Morales-García Ángel, Macià Escatllar Antoni, Illas Francesc, Bromley Stefan T
Departament de Ciència de Materials i Química Física & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, c/Martí i Franquès 1-11, 08028 Barcelona, Spain.
Nanoscale. 2019 May 9;11(18):9032-9041. doi: 10.1039/c9nr00812h.
Anatase TiO2 nanoparticles (NPs) have the potential to photocatalyse water splitting using UV light, to thus provide hydrogen fuel in a clean and sustainable manner. Such NPs have optical gaps covering a small range of relatively high energy solar photons, giving rise to low photo-efficiencies. Although anatase NPs with 10-20 nm diameters thermodynamically prefer crystalline faceted morphologies, application of physico-chemical procedures can produce more rounded NPs with amorphous shells. Such engineered metastable core-shell NPs (so-called black TiO2 NPs) have reduced band gaps due to shell-induced band edge broadening, resulting in higher photoactivities. For <5 nm diameters, TiO2 NPs typically exhibit spherical-like NP morphologies, which also display enhanced photoactivity. For smaller NPs it is difficult to experimentally determine their thermodynamic stability and internal atomic structure, to help rationalise their higher photoactivities. Employing accurate electronic structure calculations, we establish the relative stability of spherical and faceted stoichiometric TiO2 NPs with 1-3.4 nm diameters. Mirroring experimental preparation, simulated thermal annealing is found to significantly stabilise relaxed spherical cut anatase NPs. We find that the smallest spherical NPs become amorphized by annealing, but, for diameters >2 nm, annealing yields NPs with anatase-cores and amorphous-shells. Like larger black TiO2 core-shell NPs, we confirm that our core-shell NPs are metastable relative to faceted anatase NPs and have significantly smaller optical gaps than faceted NPs. Our calculated gaps are in excellent agreement with experimental data, strongly supporting the validity of our NP models. Energy gap narrowing in these core-shell NPs is found to be due to broadening of valence band states induced by the amorphous shell, analogous to the mechanism proposed for black TiO2 NPs. Our stoichiometric NPs also show that this band narrowing effect does not require the disordered shells to be non-stoichiometric or for incorporation of other atom types. Instead, we find that this effect mainly arises from 4-coordinated Ti atoms in the amorphous shell. Our careful and systematic computational investigation, using NP models of unprecedented realism, thus provides direct confirmation that the enhanced photoactivity in small spherical TiO2 NP observed in experiment is due to the formation of metastable core-shell NPs with 4-coordinated Ti centres.
锐钛矿型二氧化钛纳米颗粒（NPs）有潜力利用紫外光光催化水分解，从而以清洁且可持续的方式提供氢燃料。这类纳米颗粒的光学带隙覆盖了一小部分能量相对较高的太阳光子，导致光效率较低。尽管直径为10 - 20纳米的锐钛矿型纳米颗粒在热力学上倾向于结晶多面体形貌，但物理化学方法的应用可以产生更多带有非晶态壳层的更圆润的纳米颗粒。这种经过工程设计的亚稳态核壳纳米颗粒（所谓的黑色二氧化钛纳米颗粒）由于壳层引起的能带边缘展宽而具有减小的带隙，从而导致更高的光活性。对于直径小于5纳米的情况，二氧化钛纳米颗粒通常呈现类球形的纳米颗粒形貌，这也表现出增强的光活性。对于更小的纳米颗粒，很难通过实验确定它们的热力学稳定性和内部原子结构，以帮助解释它们更高的光活性。通过精确的电子结构计算，我们确定了直径为1 - 3.4纳米的球形和多面体形化学计量比二氧化钛纳米颗粒的相对稳定性。与实验制备情况相似，发现模拟热退火能显著稳定弛豫后的球形切割锐钛矿型纳米颗粒。我们发现最小的球形纳米颗粒在退火后会变成非晶态，但对于直径大于2纳米的情况，退火会产生具有锐钛矿型核心和非晶态壳层的纳米颗粒。与更大的黑色二氧化钛核壳纳米颗粒一样我们证实我们的核壳纳米颗粒相对于多面体形锐钛矿型纳米颗粒是亚稳态的，并且其光学带隙比多面体形纳米颗粒小得多。我们计算得到的带隙与实验数据非常吻合，有力地支持了我们纳米颗粒模型的有效性。发现这些核壳纳米颗粒中的能隙变窄是由于非晶态壳层引起的价带态展宽，这与针对黑色二氧化钛纳米颗粒提出的机制类似。我们的化学计量比纳米颗粒还表明，这种能带变窄效应并不要求无序壳层是非化学计量比的或掺入其他原子类型。相反，我们发现这种效应主要源于非晶态壳层中的四配位钛原子。因此，我们使用具有前所未有的真实性的纳米颗粒模型进行的仔细而系统的计算研究直接证实了实验中观察到小的球形二氧化钛纳米颗粒增强的光活性是由于形成了具有四配位钛中心的亚稳态核壳纳米颗粒。
