion rates of the benchmark P25 TiO₂ in both CO and CH₄ production. To further enhance the selectivity and overall performance of the HBNs, RuO₂ was incorporated into the synthesis, which enhanced the CH₄ selectivity from 67% to 74%; whereas the incorporation of CuO revealed a selectivity profile comparative to P25.

Fig. 1. Photoreduction rig set-up diagram (not to scale).

Fig. 2. X-ray diffraction pattern for the as prepared HBNs BP and HBNs AP (a), Raman spectra of the fabricated samples with potassium titanates (blue), anatase (red) and rutile (green) components (b).

Fig. 3. The HBNs BP sample under SEM (a–c) and TEM (d–f). The top view (g) and cross-section (h and i) of HBNs AP sample under SEM.

Fig. 4. SAED pattern and TEM images of HBNs BP (a and c, respectively) and HBNs AP (b and d, respectively).

Fig. 5. Diffuse reflectance spectra with a Kubelka–Munk inset for band gap calculation.

Fig. 6. Microscopy morphology investigation and elemental mapping EDX. (a and b) SEM imaging for CuO-HBNs, (c–f) EDX analysis for CuO-HBNs. (g) SEM imaging with zoomed inset for RuO₂-HBNs, (h) TEM imaging for RuO₂-HBNs, (i–l) EDX analysis for RuO₂-HBNs.

Fig. 7. XPS analysis, magnification of the peaks (a) Ti 2p of HBNs AP, (b) Ru 3d_5/2 of RuO₂-HBNs and (c) Cu 2p_3/2 of CuO-HBNs samples.