REVIEW pubs.acs.org/cm
Anatase TiO2 with Dominant High-Energy {001} Facets: Synthesis, Properties, and Applications Shengwei Liu,† Jiaguo Yu,*,† and Mietek Jaroniec*,‡ †
State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, P. R. China ‡ Department of Chemistry, Kent State University, Kent, Ohio 44242, United States ABSTRACT: Control of the surface structure of inorganic materials, in particular titania (TiO2), by chemical processes under nonequilibrium conditions is of growing interest from scientific and utilitarian viewpoints. Titania is one of the most important materials because of its unique surface, electronic, and photocatalytic properties, which make this material applicable in many areas of science and technology ranging from adsorption, catalysis and photocatalysis to biomedicine, environmental monitoring and cleanup, energy conversion and storage, etc. Here we review the existing strategies for the synthesis of anatase TiO2 micro- and nanosheets with exposed high-energy {001} facets and for the assembly of these nanosheets into various hierarchical structures. The {001} facets are stabilized by specific capping agents (typically, fluoride), which are used to control the growth of titania crystals. The presence of high-energy facets in titania improves significantly its adsorption, electronic, and photocatalytic properties, making this material attractive for various environmental and energy-related applications. KEYWORDS: anatase TiO2, {001} facets, synthesis and assembly, properties and applications
1. INTRODUCTION Titanium dioxide (TiO2) has been intensively investigated because of its importance for environmental and energy-related applications, especially for photocatalysis, solar cell devices, sensors, catalyst supports, etc.1 3 The unique physical and chemical properties of TiO2 crystals are affected not only by the intrinsic electronic structure, but also by their size, shape, organization, and surface properties.2,3 In particular, some physicochemical properties such as, adsorption, catalytic reactivity and selectivity etc., largely depend on the surface atomic configuration and the degree of exposure of reactive crystal facets.4,5 Generally, during the crystal growth processes under equilibrium conditions, the high-energy facets diminish quickly and the crystal spontaneously evolves into specific shape with exposed facets that minimize the total surface free energy. Typically, the most available anatase TiO2 crystals are dominated by the thermodynamically stable {101} facets with lower surface free energy (0.44 J/m2).6 8 The production of anatase crystals with exposed {001} high-energy facets (0.90 J/m2)8 is important and challenging. For this reason, there is a great interest in the development of controllable synthesis and assembly of anatase TiO2 micro/nanostructures with dominant {001} high-energy facets and exploiting their enhanced surface properties for photocatalytic and related applications. In this concise review, we summarize the available routes for the synthesis and assembly of anatase TiO2 micro/nanostructures with dominant {001} high-energy facets, briefly present r 2011 American Chemical Society
their photocatalytic properties and potential applications, especially those related to environmental and energy-related issues. A special attention is given to the preparation of anatase micro/ nanostructures with high percentage of {001} high-energy facets, and to their applications for photocatalytic degradation of pollutants, solar energy conversion and Li-ion batteries.
2. SYNTHESIS AND ASSEMBLY The truncated octahedral bipyramid (TOB, Figure 1 (left)), having eight {101} facets and two {001} facets, is a very popular crystal shape observed in nature8 and the most common shape of anatase crystals based on Wulff construction.7 The typical degree of truncation (B/A) is in the order of 0.3 0.4 over a wide range of conditions, giving less than 10% of the exposed {001} facets.7 To synthesize anatase TiO2 crystals with dominant high energy {001} facets (Figure 1 (right)), the crystal growth should to be confined within the kinetically controlled regime under nonequilibrium conditions.9,10 For instance, the high-temperature (1200 °C) gasphase thermal oxidation of TiCl4 or Ti(OC4H9)4 by rapid heating and quenching creates nonequilibrium conditions, which results in the growth of anatase TOBs with nearly 40% of exposed {001} facets.11,12 Notably, both temperature and the ramping rate are crucial factors in this synthesis. For example, Alivov and Fan Received: February 26, 2011 Revised: July 8, 2011 Published: July 27, 2011 4085
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Chemistry of Materials
Figure 1. Crystal evolution of anatase TiO2 nucleus (middle), typically nucleated as truncated octahedral bipyramid (TOB) seed, exposing eight {101} facets and two {001} facets. (a) Under equilibrium conditions, the high-energy {001} facets diminish quickly and the crystal spontaneously evolves into thermodynamically stable {101}-dominated TOB (left). (b) Under nonequilibrium conditions, the high-energy {001} facets can be stabilized, typically, by selective adhesion of capping agents and the crystal growth results in a metastable {001}-dominated TOB (right). The side lengths labeled A and B are used to define the degree of truncation (B/A) and to estimate the percentage of {001} facets.
