Direct Measurement of Size, Three‐Dimensional Shape, and Specific Surface Area of Anatase Nanocrystals

Abstract

At the heart of photocatalysis lies the recognition that chemical reactions are catalyzed on specific surfaces of catalyst particles. Because of the specific structure of such particles and the coordination of exposed surface atoms, the reactivity of different surfaces and their mechanisms for bonding to adsorbed molecules can vary dramatically. [1,2] Information about the size and shape of titanium dioxide (TiO 2 ) nanocrystals and, in particular, about the abundance and relative surface areas of specific crystal facets is highly desirable for the development of sophisticated photocatalytic experiments that are based on powdery materials. Unfortunately, for real catalysts, the kind of crystal facets that are exposed and their relative abundances are often not known. Profound knowledge about the relationship between processing conditions and the abundance of specific crystal facets would allow for tailoring improved catalysts that show predominantly those surfaces that are catalytically most active for specific reactions. The discovery of the decomposition of water on TiO 2 electrodes [3] stimulated many further investigations and opened up the research area of photocatalysis. Today, titanium dioxide is by far the most important photocatalyst material. It is commercially available as powders in two crystalline modifications, anatase and rutile, with surface areas ranging from 10 to more than 300 m 2 g À1. There is some evidence that the surface energy densities of anatase are generally lower than those of rutile. [4][5][6] However, theoretical predictions of the relative stability of perfect rutile and anatase surfaces do not give a coherent picture. Depending on the quantum-chemical method, the calculated rutile (110) surface energy density is found to be smaller [7] or larger [8,9] than those of all anatase low-index surfaces. Calculations based on classical pair potentials [6,10] disagree as to the relative stability of different anatase surfaces. All of these theoretical calculations are for perfect clean TiO 2 surfaces. A study for water-covered surfaces predicts anatase (101) and anatase (100) to be more stable than rutile (110), depending on the coverage with hydrogen or oxygen. [5] In another theoretical study of rutile and anatase nanoparticles of 2-6 nm diameter, [11] it was concluded that anatase particles have smaller surface energy densities than rutile up to a diameter of 2.5 nm. Consequently, for small crystals that show high surface-tovolume ratios, it is advantageous to adopt the anatase modification, and large titania crystals are thermodynamically favored if they have the rutile structure. The rutile-to-anatase transition occurs at particle sizes of a few tens of nanometers, and it depends on temperature, grain size, nanocrystal morphology, impurity content, reaction atmosphere, and synthesis conditions. [5,12]

Oxalic acid at the TiO 2 /water interface under UV(A) illumination: Surface reaction mechanisms

“…The lattice parameters and the atom positions in the bulk of the cluster were fixed to the values obtained from the previous three-dimensional calculation (CCM-3D) [2]. The (1 0 0) and (1 1 0) planes for anatase and rutile, respectively, were used as representative surfaces according to respective high resolution transmission electron microscopy (HRTEM) [9] investigations.…”

“…In particular, when working with nanoparticles, the detailed knowledge of the nature of the exposed facets and of the atomic structure of the nanoparticles is of utmost interest due to their key role in the photocatalytic reaction mechanism [7,8]. Since, in particular, commercially produced TiO 2 nanocrystallites do not exhibit the equilibrium faces naturally occurring in macroscopic single crystals [9], photocatalytic reaction models assuming the well-known crystallographic facets may be unrealistic. Different exposed surfaces will definitively lead to different chemical interactions with a given adsorbate [1,2] due to a different coordination of the surface atoms, thus likely triggering different reaction mechanisms.…”

Nitrogen(II) Oxide Charge Transfer Complexes on TiO2: A New Source for Visible-Light Activity

“…However, the anatase phase of titanium dioxide enjoys more relevance as photocatalyst due to its generally higher photocatalytic activity. [45][46][47][48] The (101) facet of anatase is found to be thermodynamically favorable. However, the high reactivity of anatase nanoparticles has been often associated with the minority (001), 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 The surfaces were modeled by finite neutral TiO 2 clusters where the peripheral oxygen atoms not belonging to the surface were saturated with hydrogen.…”

Dye adsorbates BrPDI, BrGly, and BrAsp on anataseTiO2(001)for dye-sensitized solar cell applications

“…Relative stability of various low-index surfaces of anatase nanoparticles might be controlled by surface chemistry such as pH environment or hydroxilation, or particle size. 17,18 Another recent study showed that single-crystal anatase TiO 2 with a high percentage ͑001͒ surface can be synthesized by using hydrofluoric acid as a morphology controlling agent. 19 Furthermore, fluorated anatase surface can be easily cleaned to obtain fluorine free surface without altering the crystal structure and morphology.…”