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Nanotechnology

Enhanced solar water splitting by surface engineering of titanium dioxide

Anatase titanium dioxide bounded by high-energy facets is remarkably effective in promoting sunlight-induced splitting of water into oxygen and hydrogen.
1 November 2011, SPIE Newsroom. DOI: 10.1117/2.1201109.003894

Global energy requirements today depend mostly on traditional fossil fuels such as coal, natural gas, and oil. Unfortunately, the depletion of these limited resources is foreseeable. Hydrogen is a viable alternative that can release much more heat than conventional fuels without any undesirable impact on the environment. Moreover, solar splitting of water using semiconductors offers a green process for producing hydrogen. Photoelectrochemical hydrogen production using titanium dioxide (TiO2) was first reported by Fujishima and Honda in 1972.1 Since then, a variety of photocatalysts for water splitting have been developed. Here, we highlight some of our recent work focusing on enhanced solar water splitting by surface engineering of TiO2.

Tailored synthesis of anatase (one of the three mineral forms of TiO2) nanocrystals has been widely investigated over the past decades. In 2008, we successfully synthesized anatase TiO2 single crystals with a large percentage of reactive {001} facets (the number relates to the specific crystal shape) using hydrofluoric acid (HF) as a capping agent to promote desirable crystal growth (see Figure 1).2 In this work, we investigated the adsorption effect of 12 non-metallic atoms X (where X can represent hydrogen, boron, carbon, oxygen, fluorine, silicon, phosphorus, sulfur, chlorine, bromine, or iodine) based on first-principle calculations (see Figure 2). Next, we prepared ultrathin anatase TiO2 nanosheets dominated by {001} facets.3 Figure 3(a) shows a transmission electron microscopy image of as-synthesized well-defined rectangular TiO2 nanosheets. Our findings show that the concentration of HF can significantly affect the thickness and side length of these sheets. Figure 3(b) shows the hydrogen evolution of ultrathin TiO2 nanosheets with different percentages of {001} facets.


Figure 1. High-energy facets of anatase titanium dioxide (TiO2) crystals show superior catalytic activity in solar water splitting.

Figure 2. (a) Calculated energies of (001) and (101) surfaces surrounded by X atoms. (b) Plots of the optimized value of B (the side of the square {001}‘ truncation ’ facets)/A (the side of the bipyramid) and percentage of {001} facets for anatase single crystals with various adsorbate atoms X. B: Boron. Br: Bromine. C: Carbon. Cl: Chlorine. F: Fluorine. I: Iodine. H: Hydrogen. N: Nitrogen. O: Oxygen. P: Phosphorus. S: Sulfur. Si: Silicon. S and S001: Total surface area and surface area contributed by the {001} facets. γ: Surface energies.

Figure 3. (a) Transmission electron microscopy images of ultrathin TiO2 nanosheets. (b) Hydrogen evolution properties of the obtained nanosheets with different percentages of {001} facets.

Since these efforts, researchers have exploited various reaction systems to achieve TiO2 with exposed highly reactive {001} or {100} facets. However, anatase TiO2 single-crystalline nanosheets wholly exposed with {001} and {100} facets have rarely been synthesized. We recently reported preparation of large-sized anatase TiO2 nanosheets wholly bounded by highly reactive {001} and {100} facets amounting to 98.7 and 1.3%, respectively.4 Figure 4(a) shows a field emission scanning electron microscopy image of the as-synthesized sheets. We also measured their solar water-splitting property and found that they exhibit superior photocatalytic activity compared with crystals possessing 47% of {001} facets when methanol is used as a sacrificial reagent: see Figure 4(b).

We also successfully prepared anatase TiO2 crystals predominantly exposed with {105} facets by a simple gas-phase route.5 Figure 5 shows the well-faceted surface of the product. Our research suggests that the {105} facets have the capability to cleave water through a photocatalytic process. This activity is higher than that of {101} facets but lower than that of {001} facets—see Figure 5(c)—which is consistent with our theoretical predictions.


Figure 4. (a) Field emission scanning electron microscopy (FESEM) image of as-synthesised anatase TiO2 nanosheets and (b) photocatalytic activities of the nanosheets wholly exposed by {001} and {100} facets () and the anatase TiO2 with 47% of {001} facets ().

Figure 5. FESEM (a) and schematic shape (b) of as-obtained anatase TiO2 crystals dominated by high-index {105} facets. (c) Comparison of hydrogen generation of the as-obtained high-index TiO2 and the sample dominated by {001} facets.

In summary, we prepared anatase TiO2 single crystals bounded by high-energy {001} or {105} facets. These products show photocatalytic activity for water splitting. The present work also motivates us to explore new synthetic methods for preparing tailored crystal facets of other functional materials. Our next steps will involve exploiting anatase TiO2 with new high-index facets and developing novel semiconductor materials for solar water splitting.

This work was financially supported by the Scientific Research Foundation of East China University of Science and Technology (YD0142125), the Pujiang Talents Programme and the Major Basic Research Programme of the Science and Technology Commission of Shanghai Municipality (09PJ1402800, 10JC1403200), the Shuguang Talents Programme of the Education Commission of Shanghai Municipality (09SG27), the National Natural Science Foundation of China (20973059, 91022023, 21076076), Fundamental Research Funds for the Central Universities (WJ0913001), and the Program for New Century Excellent Talents in University (NCET-09-0347).


Jun Xing, Hua Gui Yang
East China University of Science and Technology (ECUST)
Shanghai, China

Jun Xing earned his BEng from Qingdao University of Science and Technology (2009). Currently, he is a PhD candidate at ECUST under the supervision of Hua Gui Yang. His research focuses on developing novel photocatalysts for splitting water.

Hua Gui Yang is a professor. His work has been published in major research journals, including Nature.Currently, he is interested in the design and synthesis of materials for renewable clean energy.

Gao Qing (Max) Lu
University of Queensland
Brisbane, Australia

Max Lu is currently deputy vice-chancellor (research) at the University of Queensland and research director for the ARC Centre of Excellence for Functional Nanomaterials. His research expertise is in nanoparticles and nanoporous materials for clean energy and environmental technologies.


References:
1. A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238, pp. 37-38, 1972. doi:10.1038/238037a0
2. H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng, G. Q. Lu, Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453, pp. 638-641, 2008. doi:10.1038/nature06964
3. X. H. Yang, Z. Li, G. Liu, J. Xing, C. H. Sun, H. G. Yang, C. Z. Li, Ultra-thin anatase TiO2 nanosheets dominated with {001} facets: thickness-controlled synthesis, growth mechanism, and water-splitting properties, Cryst. Eng. Commun. 13, pp. 1378-1383, 2010. doi:10.1039/C0CE00233J
4. C. Z. Wen, J. Z. Zhou, H. B. Jiang, Q. H. Hu, S. Z. Qiao, H. G. Yang, Synthesis of micro-sized titanium dioxide nanosheets wholly exposed with high-energy {001} and {100} facets, Chem. Commun. 47, pp. 4400-4402, 2011. doi:10.1039/C0CC05798C
5. H. B. Jiang, Q. Cuan, C. Z. Wen, J. Xing, D. Wu, X. Q. Gong, C. Z. Li, H. G. Yang, Anatase TiO2 crystals with exposed high-index facets, Angew. Chem. Int'l Ed. 50, pp. 3764-3768, 2011. doi:10.1002/anie.201007771