On the True Photoreactivity Order of {001}, {010}, and {101} Facets of Anatase TiO₂ Crystals

Abstract

Design and morphological control of crystal facets is a commonly employed strategy to optimize the performance of various crystalline catalysts from noble metals to semiconductors. [1][2][3][4][5][6][7][8] The basis of this strategy is that surface atomic configuration and coordination, which inherently determine their heterogeneous reactivity, can be finely tuned by morphological control.[3] The conventional understanding of the surface atomic structure of a crystal is that facets with a higher percentage of undercoordinated atoms are usually more reactive in heterogeneous reactions. For instance, {001} facets of anatase TiO 2 , which is one of the most important photocatalysts, [9][10][11][12][13][14][15][16][17] are considered to be more reactive than {101}. We have now discovered, by investigating a set of anatase crystals with predominant {001}, {101}, or {010} facets, that, contrary to conventional understanding, clean {001} exhibits lower reactivity than {101} in photooxidation reactions for OH radical generation and photoreduction reactions for hydrogen evolution. Furthermore, the {010} facets showed the highest photoreactivity. However, these three facets had similar photoreactivity when partially terminated with fluorine. We concluded that a cooperative mechanism of surface atomic structure (the density of undercoordinated Ti atoms) and surface electronic structure (the power of photoexcited charge carriers) is the determining factor for photoreactivity. The findings of this work open up new opportunities for maximizing photoreactivity through morphological control of photocatalysts.The predicted shape of anatase crystals under equilibrium conditions is a slightly truncated tetragonal bipyramid, enclosed by a majority of {101} and a minority of {001} facets.[18] In contrast to {101} facets with only 50 % fivecoordinate Ti (Ti 5c ) atoms, {001} facets with 100 % Ti 5c atoms were once considered more reactive in heterogeneous reactions. [19][20][21][22] A breakthrough by Yang et al. in understanding and controlling crystal facets dramatically increased the ratio of {001} to {101} in anatase, as illustrated in Figure 1 a. [8] Other important low-index facets, namely {010} facets, which also have 100 % Ti 5c atoms, may be dominant in the elongated truncated tetragonal bipyramids with appropriate surface chemistry, as predicted by Barnard and Curtiss [23] (see the right panel in Figure 1 a), and which was realized recently. [24,25]

Spatial separation of photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4

“…It is reasonable to deduce that the accumulation of electrons and holes originates from charge separation on different facets. The origin of the charge separation on the {010} and {110} crystal facets of TiO 2 has been already estimated by theoretical calculation 19 . Similarly, we also evaluated the energy levels of {010} and {110} facets of BiVO 4 by DFT calculation method.…”

“…Thus, certain facets of a semiconductor prefer reduction while others favour oxidation [18][19][20] . However, reports on the reduction and oxidation reaction facets on the same semiconductor crystal (such as TiO 2 ) were often contradictory in the literature [21][22][23][24][25][26] .…”

Water adsorption on rutile TiO2(110) for applications in solar hydrogen production: A systematic hybrid-exchange density functional study

“…[5][6][7] However, the required solar-to-fuel efficiencies have been unattainable at a reasonable cost, 8 and there is ample motivation to enhance the photochemical conversion efficiency of the photocatalytic material. A dependency of photocatalytic activity on the surface facet has become evident from a number of experimental studies, [9][10][11][12] though the reasons for this dependency are unknown (see Ref. 13 for proposed mechanisms).…”

Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis

“…These results show that a slight change occurred along the a and b directions, but there was significant expansion in the c direction, and as a result, the unit cell volume expanded significantly as well [10]. Recent publications showed that the morphology of TiO 2 materials resulted in differences of the enhanced photocatalytic activity for the production of hydrogen between the {101} and {001} facets of anatase tetragonal bipyramidal nanocrystals [11][12][13]. Based on the XRD simulation, Titanium Dioxide -Material for a Sustainable Environmentthe length and width of the peaks were calculated to confirm the percentages of the {101} and {001} facets (Figure 3).…”