Long-lived photogenerated charge carriers of 0 0 1-facet-exposed TiO2 with enhanced thermal stability as an efficient photocatalyst
Introduction
Anatase TiO2 has been widely investigated due to its promising applications in photocatalysis, photovoltaic cells, sensors, and for photosplitting water. Recently, theoretical and experimental studies have demonstrated that, the (0 0 1) facets of anatase TiO2 are much more reactive than the thermodynamically more stable (1 0 1) surface due to higher average surface energy of the (0 0 1) facets than that of the (1 0 1) facets [1], [2]. Unfortunately, surfaces with high reactivity usually diminish rapidly during the crystal growth process as a result of the minimization of surface energy. Most available anatase TiO2 crystals are dominated by the thermodynamically stable (1 0 1) facets, rather than the much more reactive (0 0 1) facets. Until Yang et al. discovered that introducing HF could reverse the relative stability of (0 0 1) and (1 0 1) faces in 2008, a pioneering breakthrough in the synthesis of anatase TiO2 with exposed (0 0 1) facet was accomplished [3]. Following the breakthrough, other researchers further confirm that the presence of fluorine species plays an important role in the formation of 0 0 1-facet exposed TiO2 and finally could remain on the surface of TiO2 [4], [5]. Interestingly, it is clearly demonstrated that the high-energy 0 0 1 facet, along with the surface residual fluoride, is much favorable for efficient photocatalytic reactions on TiO2 [6], [7].
To expand its application, like as films and coatings, it is often required for the 0 0 1-facet exposed TiO2 obtained to be subjected to the high temperature thermal treatment. It is widely accepted that fluorine species play a vital role in keeping the stability of (0 0 1) facets during the growing process of TiO2 crystallite. However, the residual fluoride species on the surfaces of the resulting TiO2 is not thermally stable so that it is easily left away after thermal treatment at high temperature of over 450 °C, based on our previous work and other reports [8], [9]. Thus, we naturally come to predict that the disappearance of surface residual fluoride would influence the thermal stability of high-energy 0 0 1 facet so as to greatly decrease its photocatalytic performance. To the best of our knowledge, the thermal stability of exposed 0 0 1 facet of nanocrystalline anatase TiO2 is often ignored. Different from hydrofluoric acid, phosphoric acid is thermally stable so that it could be firmly fixed on the surfaces of TiO2 after thermal treatment at the high temperature of 800 °C [10]. Worthy of speaking, the phosphate modification could promote the O2 adsorption so as to improve the photogenerated charge separation of TiO2 [11]. Based on the above consideration, we try to modify the synthesized high-percentage 0 0 1-facet exposed nanocrystalline TiO2 with phosphoric acid, aiming to overcome the expected issues so as to keep high or even much high photocatalytic activity after thermal treatment at high temperature. It is of great significance for environmental photocatalysis from engineering and scientific points.
Section snippets
Experimental
All substances used in this study are analytical grade and used without further purification. Deionized water is used in all experiments.
Structural characterization and surface composition
Fig. 1 and Table 1 show the XRD patterns and TEM images of unmodified and phosphate-modified TiO2, and their corresponding XRD data and BET surface areas, respectively. It can be seen that the as-synthesized unmodified TiO2 dried at 100 °C (T-100) is pure anatase TiO2 with low crystallinity, small crystallite size of about 12 nm and large surface areas of about 72 m2 g−1. According to its anatase (0 0 4) and (2 0 0) XRD diffraction peaks and the thickness and the side length of the nanosheet by the TEM
Conclusions
In conclusion, the thermal stability of 0 0 1-facet-exposed nanocrystalline anatase TiO2 is greatly enhanced by phosphate modification, and the modified TiO2 treated at high temperature exhibits obviously lifetime-prolonged and separation-enhanced features of photogenerated charge carriers, which are mainly attributed to the synergetic effects of high-percentage 0 0 1-facet exposure, high anatase crystallinity, and strong ability to adsorb O2. This, interestingly, leads to remarkably high
Acknowledgements
We are grateful for financial support from NSFC (21071048), the Program for Innovative Research Team in Chinese Universities (IRT1237), the Specialized Research Fund for the Doctoral Program of Higher Education (20122301110002) and the Chang Jiang Scholar Candidate Program for Heilongjiang Universities (2012CJHB003).
References (25)
- et al.
Appl. Catal. B Environ.
(2010) - et al.
Appl. Catal. B Environ.
(2011) - et al.
J. Hazard. Mater.
(2009) - et al.
Appl. Surf. Sci.
(2012) - et al.
J. Phys. Chem. B
(2005) - et al.
Chem. Commun.
(2011) - et al.
Nature
(2008) - et al.
Chem. Commun.
(2009) - et al.
ACS Appl. Mater. Interfaces
(2011) - et al.
Cryst. Growth Des.
(2010)