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利用光催化技术将太阳能转化为化学能，是解决能源危机和环境污染的一种有前途的路径，光分解水成氢气和氧气是其中的一个思路。要提高光驱动分解水产生氢气和氧气的效率，有两个关键的过程需要能有效调控：一是要使光激发产生的电子和空穴能有效分离；二是要使产氢反应和产氧反应在空间能够上分离，从而有效避免水分解反应的逆反应（2H2+O2→2H2O）。实现这些目标的一个可行的策略是利用晶体表面工程，即采用合适的合成方法制备出具有不同极性的半导体催化表面，电子和空穴倾向于迁移到不同的暴露面，从而实现有效的空间分离。沿着这个方向已经有不少有意义的探索，遗憾的是之前所研究的材料，要么只具有产氢功能，要么只具有产氧功能，几乎没有利用单一半导体光催化剂能够同时具好产氢和产氧功能选择的报道，寻找相应的新材料就成为一个关键。
近年来陆亚林课题组利用层状氧化物的结构和组成的丰富可调性，利用其所提出的原子层间嵌入技术，成功的把该材料体系拓展到铁电铁磁等多参量共存以及室温下耦合领域，发明了多种室温磁电耦合、交换偏置、铁磁绝缘等新材料。在这些广泛研究的基础上，如何把这类新材料应用到能源环境领域就成为一个自然的课题选择，例如，利用其中的铁电半导体特性，是非常有可能发展一种极具吸引力的光催化剂。这是因为：一、内极化可能会有效降低产氢和产氧反应对光催化剂能带结构的要求，因此有可在单相光催化剂中实现高效、选择性地产氢和产氧；二、铁电体内的内置自发极化所产生的内建电场有可能极大地抑制光生电子空穴对的复合。第三，内建电场也将有可能引导光生电子空穴迁移到不同的表面，从而抑制水分解的逆反应；最后，这类新材料相对偏低的对称性使得使用简单的合成方法成为可能，因而就有可以获得不同的极性暴露面。
沿着这个思路，在前期充分的工作基础上（ACS Appl. Mater.Interfaces, 2017, 9, 19908; Nanotechnology, 2018, DOI:10.1088/1361-6528/aabdba），近日中国科学技术大学陆亚林课题组傅正平副教授采用改进的熔盐法和固态法合成了该层状材料体系中的一种层状铁电纳米片新材料Bi3TiNbO9，成功的选择制备了{001}和{110}暴露面，并巧妙的利用合成过程温度控制调控暴露面的比例。发现Bi3TiNbO9纳米片在光照下既能分解水产氢，也能产氧，最有意思的是发现通过温度调节{001}/{110}面的比例，可以实现有选择性地优化产氢或产氧。{001}暴露面比例最高的样品具有最好的产氢能力(342.6μmol h−1g−1)，而{110}暴露面比例最高的样品具有最好的产氧性能(275.2μmol h−1g−1)，表明{001}面为产氢活性面，而{110}为产氧活性面。本研究对类似的光催化剂中合理设计并高效产氢或产氧具有指导意义，该文章发表在Nano Energy上: Xiaofeng Yin et al., Realizing selective water splitting hydrogen/oxygen evolution on ferroelectric Bi3TiNbO9 nanosheets, Nano Energy, 19， 489-497， 2018. DOI：10.1016/j.nanoen.2018.05.001。傅正平副教授和陆亚林教授为共同通讯作者，文章第一作者为殷小丰博士研究生。 样品制备：Bi3TiNbO9 nanosheets were synthesized by modified molten salt method. The experimental procedure is as follows: First, the stoichiometric ratios Bi(NO3)3·5H2O, Ti(OC4H9)4 and Nb2O5 were added to 20 mL HNO3 (4 M), 10 mL absolute ethanol, 25 mL deionized water, respectively. After 0.5 h magnetic stirring, the ethanolic solution of Ti(OC4H9)4 and the aqueous solution of Nb2O5 were added dropwise to the Bi(NO3)3·5H2O nitric acid solution in turn. The mixed solution was continuously stirred for another half an hour and then concentrated ammonia was added dropwise to the mixed solution until the solution PH value of 9-10. The resulting suspension was stirred for an additional hour and followed by repeatedly washing with deionized water until neutral. The washed precursors were dried at 70 °C for 12h in an oven. Then, the precursor and NaCl-KCl mixed molten salt with a mass ratio of 1: 8 were ground evenly and the mixtures were calcined at the set temperature for 1 h in a muffle furnace. The calcination temperatures were set at 700 °C, 750 °C, 800 °C and 900 °C as required. The corresponding samples were labelled as BTNO-M700, BTNO-M750, BTNO-M800 and BTNO-M900, respectively. Finally, the calcined products were washed repeatedly with hot deionized water to remove the molten salt and dried to obtain the final target samples. As a comparative sample, Bi3TiNbO9 ferroelectric was also synthesized by solid state method labelled as BTNO-SS. The details of the solid state method are described as follows. First, the stoichiometric ratios of Bi2O3, Nb2O5 and TiO2 were repeatedly ground in a mortar for half an hour. Subsequently, the resulting mixture was transferred into an alumina crucible and calcined (800 °C, 2 h) in a muffle furnace. The heating rate was set to 3 °C/min and the cooling rate was set to 5 °C/min. Fig. 1 (a) Crystal structure diagram of Bi3TiNbO9; (b) Total density of states (TDOS) of Bi3TiNbO9 calculated by density functional theory (DFT); (c) XRD diffraction patterns of the powders synthesizedby molten salt method and solid state method; (d) High resolution XPS spectraof O 1s. Fig. 2 (a and b) SEM images of BTNO-SS and BTNO-M800; (c) TEM images of BTNO-M800; (d) Lattice-fringe phase of the white marking area in Fig 2c; (e) Selected area electron diffraction (SAED) perpendicular to the upper facet of BTNO-M800 nanosheet; (f) Simulation diagram for the crystal orientation of BTNO-M800 nanosheet. Fig. 3 (a) UV-vis diffuse reflectance spectra of BTNO-SS and BTNO-M800; (b) Water splitting hydrogen evolution and oxygen evolution of BTNO-SS and BTNO-M800; (c) Photocurrent - time curve; (d) Normalized hydrogen evolution and oxygen evolution activity. Fig.4 SEM images of the as-prepared Bi3TiNbO9 under different molten salt temperature: (a) BTNO-M900; (b) BTNO-M800; (c) BTNO-M750; (d)BTNO-M700. Fig. 5 (a) UV-vis diffuse reflectance spectra of BTNO-M700, BTNO-M750, BTNO-M800 and BTNO-M900; (b) (αhv)2 vs photonenergy curves; (c and d) Local magnification of the XRD patterns of the foursamples; (e) Water splitting hydrogen evolution and oxygen evolution of theabove samples; (f) The relation chart between the normalized hydrogen evolutionand oxygen evolution with their corresponding {001}/{110} ratio; (g) Therelation chart between the normalized hydrogen evolution and oxygen evolutionwith their corresponding{001}/{110} ratio. Fig. 6 Schematic diagram of water splitting hydrogen evolution and oxygen evolution process of Bi3TiNbO9 nanosheets. |
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