Ultrathin-Film Titania Photocatalyst on Nanocavity for CO2 Reduction with Boosted Catalytic Efficiencies

Abstract

Photocatalytic CO₂ reduction with water to hydrocarbons represents a viable and sustainable process toward greenhouse gas reduction and fuel/chemical production. Development of more efficient catalysts is the key to mitigate the limits in photocatalytic processes. Here, a novel ultrathin-film photocatalytic light absorber (UFPLA) with TiO₂ films to design efficient photocatalytic CO₂ conversion processes is created. The UFPLA structure conquers the intrinsic trade-off between optical absorption and charge carrier extraction efficiency, that is, a solar absorber should be thick enough to absorb majority of the light allowable by its bandgap but thin enough to allow charge carrier extraction for reactions. The as-obtained structures significantly improve TiO₂ photocatalytic activity and selectivity to oxygenated hydrocarbons than the benchmark photocatalyst (Aeroxide P25). Remarkably, UFPLAs with 2-nm-thick TiO₂ films result in hydrocarbon formation rates of 0.967 mmol g^-1 h^-1, corresponding to 1145 times higher activity than Aeroxide P25. This observation is confirmed by femtosecond transient absorption spectroscopic experiments where longer charge carrier lifetimes are recorded for the thinner films. The current work demonstrates a powerful strategy to control light absorption and catalysis in CO₂ conversion and, therefore, creates new and transformative ways of converting solar energy and greenhouse gas to alcohol fuels/chemicals.

Keywords: CO2 photoreduction; light absorption; nanocavity; photocatalysis; ultrathin‐film titania.

Figure 1

Light trapping within planar nanocavity enhanced ultrathin TiO₂ films. a) Schematic of a two‐layered UFPLA comprised of sequential photocatalytic/absorptive TiO₂ thin films and a bottom aluminum (Al) reflector layer. b) Modeled and c) measured total absorption spectra of TiO₂/Al as a function of the thickness of TiO₂. d) The spatial absorption distribution in UFPLAs with different TiO₂ films. e) The measured optical absorption spectra of different TiO₂ layers normalized by their thicknesses.

Figure 2

CO₂ photocatalytic reduction by water on ultrathin TiO₂ films. a) Measured product type and quantity in each UFPLA sample. b) Product formation rates normalized to per unit mass of TiO₂ catalyst. c) Product selectivity versus TiO₂ thin‐film thickness in the UFPLAs.

Figure 3

Physicochemical properties of TiO₂ thin films on planar nanocavity. a) FTIRs and b) XPS spectra of TiO₂ thin films. c–e) XPS spectra of Ti 2p, O 1s, and Al 2p, respectively, of TiO₂/Al/glass light absorber structures. f) VBM positions determined from XPS data. g) fs‐TA kinetics traces probed at 860 nm of different thickness of TiO₂ on glass substrate following 365 nm optical excitation. Solid red line shows exponential fit of the experimental data. h) Lifetime as a function of TiO₂ thickness.

Figure 4

Light trapping within planar nanocavity enhanced ultrathin TiO₂ films. a) Schematic of a three‐layered UFPLA with an optional spacer layer. b) Measured and c) modeled absorption spectra of Al/Al₂O₃/2 nm TiO₂ as a function of the thickness of Al₂O₃. d) Modeled exclusive absorption in TiO₂ layer as a function of the thickness of TiO₂. e) The modeled exclusive absorption in the 2‐nm‐thick TiO₂ layer on 15 nm Al₂O₃/Al cavity. The product formation and selectivity are shown in (f) and (g), respectively. h) The FTIRs spectra of the UFPLAs with various Al₂O₃ spacer thicknesses.