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


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.

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    • "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. "
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    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.
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    • "The (001) crystal facets have higher photocatalytic activity (oxidative power) compared to the (100) or (101) crystallographic surfaces, especially at nanometer scale [31] [32] [33] [34]. The (001) planes can be grown in the presence of F À , which are very efficient species in the shape tailoring of TiO 2 nanoparticles [35] [36] [37]. There are some papers regarding the improvement of rutile's photocatalytic activity by the formation of (111) facet dominated pyramid-shaped nanoparticles [38]. "
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    ABSTRACT: The photocatalytic activity of TiO2 photocatalysts depends mainly on its crystal phase composition, primary particle size and specific surface area. Shape manipulation is an interesting way to increase the photocatalytic efficiency. The shape-tuning can be carried out at different levels, both at single crystal and polycrystalline agglomeration levels. The aim of our present study was to compare the structural and photocatalytic performances of two type/level of crystal organization of TiO2, namely single crystal shaping vs. polycrystalline/shape tailored agglomeration. The morphological analysis was achieved by XRD, SEM, TEM, Raman spectroscopy, DRS. The photocatalytic performance of the materials was evaluated by the degradation of a model pollutant (phenol). It was found, that both shape manipulating approaches bear the necessary potential which can be exploited in future development of efficient photocatalysts’ synthesis procedures.
    Ceramics International 10/2015; DOI:10.1016/j.ceramint.2015.10.095 · 2.61 Impact Factor
    • "It has been reported that the photoactivity of TiO 2 anatase, the most active photocatalyst [5] [6] [7] [8] [9] [10] [11], is influenced by different properties such as surface area, crystallinity, crystallite size and crystal structure [12] [13]. But, this photoactivity depends not only on these properties, but also on the specific morphology of the particles, which determines which crystal facets are exposed, having a strong influence on the photocatalytic performance, e.g. they can lead to a reduction and the formation of electron traps and thus facilitate the electron transfer in the semiconductor structure [14] [15] [16] [17] [18] [19] [20] [21]. Both theoretical and experimental studies have shown that the (0 0 1) surface of anatase TiO 2 with 100% Ti five-fold coordinated (Ti 5c ) is much more reactive than the thermodynamically more stable {1 0 1} facets with 50% Ti 5c and 50% Ti 6c (Fig. 1a) [22] [23] [24] [25] [26]. "
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    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
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