Environ. Sci. Technol. 2009, 43, 5423–5428
Highly Thermal Stable and Highly Crystalline Anatase TiO2 for Photocatalysis W E I L I , †,‡ Y A N G B A I , † C H A N G L I U , † ZHUHONG YANG,† XIN FENG,† X I A O H U A L U , * ,† NICOLE K. VAN DER LAAK,‡ AND KWONG-YU CHAN‡ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China, and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, China
Received December 31, 2008. Revised manuscript received April 2, 2009. Accepted May 14, 2009.
In the absence of any doping and modification, the anataseto-rutile phase transformation was inhibited at high temperatures giving rise to highly thermal stable and highly crystalline anatase TiO2 fibers. The initial formation of the TiO2(B) phase is found to be key in inhibiting this transformation. The intermediate structure of the TiO2 fiber comprises an inner anatase core with an outer TiO2(B) shell, which has a specific crystallographic orientation with respect to the anatase structure. During the calcination process from 300 to 800 °C, both the TiO2(B) shell and the bulk anatase crystal structure was preserved. At temperatures of 800-900 °C the TiO2(B)-to-anatase transformation was finished and a near-pure and thermally stable anatase fiber was obtained. This final product shows the same activity as a standard commercial photocatalyst Degussa P-25 when measured against unit mass, and 5 times the activity when measured with respect to the unit surface area. The anatase TiO2 fibers presented here have considerable interest as practical photocatalysts for water purification, as they can be easily recycled without a decrease in their photocatalytic activity and can be prepared at large scale and at low cost.
1. Introduction Titanium dioxide (TiO2) is one of the most prominent oxide materials for use in various kinds of industrial applications such as photocatalysis (1, 2) and photovoltaics (3, 4). Its low cost, ease of handling, and high resistance to photoinduced corrosion make it one of the most suitable candidates for industrial use (4, 5). Among the common crystal phases of titania, anatase is generally recognized to be the most active (6, 7). Research unambiguously shows that improving the crystallinity of anatase is critical to obtaining high photocatalytic activity by reducing lattice defect sites which act as recombination traps for electron-hole pairs (8-11). Generally, heat treatment promotes the crystallinity of anatase. However, the anatase-to-rutile transformation in pure titania usually occurs around 600 °C (12, 13) and rutile with its poor * Corresponding author e-mail:
[email protected]; phone: +8625-83588063; fax: +86-25-83588063. † Nanjing University of Technology. ‡ The University of Hong Kong. 10.1021/es8037005 CCC: $40.75
Published on Web 06/08/2009
2009 American Chemical Society
photocatalytic activity is formed irreversibly during further heat treatment due to its greater thermodynamic stability (7, 14). Many approaches have been developed to extend the anatase-to-rutile transformation temperature above 600 °C so that the crystallinity of anatase can be promoted. Banfield et al. found that the creation of interfacial boundaries via the introduction of surface impurities inhibited crystal growth so that the anatase phase was stabilized and the anatase-to-rutile transformation was shifted to higher temperatures (15). Methods for inhibiting the phase transformation have been developed including the use of cationic dopants (16, 17), metal oxide doping, or coating (18-20). However, formation of secondary impurity phases which degrades the photoactivity is the main disadvantages of these techniques (21). Very recently, Pillai et al. developed a method for fabricating pure anatase which was stable at high temperatures. Trifluoroacetic acid was used to introduce fluorine which acted as a dopant. The fluorine was later removed by heat treatment (22). In a very different synthesis route, TiO2 with an anatase crystal structure stable up to 900 °C was synthesized from layered titanates without any doping or modification (23, 24). At low postheat treatments at 300-500 °C an unusual intermediate phase of TiO2, namely, TiO2(B), was observed. On further heating to 900 °C, TiO2(B) was found to transform to anatase. TiO2(B) was first synthesized in 1980 by Marchand et al. (25) from a layered titanate K2Ti4O9 via K+/H+ ion exchange and subsequent calcination. Feist et al. (26) have also reported the formation of TiO2(B) from layered titanates with the formula A2TinO2n+1 (A ) Na, K, Cs; 3 e n e 6) upon ion exchange followed by thermal dehydration. They found that TiO2(B) is a metastable monoclinic modification of titanium dioxide, and the TiO2(B)-to-anatase transformation proceeds slowly over a broad range of temperatures from 300 to 800 °C (26, 27). Thus, the presence of TiO2(B) and the subsequent TiO2(B)-to-anatase transformation up to temperatures of ca. 800 °C is suggested to be key to suppressing the formation of rutile, which normally occurs from ca. 600 °C. Recently, the research groups of Yoshikawa (23) and Yin (24) have investigated the photocatalytic properties of TiO2 materials formed from layered H2Ti4O9 and H2Ti3O7, respectively. However, the photodegradation of organics was poor due to low surface area and an inadequate microstructure. Recently we have reported a new synthetic route using a layered K2Ti2O5 precursor to obtain mesoporous TiO2 with a high surface area (28) and bicrystalline TiO2 with a core-shell structure (29). It is well-known that K2Ti2O5 exhibits a unique layered structure composed of edge-sharing trigonal bipyramids with TiO5 units comprising five oxygen anions coordinate to one titanium atom, which is very different from all other layered titanates (30). We concluded that our novel TiO2 materials could be attributed to the distinctive crystal structure of the precursor. On this basis, we have fabricated thermally stable TiO2 anatase from K2Ti2O5 using a similar approach and have investigated in detail the structure of TiO2 fibers and the role of TiO2(B) in preserving the anatase phase to higher than expected temperatures. We also present results which assess the performance of the fibers for use in practical applications such as water purification.
2. Experimental Section 2.1. Sample Preparation. A reagent mixture of TiO2/K2O (M ) 1.9) was prepared by uniformly adding K2CO3 (reagent grade) to TiO2 · nH2O (prepared by hydrolyzing TiOSO4 in hot water while stirring vigorously) and then sintered at 810 VOL. 43, NO. 14, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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dark for 30 min. The concentration of MO was monitored using UV-vis spectroscopy (Hitachi U-2001) at 365 nm while the concentration of phenol was determined by measuring the absorbance change at 270 nm. The mineralization of MO was monitored by a Shimadzu TOC-VCPH analyzer. Degussa P-25 was used as a benchmark in the same concentration (0.5 g · L-1) as the other TiO2 photocatalysts. For the cycling experiments, the samples were left to separate from solution by sedimentation for 1 h. The upper clear solution was removed and then the samples were dispersed back into the reactor for another cycle.
3. Results and Discussion
FIGURE 1. XRD of samples derived from K2Ti2O5. (1: K2Ti2O5 (PDF#51-1890); b: H2Ti2O5 · H2O (PDF# 47-0124); O: H2Ti5O11 · H2O (PDF#44-0131); 0: anatase (PDF#21-1272); 9: rutile (PDF#211276)). °C for 2 h. Ten g of the product was suspended in 100 mL of water and vigorously stirred, HCl solution was added to pH ) 1.0 for H+/K+ ion-exchange. Inductively coupled plasma-mass spectrometry (ICP-MS) confirmed that the K+ residual was
