Enhancement of Photoexcited Charge Transfer by {001} Facet-Dominating TiO₂ Nanoparticles

Abstract

b S Supporting Information N anosized metal oxide semiconductors have been actively studied for the past few decades, especially for their potential applications for photocatalytic reactions, photobreaching of toxic compounds, artificial photosynthesis, and photovoltaics. 1À5 Among all metal oxide semiconductors, TiO 2 is one of the most important materials because of the abundance of titanium as a natural resource, corrosion resistivity, transparency in the visible region, and nontoxicity. The kinetically favorable anatase crystalline phase is the most widely studied due to its ease of preparation, stability of the crystal up to ∼600°C, and advantages for catalytic and electronic properties. The anatase crystal of TiO 2 has a few fundamental low-index facet systems, such as {101}, {001}, {100}, {110}, and {103}. 2 Typical anatase crystal of TiO 2 , including naturally grown crystals, consists of mostly the {101} facet due to the much greater stability of this surface than other surfaces due to its lower surface energy. Therefore the characteristics of TiO 2 nanoparticles, in most cases, have been supposed to be attributed to the {101} facet, regradless of whether it is mentioned in the literature, since it had been extremely difficult to selectively synthesize other unstable surfaces, such as the (001) surface. 2,6 Recently, however, Lu et al. proposed a novel synthesis to selectively increase the fraction of {001} facet and thus introduced a new area of study, the chemically reactive (001) surface, into the widely studied TiO 2 nanoparticles. 7 The proposed novel process is composed of the addition of a relatively large amount of hydrofluoric acid (HF) for hydrothermal growth of TiO 2 . Although a few groups also reported the follow-up work of {001} facet-dominating TiO 2 preparation, most groups used the same procedure using HF. 8À11 The role of HF in TiO 2 synthesis was also suggested to be that the fluorine-termination of TiO 2 surface stabilizes the (001) surface more than the other facets and results in a larger fraction of intrinsically unstable (001) surface; the result of the first-principle calculations also supported this explanation. 7 The typical fraction of {001} facet in reports varies from 40 to 80%, usually determined by the characteristic bipyramidal structure of particles revealing the trapezoidal (101) and rectangular (001) surfaces observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). 7À12 Since the surface energy of the {001} facet is higher than that of the {101} facet, recent reports indicated a few characteristic features of the {001} facet, a site-selective reaction, enhancement of reactivity, adsorption of molecules, and water dissociation with {001} facetdominating TiO 2 nanoparticles 13À15 for application to photocatalytic reactions 10,11 and photovoltaics. 12,16 Additionally, a few theoretical investigations based on first-principle calculations also reported the characteristic reactivity of the {001} facet. 17,18 Nevertheless, detailed analysi...

One-step uniformly hybrid carbon quantum dots with high-reactive TiO2 for photocatalytic application

“…96% of the surface was covered by the {0 0 1} facet which possesses the highest thermodynamic stability [2,7,8]. By employing acid environment and F as the controlling agent, recent investigations have proven that introduce high-energetic facets ({1 1 0} (1.09 J/m 2 ) > {0 0 1} (0.90 J/ m 2 ) > {0 1 0} (0.53 J/m 2 ) > {1 0 1} (0.44 J/m 2 )) to replace the {1 0 1} facet can significantly enhance the photocatalytic activity [1,[9][10][11]. However, even various reaction systems have been discovered to synthesize such kinds of TiO 2 , the wide band gap (ca.…”

Structural, electronic and optical features of molybdenum-doped bismuth vanadium oxide

“…Last two decades, TiO 2 (titanium di-oxide) has been highly investigated photocatalyst because of its low fabrication cost, high photocatalytic efficiency, and high chemical stability in aqueous media [1]. However, TiO 2 with the band gap of 3.2 eV responds only to UV light, which absorbs only ca.…”

The effect of hydrothermal temperature on the structure and photocatalytic activity of {001} faceted anatase TiO2

“…Improving the light absorption and separation efficiency of the photogenerated electron-hole pairs are two key factors in improving the activities of the photocatalysts [1,2]. Nowadays, there is an enormous interest in controlling the anatase TiO 2 crystal facets (e.g., {010}, {001}, {100}) due to the interesting facet-dependent physicochemical properties [3,4]. The most investigated issue is to increase the percentage of the exposed {001} facet.…”

“…(2) The formed surface heterojunction increases the separation efficiency of electrons and holes [6]. (3) Different abilities in absorbing the pollutant molecules [7]. All the properties are closely related to the crystal structure.…”