Figure 2. Fluoride-mediated stabilization of anatase {001} facets. (a, b) The calculated surface energy (γ) and the optimized degree of truncation (B/A, A and B denote side lengths of anatase crystal as shown in Figure 1) and the percentage of {001} facets (S001/S, S and S001 are, respectively, the total surface area and the surface area of the {001} facets) plotted for different adsorbate atoms (X). (a) The lowest value of γ for both the {001} and {101} facets is for the F-terminated surface; among 12 X-terminated surfaces and the clean one, the F-terminated {001} facets are more stable than {101}. (b) F-terminated surfaces exhibit the highest degree of truncation (B/A) and the highest percentage of exposed {001} facets (S001/S). (c, d) Electron microscopy (EM) images of discrete anatase TiO2 sheets with exposed high-energy {001} facets. (c) Anatase TiO2 microsheets with large {001} facets (∼47%) synthesized by hydrothermal treatment of titanium tetrafluoride (TiF4) aqueous solution in the presence of hydrofluoric acid as crystallographic controlling agent. (d) Anatase TiO2 nanosheets with high percentage (∼89%) of {001} facets synthesized by hydrothermal hydrolysis of tetrabutyl titanate, instead of TiF4, mixed with concentrated HF solution. Panels a c reprinted with permission from ref 6. Copyright 2008 Nature Publishing Group. Image d reprinted with permission from ref 50a. Copyright 2010 Royal Society of Chemistry.
demonstrated that both the annealing temperature above 500 °C and the ramping rate above 16 °C/min are necessary for the formation of TOBs by annealing ordered TiO2 nanotubes arrays in ambient fluorine.13 Unfortunately, high temperature gas-phase reaction usually gives rise to the concomitant rutile phase as byproduct.11
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Alternatively, the majority of syntheses of titania are carried out either in aqueous or nonaqueous liquid phases. Under these conditions the reaction dynamics, and thus the nucleation process, can be easily controlled by selecting the suitable titania precursors, the reaction medium (solvents) as well as the capping agents in combination with controlling the reaction temperature and pressure. Anatase TiO2 has a tetragonal structure (the c-axis being 2.7 times of the a-axis) and has been shown to nucleate as TOB seeds with eight {101} facets and two {001} facets (Figure 1 (middle)).14 The surface free energy of the {001} facets (0.90 J/m2) is ca. 1.4 times larger than that of the {101} facets (0.44 J/m2).14 One of the crucial parameters influencing the growth pattern of nanocrystals is the surface energy of the crystallographic faces of the seeds.10 The crystal growth rate is exponentially correlated to the crystal surface energy; therefore, the kinetic crystal growth and the targeted crystal shape emerge as a result of enhancing or reducing the surface energy by specific surface modification. A general approach for modifying the surface energy and thus controlling the final shape is to grow the crystal in the presence of adsorbing species that interact differently with the different crystalline facets.7 The dynamic solvation processes and selective adhesion effects of specific capping agents occur on the crystal surface.9 The exchange rates on different facets determine the relative stabilities of the facets (or the growth rate in different directions), resulting in a shape that is uniquely defined by the nature and concentration of the modifier (adsorbate). One typical example is the strong preferential interaction between fluorine and the {001} facets of anatase TiO2 crystals. On the basis of the first principle calculations, Lu and co-workers first pointed out that the adsorbed fluorine atoms could cause an exceptional stabilization of the {001} facets in fluorine-terminated anatase TiO2 crystals (Figure 2a,b).6 Subsequently, they confirmed this predication by successful synthesis of anatase TiO2 TOB single crystals with 47% of {001} facets (Figure 2c) using HF as a capping agent and TiF4 as titania precursor under specific hydrothermal conditions. This pioneering work has stimulated a great interest in the synthesis of anatase TiO2 crystals with dominant {001} facets and pointed out research efforts in this area on the increase of percentage of the exposed {001} facets as well as on the reduction of the particle size and the increase of the specific surface area.15 28 Instinctively, various titanium sources, e.g., TiN,15 TiB2,16 TiC,17 etc. have been examined as titanium precursors instead of TiF4 (Table 1). However, in the most cases, the resulting anatase TiO2 samples were in the form of randomly aggregated microsized TOBs with low specific surface area. The nanosized TOBs were only obtained in a few cases.19 21 Remarkably, Xie et al. synthesized anatase TiO2 nanosheets with exposed {001} facets up to 89% (Figure 2d) by hydrothermal hydrolysis of tetrabutyl titanate mixed with concentrated HF solution.20 Notably, HF was most often used as the capping agent; however, it is undesirable for a large scale production due to its high corrosion and toxicity in either liquid or vapor forms. Subsequently, solvothermal methods were developed with aim of reducing or avoiding the usage of HF. Usually, alcohol or ionic liquid (IL) were used in combination with HF in lower concentration.23 28 Typical examples of the systems studied are listed in Table 1. Lu et al. demonstrated that the percentage of the exposed {001} facets can be increased to some extent because of the synergetic effect of alcohol and fluorine in stabilizing {001} facets.23 Another benefit of using alcohol is a greater flexibility in controlling the degree of dispersion and the particle size.24 Moreover, the organization of primary building 4086
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Table 1. Chemical Composition and Experimental Conditions of Exemplary Anatase Titania Micro-/Nanosheets with Exposed {001} Facetsa precursor
capping agents
TiF4/TiN/TiB2/ TiS2/TiC/Ti-powder
HF/NH4HF2/ NH4F
H2O
Ti(SO4)2/TBOT
HF
H2O
side length thickness
solvents
% of {001} facets
ref
1 5 μm 1 μm (microsize)
40 60%
6,15 18,22
50 nm 10 nm (nanosize)
18 89%
19,20
Fluoride-Mediated Hydrothermal Synthesis
Fluoride-Mediated Solvothermal Synthesis TiF4/TiCl4
HF
H2O-alcohol
1.09 mm 260 nm
50 65%
23 25
TiF4/TiCl4
HF
H2O-IL
microsize
27 80%
27,28
TIP
OM
BA(H2O)
