Hydrogenation of TiO₂ nanosheets with exposed {001} facets for enhanced photocatalytc activity

Abstract

Hydrogenated {001}-facets-dominated anatase TiO₂ nanosheets have O–H bonds and O⁻ on their surface. The high reactive {001} facets were maintained by the formation of Ti–H bonds. A large number of Ti³⁺ ions and oxygen vacancies were produced by hydrogenation, resulting in improved light absorption and enhanced photocatalytic activity.

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TiO₂, the most popular wide-band-gap photocatalysts, which can use the UV part of solar light to degrade organic pollutants and produce hydrogen from water, have attracted world-wide attention in the last two decades.^1,2 The photoactivity performance of TiO₂ is mainly dependent on its morphology, crystal structure, light absorption, and surface chemical environment. The {001} facets of anatase TiO₂ have been considered to have higher photocatalytic activity than the traditional {101} facets because of their higher surface energy and the 100% unsaturated Ti_5c atoms located on the {001} facets.^3,4 To obtain highly reactive TiO₂, most works have been focused on the controllable growth of {001}-facets-dominated TiO₂. Liu et al. prepared {001}-facets-dominated TiO₂ microspheres, resulting in high photocatalytic acitivity.⁵ Xie et al. successfully synthesized hollow TiO₂ boxes with a high percentage of exposed {001} facets.⁶ However, systematic studies on the location position and effect of F, the most commonly used agent to control the exposure of the {001} facets, are scarce. On the other hand, it is also very important to enhance the visible light, even infrared light, absorption of TiO₂. Recently, Chen et al. prepared hydrogenated TiO₂ nanocrystals to introduce a surface disorder structure, extending the light absorption to the near infrared range (∼1200 nm).⁷ Wang et al. reported that the light absorption and water splitting ability of TiO₂ nanowire arrays were significantly enhanced by hydrogenation.⁸ To date, there is no systematic experimental study on the hydrogenation of {001}-facets-dominated anatase TiO₂ nanosheets, especially those that are F-modified (surface adsorption and inner substitution), to further improve their photocatalytic activity. Systematic studies on the nature of hydrogenation treatment and its correlation with the highly reactive {001} facets are very interesting topics.

The hydrothermal method is very effective in controlling the growth of crystal particles with certain morphologies. Herein, we report the hydrogenation of F-modified anatase TiO₂ nanosheets with a high percentage of exposed {001} facets by annealing the fine shaped pristine hydrothermal product under a high pressure hydrogen atmosphere. The photocatalysts were characterized by UV-visble diffuse reflectance spectroscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS). We show that TiO₂–H has Ti–H and O–H chemical bonds and O⁻ species on its surface, and Ti³⁺ and an oxygen vacancy located inside the crystal structure. The {001} facets of TiO₂–H are maintained by the formation of Ti–H chemical bonds, resulting in the change from Ti_5c to Ti_6c. The light absorption of TiO₂–H is extended to the infrared range, and its photocatalytic activity is highly enhanced.

The TEM images of TiO₂ (Fig. 1a) and TiO₂–H (Fig. 1b) reveal that both of the photocatalysts are square sheets in shape. After hydrogenation, the particles of TiO₂–H are mutually bonded to each other slightly, and the exposed {001} facets are maintained. The HRTEM (Fig. 1c and d) images of TiO₂ further depict that the thickness of TiO₂ is ca. 20 nm and prove the exposure of the {001} facets.⁹ Besides the TEM analysis, the XRD patterns (Fig. 1e) also prove the anatase phase of TiO₂ and TiO₂–H (JCPDS No. 21-1272). More importantly, the (101) diffraction peak of TiO₂–H gives a slight shift toward a higher diffraction angle, indicating a smaller interplanar crystal spacing (Fig. S1, ESI†). This result demonstrates that structural changes have occured in TiO₂ during the hydrogenation process. The UV-visible diffuse reflectance spectra show that {001}-facets-dominated TiO₂–H exhibits a high and broad light absorption band toward the infrared range (Fig. 1f). This result is in good agreement with the color change of the photocatalysts from white TiO₂ to dark blue TiO₂ (inset of Fig. 1f). The XPS valence band spectra (Fig. S2, ESI†) depict that TiO₂ and TiO₂–H have a nearly overlapped band edge below the Fermi energy, which confirms that hydrogenation has a negligible effect on the valence band position of TiO₂–H.⁸

