Effective increasing of optical absorption and energy conversion efficiency of anatase TiO2 nanocrystals by hydrogenation.
Effective increasing of optical absorption and energy conversion efficiency of anatase TiO2 nanocrystals by hydrogenation.
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DOI：
10.1039/c1cp22726b
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发表时间：
2011-10
期刊：
影响因子：
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通讯作者：
Jibao Lu;Ying Dai;Hao Jin;Baibiao Huang
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文献类型：
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作者：
Jibao Lu;Ying Dai;Hao Jin;Baibiao Huang
通过氢化法制备的无序纳米TiO_2具有良好的太阳光催化活性[X.陈湖，澳-地Liu，P. Y. Yu，S. S. Mao，Science，2011，331，746]，而无序的详细图像尚不清楚，并且氢化的作用以及高光活性的机制仍不明确。基于第一性原理计算，考虑到Ti-H键和O-H键的协同作用，我们发现在（101），（001）和（100）面，氢原子可以化学吸附在Ti（5c）和O（2c）原子上，而以前的研究预测H在Ti（5c）和O（2c）上的化学吸附仅发生在（001）表面，这是由于忽略了协同效应。氢化导致纳米粒子的（101）和（100）面晶格畸变，增强了价带内的带内耦合，而（001）面没有受到很大影响。与以往认为晶格无序是导致中间带隙态的主要原因，而氢原子只是通过钝化悬挂键来稳定晶格无序不同，我们发现吸附原子不仅导致晶格无序，而且与Ti 3d和O 2 p态发生强烈的相互作用，对中间带隙态有相当大的贡献.由于中间带隙态，光吸收显著红移，并且由于氢化纳米颗粒的不同面之间的电子-空穴流动，光生电子-空穴分离被显著促进，这可以解释在太阳照射下异常高的能量转换效率。更有趣的是，我们发现氢化逆转了纳米颗粒不同表面的氧化还原行为，这提供了新的提示，人们可以根据自己的要求通过适当的化学表面处理来调节纳米颗粒不同表面之间的光激发电子-空穴流。我们相信，纳米晶体的不同面之间的带偏移工程可以是一种有效的方式，以促进能量转换效率，并应适用于其他纳米材料。
Disorder-engineered nanophase anatase TiO(2) through hydrogenation has been demonstrated to exhibit substantial solar-driven photocatalytic activities [X. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science, 2011, 331, 746], while the detailed image of the disorder is unclear, and the role of the hydrogenation as well as the mechanism of high photoactivity is still ambiguous. Based on first-principles calculations, we find by taking into account the synergic effect of Ti-H and O-H bonds that hydrogen atoms can be chemically absorbed both on Ti(5c) and O(2c) atoms for (101), (001), and (100) surfaces, while previous studies predicted that chemical absorption of H on both Ti(5c) and O(2c) only takes place on the (001) surface due to overlooking the synergic effect. The hydrogenation induces obvious lattice distortions on (101) and (100) surfaces of nanoparticles enhancing the intraband coupling within the valence band, while the (001) surface is not largely affected. Different from the previous understanding that the lattice disorder accounts for the induced mid-gap states while the hydrogen only stabilizes the lattice disorders by passivating their dangling bonds, we find that the adatoms not only induce the lattice disorders but also interact strongly with the Ti 3d and O 2p states, resulting in a considerable contribution to the mid-gap states. The optical absorption is dramatically red shifted due to the mid-gap states and the photogenerated electron-hole separation is substantially promoted as a result of electron-hole flow between different facets of hydrogenated nanoparticles, which may account for the exceptional high energy conversion efficiency under solar irradiation. Even more interestingly, we find that hydrogenation reverses the redox behavior of different surfaces of nanoparticles, which provides new hints that one can tune the photoexcited electron-hole flow between different surfaces of nanoparticles in accordance to one's request by appropriate chemical surface treatment. We believe that band-offset-engineering between different facets of nanocrystals can be an effective way to facilitate energy conversion efficiency and should be applicable to other nanophase materials.