Zinc Orthogermanate Nanoribbons Support Improved Photocatalytic Reduction of CO2 to Renewable Methane

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CH4 generation over (a) bulk material, (b) nanoribbons, (c) 1 wt % Pt-loaded nanoribbons, (d) 1 wt % RuO2-loaded nanoribbons, and (e) 1 wt % RuO2 + 1 wt % Pt-coloaded nanoribbons as a function of light irradiation time. Credit: ACS, Liu et al. Click to enlarge.

Researchers at Nanjing University and Anhui Polytechnic University in China have synthesized zinc orthogermanate (Zn2GeO4) ultralong nanoribbons which show promising photocatalytic activity toward the reduction of CO2 into renewable methane (CH4) and water.

The team used a En/H2O binary solvent system for the synthesis, and noted that this binary solvent system may provide a new route for preparing other 1D ternary oxides. A paper on their work was published online 24 September in the Journal of the American Chemical Society.

Studies of 1D ternary nanostructures, in comparison with 1D binary ones, are relatively more meaningful because the ternary nanostructures exhibit not only more complex functions but also properties that are readily tunable by changing the ratio of the component elements. Reports of 1D ternary nanostructures, specifically nanoribbons, however, have been limited because of the considerable difficulty of their synthesis.

Zinc orthogermanate (Zn2GeO4) is an important ternary oxide that exhibits negative thermal expansion below ambient temperature. It also exhibits high-wavelength selectivity in UV photodetectors with fast response and recovery time, bright white-bluish luminescence, photocatalytic water splitting, and mineralization of volatile aromatic hydrocarbons. Several solution routes and gas phase evaporation techniques have been developed for the preparation of 1D nanorods and nanowires of Zn2GeO4.

Herein we report for the first time the high-yield synthesis of single-crystalline Zn2GeO4 nanobelts with lengths of hundreds of micrometers, thickness as small as ~7 nm (corresponding to five repeating cell units), and aspect ratios of up to 10,000 in a binary ethylenediamine (En)/water solvent system using a solvothermal route. The ultralong and ultrathin geometry of the Zn2GeO4 nanoribbon greatly improves the photocatalytic activity toward reduction of CO2 into renewable hydrocarbon fuel (CH4) in the presence of water vapor.

—Liu et al.

The nanoribbons delivered a CH4 yield of ~1.5 &icro;mol g-1 during the first hour under light illumination. Bulk Zn2GeO2 obtained by conventional solid-state reaction (SSR) produced only a trace amount of CH4.

The researchers attributed the higher photocatalytic activity of the nanoribbons toward reduction of CO2 relative to that of the SSR sample to four reasons:

  • Reducing the lateral dimension to the nanometer length scale as in the nanobelts offers a high specific surface area of 28.27 m2/g, which is more than 37 times larger than the area for the SSR material.

  • The superb crystal quality excludes the possibility of any grain boundaries and/or other interfaces (which usually act as recombination sites in polycrystalline materials). This should favor improved separation of the photogenerated electron and hole and decrease the electron-hole recombination rate.

  • The ultralong longitudinal dimension provides a sufficiently spacious transport channel for charge separation.

  • The ultrathin geometry of the nanoribbons allows charge carriers to move rapidly from the interior to the surface to participate in the photoreduction reaction.

The team found that the rate of CH4 generation over the nanoribbon could be significantly enhanced by loading of Pt or RuO2 and especially by co-loading of Pt and RuO2 as a co-catalyst to improve the separation of the photogenerated electron-hole pairs, as demonstrated in photocatalytic water splitting.

Resources

  • Qi Liu, Yong Zhou, Jiahui Kou, Xiaoyu Chen, Zhongping Tian, Jun Gao, Shicheng Yan, and Zhigang Zou (200) High-Yield Synthesis of Ultralong and Ultrathin Zn2GeO4 Nanoribbons toward Improved Photocatalytic Reduction of CO2 into Renewable Hydrocarbon Fuel. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja1068596


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