Nanoclusters-Grafted Photocatalyst to Drive Multi-Electron Reduction for Energy Production and Environmental Purification
Authors: Miyauchi Masahiro , Shoji Shusaku , Yin Ge 
Published: February 2020

Redox potential of multi-electron reaction is generally lower than that of single-electron process, thus multi-electron processes are seen in metabolism or photosynthesis reactions. Herein, we have developed copper oxide nanocluster cocatalysts grafted onto light harvesting semiconductors to drive efficient multi-electron reduction of oxygen (O2) or carbon dioxide (CO2).1 O2 reduction is important reaction for photocatalytic air purification, while CO2 reduction reaction can produce fuel molecules, such as carbon monoxide (CO), formic acid (HCOOH), and methane (CH4) etc. like natural plants for energy production.
We have developed copper oxide grafted and metal ion doped titanium dioxide (TiO2) as an environmental purification photocatalyst.2 Excited holes were generated in the valence band of TiO2, while excited electrons in dopant levels were injected into copper oxide nanoclusters, which acted as active site to reduce oxygen molecules in air into hydrogen peroxide.3 In addition to TiO2 light harvester, the copper oxide cocatalyst was also very effective to improve the photocatalytic activity of tungsten oxide (WO3) for decomposition of organic contaminant in air.4 The developed photocatalysts in the present study are useful for indoor environmental purification, and they exhibited significant anti-bacteria and self-cleaning properties even under visible light irradiation.5 To evaluate the practical application of this photocatalytic material, we conducted field tests of these coated products in indoor public spaces, including hospitals and airports. In a recent field test, we installed nanocluster-grafted TiO2 photocatalyst products in a washroom at International Airport in Vietnam, and these photocatalysts exhibited excellent antibacterial and deodorization functions, even in an indoor environment, with greater than 90% decrease of bacteria and ammonia levels.1

In addition to O2 reduction for environmental purification, our copper oxide nanoclusters can also drive CO2 reduction for fuel production. In case of CO2 photoreduction, we grafted copper oxide nanoclusters onto high conduction band semiconductors, such as niobate nanosheet (CuxO-Nb3O8
-) 6 and strontium titanate (CuxO-SrTiO3). Our CuxO-Nb3O8
- and CuxO-SrTiO3 could reduce CO2 into CO under UV irradiation. To prove the fuel production from CO2 and water like natural plants, the isotope labeling analysis was performed. Under the existence of H2
18O and 13CO2 isotopes, CuxO grafted photocatalysts produced 18O2 and 13CO under UV irradiation, which proved that the water molecules behaved as electron donor to reduce CO2 into CO. Electron spin resonance (ESR) spectra also proved that the electron injection from the conduction band of Nb3O8
- or SrTiO3 into CuxO nanoclusters to drive CO2 reduction. Similar to natural plants, fuel molecules were successfully generated from CO2 and water under photon irradiation. Key points of our development are high conduction band of light harvester, highly dispersed CuxO clusters loading onto light harvester, and causing efficient multi-electron reduction in CuxO nanoclusters.
Our CuxO nanoclusters promote the multi-electron process because of their nanoscale size and amorphous nature, which provide high structural flexibility even in excited states. Nanoclusters in the present study can be facilely grafted onto various semiconductor particles like TiO2 or Nb3O8
- by a simple wet chemical method, which is readily applicable for large-scale production processes. The nanoclusters grafted photocatalyst reported in the present study can be expected to be applied to environmental purification and/or artificial photosynthesis for energy production.

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