Janus Au-TiO2 Photocatalysts with Strong Localization of Plasmonic Near-Fields for Efficient Visible-Light Hydrogen Generation
Janus Au-TiO2 Photocatalysts with Strong Localization of Plasmonic Near-Fields for Efficient Visible-Light Hydrogen Generation
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DOI：
10.1002/adma.201104241
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发表时间：
2012-05-02
影响因子：
29.4
通讯作者：
Han, Ming-Yong
中科院分区：
文献类型：
--
作者：
Seh, Zhi Wei;Liu, Shuhua;Han, Ming-Yong
最近在精确控制金属纳米颗粒的形状和尺寸方面的进展使得能够系统地设计其局部表面等离子体共振（LSPR）特性。[1-3]为了结合新的功能，这些等离子体金属纳米颗粒与各种介电氧化物材料（例如，SiO 2，[4] TiO 2，[5] ZrO 2 [6]和Fe 3 O 4 [7]）结合，形成具有光学，电子和磁性的金属氧化物混合纳米结构。在理解和微调对称核壳金属氧化物纳米结构的LSPR方面已经取得了很大进展，已知其强烈依赖于材料组成、尺寸和形状以及局部介电环境。[8]从等离子体的角度来看，Janus形态越来越重要[9]，因为金属和电介质氧化物纳米颗粒在其小的互连结处的非中心对称耦合，这可能通过颗粒间耦合产生极强的局部电近场。[10]虽然核-壳金属氧化物纳米结构的LSPR特性已经被广泛研究，但它们的Janus对应物的等离子体性质仍然相对未被探索。本文研究了非中心对称的Janus Au 50 nm-TiO 2纳米结构与50 nm金纳米颗粒的LSPR性质，与核壳结构Au 50 nm-TiO 2纳米结构相比，其LSPR红移较小，与基于离散偶极近似（DDA）的模拟结果一致.这是由于Janus形态中等离子体金纳米颗粒周围有两种不同的介电环境：一侧是高折射率的TiO 2，另一侧是低折射率的溶剂。由于其有趣的LSPR特性，非中心对称的Janus Au-TiO 2纳米结构被用作高效等离子体增强可见光制氢的光催化剂，这是第一次报告，据我们所知。与对称核壳结构的光催化剂相比，Janus光催化剂具有更高的产氢速率，这是由于在金纳米颗粒一侧靠近Au-TiO 2界面的等离子体近场具有更强的局域化能力。在该区域，等离子体激元近场与非晶TiO 2中局域电子态的光学跃迁强烈耦合，导致了有效的光吸收和电子空穴对的产生。实验上，以二异丙氧基二氧化钛（TiO 2）为前驱体，通过控制水解反应制备了Au 50 nm TiO 2光催化剂，考察了其光催化活性，并对制备的催化剂进行了表征。（乙酰丙酮化物），在室温下在50 nm金纳米颗粒的4：1 v/v异丙醇/水溶液（pH= 9-10）中进行（详情参见支持信息）。[9e]两种不同的形态，Janus和核壳，通过控制添加相同体积的TiO 2前体溶液，分别在一个和三个部分，实现（图1a，B）。使用X射线衍射和拉曼散射，确定Janus和核-壳纳米结构中的所得TiO 2为无定形（图S1，支持信息）。为了比较，还以与上述Janus Au-TiO 2相同的方式制备无定形TiO 2纳米颗粒，不同之处在于用去离子水代替金纳米颗粒的水溶液（图S2 a，支持信息）。
Recent advances in the precise control over the shape and size of metal nanoparticles have enabled systematic engineering of their localized-surface-plasmon-resonance (LSPR) properties.[1–3] To incorporate new functionalities, these plasmonic metal nanoparticles have been coupled with various dielectric oxide materials (eg, SiO 2,[4] TiO 2,[5] ZrO 2 [6] and Fe 3O 4 [7]) to form metal-oxide hybrid nanostructures with combined optical, electronic and magnetic properties. Much progress has been made in understanding and fine-tuning the LSPR of symmetric coreshell metal-oxide nanostructures, which is known to be strongly dependent on the material composition, size and shape and the local dielectric environment.[8] From a plasmonic point of view, the Janus morphology is gaining importance [9] because of the non-centrosymmetric coupling of the metal and dielectric oxide nanoparticles at their small, interconnecting junction, which can potentially generate extremely strong local electric nearfields through interparticle coupling.[10] Although the LSPR characteristics of core-shell metal-oxide nanostructures have been widely studied, the plasmonic properties of their Janus counterparts remain relatively unexplored. Herein, we investigate the LSPR properties of non-centrosymmetric Janus Au 50 nm-TiO 2 nanostructures with 50 nm gold nanoparticles, which experienced a smaller red-shift in the LSPR upon TiO 2 coating, compared with their core-shell Au 50 nm-TiO 2 counterparts, in agreement with simulation results based on the discrete-dipole approximation (DDA). This is due to the two different dielectric environments surrounding the plasmonic gold nanoparticles in the Janus morphology: higherrefractive-index TiO 2 on one side and lower-refractive-index solvent on the other. Because of their interesting LSPR properties, the non-centrosymmetric Janus Au-TiO 2 nanostructures were employed as photocatalysts in efficient plasmon-enhanced visible-light hydrogen generation, for which this is the first report, to the best of our knowledge. Compared with their symmetric core-shell counterparts, the Janus photocatalysts exhibited a much-higher rate of hydrogen generation due to the stronger localization of plasmonic near-fields close to the Au-TiO 2 interface on one side of the gold nanoparticles. In this region, the plasmonic near-fields are strongly coupled to optical transitions involving the localized electronic states in amorphous TiO 2, leading to an efficient optical absorption and generation of electron-hole pairs for photocatalysis. The use of larger gold nanoparticles in the Janus nanostructures was found to result in even-higher photocatalytic activity due to their stronger plasmonic near-fields.Experimentally, the Au 50 nm-TiO 2 photocatalysts were synthesized by controlled hydrolysis of a TiO 2 precursor, titanium diisopropoxide bis (acetylacetonate), in a 4: 1 v/v isopropyl alcohol/aqueous solution (pH= 9–10) of 50 nm gold nanoparticles at room temperature (see Supporting Information for details).[9e] The two different morphologies, Janus and core-shell, were achieved by controlling the addition of the same volume of the TiO 2 precursor solution in one and three portions, respectively (Figure 1a, b). The resulting TiO 2 in the Janus and core-shell nanostructures was determined to be amorphous, using X-ray diffraction and Raman scattering (Figure S1, Supporting Information). For comparison, amorphous TiO 2 nanoparticles were also prepared in the same way as that of the Janus Au-TiO 2 described above, except that the aqueous solution of the gold nanoparticles was replaced with deionized water (Figure S2a, Supporting Information).