Anatase TiO2 Single Crystals With a Large Percentage of Reactive Facets

ARC Centre of Excellence for Functional Nanomaterials, School of Engineering and Australian Institute for Bioengineering and Nanotechnology, Queensland 4072, Australia.
Nature (Impact Factor: 41.46). 06/2008; 453(7195):638-41. DOI: 10.1038/nature06964
Source: PubMed

ABSTRACT Owing to their scientific and technological importance, inorganic single crystals with highly reactive surfaces have long been studied. Unfortunately, surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. A typical example is titanium dioxide (TiO2), which has promising energy and environmental applications. Most available anatase TiO(2) crystals are dominated by the thermodynamically stable {101} facets (more than 94 per cent, according to the Wulff construction), rather than the much more reactive {001} facets. Here we demonstrate that for fluorine-terminated surfaces this relative stability is reversed: {001} is energetically preferable to {101}. We explored this effect systematically for a range of non-metallic adsorbate atoms by first-principle quantum chemical calculations. On the basis of theoretical predictions, we have synthesized uniform anatase TiO(2) single crystals with a high percentage (47 per cent) of {001} facets using hydrofluoric acid as a morphology controlling agent. Moreover, the fluorated surface of anatase single crystals can easily be cleaned using heat treatment to render a fluorine-free surface without altering the crystal structure and morphology.

Download full-text


Available from: Gang Liu, Sep 26, 2015
622 Reads
    • "These will be discussed in detail in Section 3.5 " Thin film structure and crystallization " . Aside from phase transformation, texture is an important aspect for photocatalysis, because the crystal planes at the surface – often referred to as facets – are known to have significantly different photocatalytic efficiencies [27] [28] [29]. While the sputtered thin films show the typical anatase peak distribution of a powder sample matching nicely the reference, the evaporated thin films have distinct texture. "
    [Show abstract] [Hide abstract]
    ABSTRACT: In order to understand the variations in photocatalytic efficiencies of titania thin films prepared by different physical vapor deposition techniques, we studied the microstructure and resulting properties for the two widely used PVD methods, electron beam evaporation and reactive pulsed DC magnetron sputtering. In addition, we investigated the effect of oxygen vacancy defects induced by tempering in reducing atmospheres and the aging behavior. After deposition, several tempering series in oxygen, air, argon, and forming gas were carried out to control the amount of oxygen vacancy defects. The films were characterized with respect to crystallinity, texture, grain growth, grain structure, surface roughness, light transmission as well as band gap energy; the photocatalytic efficiency was measured via methylene blue degradation. The results show different nucleation and growth mechanisms between evaporated and sputtered titania thin films, resulting in severe influence on photocatalytic efficiency. The evaporated thin films exhibited homogeneous nucleation and growth, and stayed in the anatase structure even after tempering at 800 °C for 1 h. In contrast, the sputtered thin films started to form grains at the interface to the substrate and showed heterogeneous nucleation and growth. Moreover, the sputtered films already formed rutile when tempered at 600 °C for 1 h. The gain in surface area due to tempering, which promotes adsorption of photocatalytic reactants, was more pronounced in sputtered thin films (+29%) compared to evaporated thin films (+6%). Films with oxygen vacancy defects, preserved by tempering in argon or induced by tempering in forming gas, showed a large improvement in photocatalytic efficiency. However, aging of the samples over a period of 19 months lead to a progressive decline in efficiency, finally reaching the level of thin films tempered in air, which had remained stable over the same 19 month period. This strongly questions widely applied concepts based on improving the photocatalytic efficiency via oxygen vacancy defects.
    • "Therefore, the Ti-HF composition has the highest percentage of {0 0 1} facets, leading to the formation of nanosheets where this facet is predominant. This is probably because the F atoms exert a stronger stabilizing effect than the O atoms on {0 0 1} facets [16]. In the case of Ti-TFAA a higher stabilization of this facet with respect to the other products is also observed, due to TFAA being degraded during the synthesis releasing F ions, and therefore truncated octahedrons and nanosheets are observed. "
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper provides direct evidence to support the role of capping agents in controlling the evolution of TiO2 seeds into nanocrystals with a specific shape. Starting with Ti(OBut)4 and using oleid acid, oleylamine, dioleamide, 11-aminoundecanoic acid, arginine, trifluroacetic acid or HF as capping agents, mainly TiO2 truncated octahedrons enclosed by {101} and {001} facets were obtained. We could also selectively obtain square, rods and rounded rhombic-shaped nanoparticles by growing of {010} facets by adding oleic acid and oleylamine in ratio 6:4, respectively, while all other parameters were kept the same. This research not only offers new insights into the role played by a capping agent in shape-controlled synthesis but also provides, a versatile approach to controlling the shape of metal oxide nanocrystals.
    Boletin de la Sociedad Espanola de Ceramica y Vidrio 07/2015; 54(4):159-165. DOI:10.1016/j.bsecv.2015.07.001 · 0.29 Impact Factor
  • Source
    • "Recently, theoretical and experimental studies have demonstrated that, the {0 0 1} facets of anatase TiO 2 are much more reactive than the thermodynamically more stable {1 0 1} surface due to higher average surface energy of the {0 0 1} facets (0.90 J/m 2 ) than that of the {1 0 1} facets (0.44 J/m 2 ) [8] [9]. A pioneering breakthrough in the synthesis of anatase TiO 2 with exposed {0 0 1} facet was accomplished since Yang et al. [10] [11] synthesized anatase TiO 2 microcrystals with 47% {0 0 1} facets on the surface by reversing the relative stability of {1 0 1} and {0 0 1} facets using HF as morphology-controlling agent. Afterward, Han et al. [12] found that {0 0 1} facets exhibit higher photocatalytic activity than {1 0 1} facets due to the high density of active unsaturated Ti atoms and active surface oxygen atoms on {0 0 1} facets. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Pt, Pd, Ag and Au nanoparticles were photodeposited on the {001} crystal facets of the TiO2 anatase nanosheets. Morphological and surface characterization of the samples as well as photocatalytic activity was studied. The influence of metal precursor concentration used during photodeposition (0.05 to 0.5%) on size of formed metal nanoparticles together with UV and vis-mediated activity of Pt, Pd, Ag or Au-TiO2 was investigated. Generally, samples obtained by photodeposition of noble metal nanoparticles using their 0.2% precursor solutions revealed highest activity in phenol degradation reaction under visible light (λ > 420 nm). The photoactivity of the as-prepared samples with respect to the modified metal species was ordered Ag≅Pd > Au > Pt. TEM analysis shown that photodeposited metal nanoparticles appeared only on {001} facets of TiO2. The average degradation rate of phenol in the presence of Pd and Ag-TiO2 was 0.5 μmol·dm−3·min−1 after 60 min of irradiation under visible light, and was 5 times higher than that of pure TiO2 nanosheets.
    Applied Surface Science 04/2015; 347. DOI:10.1016/j.apsusc.2015.04.103 · 2.71 Impact Factor
Show more