(Invited) CO2 Photoreduction By Cux０ Nanocluster Loaded SrTiO3 Nanorod Thin Film

Abstract

Introduction Photocatalytic carbon dioxide reduction, in which CO2 is converted into organic compounds, such as methane, methanol, formic acid, and formaldehyde, has the potential to greatly contribute to reducing global warming and fossil fuel dependence. In artificial photosynthesis systems designed for the reduction of CO2 for solar fuel production, water molecules should ideally serve as the electron donor, similar to photosystems I and II in natural plants. Recently, our group reported that a copper oxide nanocluster cocatalyst drives the multi-electron reduction of both oxygen and CO2, i. e. the production of hydrogen peroxide from oxygen and carbon monoxide (CO) production from CO2.1 Our copper oxide consists of Cu(I) and Cu(II) valence states during the CO2 reduction reaction, and it is designated as CuxO (x=1 or 2). Copper compounds have balanced chemisorption-desorption properties for CO2 and CO, and therefore have high CO2 reduction activity and high selectivity of products, such as CO and formic acid. Herein, we examined the potential of strontium titanate (SrTiO3: STO) to function as a light harvester with loading of CuxO cocatalysts, as the STO has high chemical stability and sufficient negative conduction band for CO2 reduction. In the present research, we used STO nanorod thin films as a UV-light harvester due to its higher crystallinity and increased surface area for the loading of CuxO cocatalyst. The photocatalytic CO2 reduction activity of CuxO cocatalyst loaded STO nanorod (CuxO-STO) thin films was evaluated under UV-light irradiation in the present study. Experimental STO nanorod thin films were synthesized by the annealing of strontium ion-exchanged titanate nanotube thin films. Titanate nanotube thin film was prepared by the hydrothermal treatment of titanium (Ti) foil (2×2×0.1 cm3, Nilaco Co.).2 CuxO nanocluster cocatalyst was loaded onto the STO nanorod thin film using an impregnation method.3 The sample morphologies were observed by SEM and TEM. Chemical compositions of the sample were investigated using an EDX equipped with a TEM apparatus. The valence number of copper nanoclusters was determined by measuring ESR. The prepared CuxO-STO film was immersed into an electrolyte solution in a sealed quartz glass reactor. To purge air in the reactor, CO2 or Ar gasses were bubbled into the reactor through a Teflon tube. Products in the headspace of the reactor were evaluated using a gas chromatograph (FID) equipped with a methanizer unit under UV light irradiation. Results and discussion The TEM image analysis revealed that amorphous nanoclusters of 2-3 nm in size were attached and highly dispersed on the STO nanorods. In the obtained EDX spectrum, a signal corresponding to Cu was clearly observed. ESR spectrum of bare STO nanorod thin film did not showed any obvious peaks corresponding to copper ions or any changes to the spectrum after UV irradiation. In contrast, the CuxO-STO film exhibited a broad ESR peak, which was assigned to Cu2+ species, around 345 mT under dark conditions, and this peak decreased in intensity after UV irradiation. Thus, under UV-light irradiation, photo-excited electrons of STO were injected into the CuxO nanocluster cocatalyst. The time course of photocatalytic CO generation for bare STO and CuxO-STO film under UV irradiation with Ar or CO2 bubbling conditions were measured. The amount of CuxO in the film was optimized to be 0.0005 wt%. The loading of the CuxO-cocatalysts onto STO nanorods clearly improved the photocatalytic CO2 reduction activity compared to bare STO nanorods. We also conducted isotope tracer analysis on the optimized CuxO-STO film. The peaks in the mass chromatography spectra at m/z = 29 and 36 were assigned to 13CO and 18O2, respectively. These results confirmed that the CO generated by the CuxO-STO film originated from CO2, and that O2 molecules were produced through water oxidation. Conclusion We have demonstrated the applicability of modifying SrTiO3 with CuxO nanoclusters as a cocatalyst for CO2 photoreduction. The CuxO-STO film exhibited high activity and selectivity for CO evolution under UV irradiation. The isotope tracer analysis confirmed that water served as an electron donor and that CO molecules were produced by the reduction of CO2 under UV irradiation. As the CuxO-STO material is composed of non-toxic and abundant elements, it has the potential to be an efficient photocatalyst for CO2 reduction.