ARTICLE pubs.acs.org/JPCC
Hydrogen Incorporation and Storage in Well-Defined Nanocrystals of Anatase Titanium Dioxide Chenghua Sun,*,†,‡,§ Yi Jia,‡ Xiao-Hua Yang,|| Hua-Gui Yang,|| Xiangdong Yao,^ Gao Qing (Max) Lu,‡ Annabella Selloni,§ and Sean C. Smith*,# †
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Centre for Computational Molecular Science, Australia Institute for Bioengineering and Nanotechnology, The University of Queensland, Queensland 4072, Australia ‡ ARC Centre of Excellence for Functional Nanomaterials, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Queensland 4072, Australia § Department of Chemistry, Princeton University, New Jersey 08542, United States Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China ^ Queensland Micro- and Nanotechnology Centre, Griffith University, Australia # Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Tennessee 37831-6494, United States ABSTRACT: Hydrogen incorporation into well-defined nanocrystals of anatase titanium dioxide (TiO2) has been investigated by a combination of experimental studies and density functional theory (DFT) calculations. The hydrogenation of TiO2 nanocrystals was determined at 450 C with an initial hydrogen pressure of 7.0 MP, and storage capacities of 1.0 wt % and 1.4 wt % were achieved for nanocrystals with predominant (001) and (101) surface terminations, respectively. X-ray diffraction and Raman spectroscopy measurements indicate that the TiO2 crystal structure is very well preserved during the hydrogenation. DFT calculations show that hydrogen occupies the interstitial sites between titanium oxygen octahedra and the energy barrier for hydrogen incorporation through the anatase (101) surface is lower than that through (001).
1. INTRODUCTION As a versatile functional material, titanium dioxide (TiO2) has been extensively studied and widely used in solar-hydrogen production, photocatalytic decomposition of pollutants, solar cells, and so forth.1 6 A major issue for TiO2-based photocatalysis is however the wide band gap (typically 3.0 3.4 eV), which makes TiO2 effective only under UV light. This limitation has stimulated considerable research efforts aimed at reducing the TiO2 band gap via incorporation of suitable dopant impurities.7 15 It has been recently reported that hydrogen incorporation into anatase TiO2 can lead to a significant band gap reduction, hence efficient photocatalytic activity under visible light.16 More specifically, it was observed that white TiO2 becomes black upon hydrogenation, while at the same time the surface region becomes amorphous and the band gap is reduced by more than 1.20 eV.16 Clearly such hydrogen incorporation is different from hydrogen adsorption and storage due to unsaturated surface atoms of tubular or porous TiO2 as reported previously.17 Hydrogen incorporation in anatase TiO2 is also supported by recent computational studies.18 According to Islam et al., hydrogen can pass through the anatase TiO2(101) surface and incorporate into the TiO2 lattice with barriers varying from 0.70 to 2.25 eV, depending on the diffusion path.18 In fact, hydrogen diffusion into rutile TiO2(110), with a barrier of 1.03 eV from TDS measurement and 1.11 eV from DFT calculations, has been reported by r 2011 American Chemical Society
W€oll et al.,19 suggesting that hydrogen incorporation into TiO2 lattice should be considered even at the room temperature. Theoretically hydrogen may migrate on the surface, leave the surface, or incorporate into the bulk, depending on which process is easier, which underlines the importance of the exposed surfaces. Anatase TiO2 crystals typically expose the majority (101) surfaces as well as minority (001) facets.2,20 Both experimental and theoretical studies indicate that the anatase (001) surface is very reactive due to the low-coordination of its surface atoms; in particular, this surface is believed to have an important role in water dissociation on TiO2 surfaces.21 Previous studies by our group also indicated that lithium diffusion through the (001) surface is more favorable than that through the (101) one.22 In addition, gold incorporation through (001) was also predicted recently.23 On the basis of this evidence, it appears interesting to understand whether hydrogen incorporation into anatase TiO2 depends on the orientation, (001) vs (101), of the exposed surfaces. To investigate the dependence of hydrogen incorporation on crystal facets, well-defined single crystals are essential. Anatase TiO2 crystals are typically dominated (>90%) by {101} facets, which are the lowest energy facets if no special controlling agent Received: October 31, 2011 Revised: November 22, 2011 Published: November 28, 2011 25590
dx.doi.org/10.1021/jp210472p | J. Phys. Chem. C 2011, 115, 25590–25594
The Journal of Physical Chemistry C
ARTICLE
Figure 1. TEM images of synthesized anatase TiO2 nanocrystals. (a) TiO2-(001) and (b) TiO2-(101).
is employed.24 Recently Yang et al. developed an effective approach to synthesize large (001)-dominated anatase TiO2 crystals.25 These crystals offer the opportunity to investigate the incorporation of hydrogen through the (001) surface. In this article, anatase TiO2 crystals dominated by (001) and (101) surfaces, indicated as TiO2-(001) and TiO2-(101), respectively, are considered, and hydrogen incorporation through these two surfaces is studied.
2. EXPERIMENTAL SECTION Materials Synthesis. TiO2-(001) was prepared by using hydrofluoric acid (HF) as a capping agent, as described by Yang et al.25,26 In a typical synthesis, 5 mL of titanate isopropoxide or tetrabutyl titanate and 0.4 0.6 mL of HF (48% w/w) were put into a Teflon-lined stainless steel autoclaves with a capacity of 30 50 mL and then kept at 180 C for 24 h. After reaction, the anatase TiO2 single crystals were harvested by centrifugation, washed with ethanol and deionized water three times, respectively, and then dried in vacuum overnight. To prepare TiO2-(101), titanium tetrachloride aqueous solution (5.33 mM) and hydrochloric acid (HCl, 10% w/w, 0.73 mL in 30 mL of TiCl4 aqueous solution) were used as the anatase TiO2 precursor and the crystallographic controlling agent, respectively. The reaction was carried out in a Teflon-lined autoclave at 180 C for 14 h. The synthesized products are typically in the range of 10 30 nm, with the percentage of (001) being around 2%. X-ray diffraction (XRD) confirms that both TiO2-(001) and TiO2-(101) are wellfaceted nanocrystals with 100% pure anatase phase. The transmission electron microscopy (TEM) images of TiO2-(001) and TiO2-(101) samples are presented in Figure 1a,b, respectively. Materials Characterization. The shape and crystal structure of the resulting anatase TiO2 were investigated by X-ray spectroscopy (Rigaku Miniflex diffractometer, Co Kα radiation, 40 kV) and TEM (Philips Tecnai T30F FEG Cryo AEM). Raman spectra were obtained by Renishaw's inVia Raman microscope using a 514 nm green laser. The laser beam is corrected using a single crystal of silicon before and after the tests. Hydrogen incorporation and storage properties of the TiO2 samples were examined by an automated Sieverts's apparatus (Suzuki Shokan PCT H2 absorption rig). Absorption/desorption measurements were performed at 450 C for 24 h. The initial hydrogen pressures for hydrogenation and dehydrogenation are 7 MPa and