Fig. 1 TEM images of TiO₂ (a), TiO₂–H (b) and high resolution TEM images of TiO₂ correlating to the (101) (c) and (001) (d) interplanar crystal spacing. XRD patterns (e) and UV-visible diffuse reflectance spectra (f) of TiO₂ and TiO₂–H.

Fig. 2a and b compares the Raman spectra differences of TiO₂ and TiO₂–H. Two features can be revealed by comparing the spectra: the first feature is the weakening of all the diffraction peaks at 144 cm⁻¹, 396 cm⁻¹, 515 cm⁻¹ and 636 cm⁻¹ in TiO₂–H. Second, the two E_g modes at 144 cm⁻¹ and 636 cm⁻¹ of TiO₂–H are shifted to a higher frequency. These two features demonstrate the increase in the number of oxygen vacancies in the lattice structure of TiO₂–H.^9,10 According to the XRD analysis, the shift of the diffraction peak to the higher angle indicates the reduction of the interplanar distance. This can be ascribed to the escape of O atoms from the crystal lattice, which often results in Ti³⁺.¹¹Fig. 2c shows the Ti 2p core level XPS spectra of TiO₂ and TiO₂–H. The two peaks centered at ∼458.5 eV and ∼464.4 eV correspond to the characteristic Ti 2p_3/2 and Ti 2p_1/2 peaks of Ti⁴⁺.^7,12 The peak of TiO₂–H shows a shift towards the lower binding energy, indicating that Ti atoms in TiO₂ and TiO₂–H have different chemical environments. More importantly, a detectable shoulder, which can be ascribed to Ti³⁺,⁹ appears in the low-energy range in both of the samples. Regarding the origin of Ti³⁺ centers in both of the samples, low temperature EPR analysis (Fig. 2d) is conducted to give a systematic study. It is well known that HF will serve as the morphology control agent during the preparation of pristine {001}-facets-dominated TiO₂. Thus the surface of TiO₂ is adsorbed by the F atoms. However, TiO₂ gives a strong EPR signal and the calculated g values are g_⊥ = 1.992 and g_∥ = 1.962, which are the typical g values for a paramagnetic Ti³⁺ center.¹³ This type of Ti³⁺ signal can be ascribed to originate from the substitution of O²⁻ ions by the F⁻ ions.¹⁴ The phenomenon shows that the F atoms not only serve as morphology control agents but also modify the inner structure of TiO₂. No other signal, such as O⁻ or O²⁻, is detected, indicating the Ti³⁺ are located in the bulk of the TiO₂ because surface Ti³⁺ will adsorb O₂.¹³ Compared to TiO₂, TiO₂–H gives a very different and stronger EPR signal which can be ascribed to the Ti³⁺ and O⁻.¹⁵ The stronger Ti³⁺ signal is in good agreement with the XPS analysis. The broadened Ti³⁺ signal of TiO₂–H may result from the different coordinations of Ti³⁺. In TiO₂, the Ti³⁺ is produced by F substitution, which induces the chemical bond Ti–F–Ti.¹⁴ In contrast, there will be Ti–□–Ti chemical bond in TiO₂–H as a result of the escape of the O and F atoms. The oxygen atoms in TiO₂ will interact with the H atoms, which will also produce one kind of Ti³⁺. This result can also be obtained from the O 1s XPS spectra (Fig. S3, ESI†). The signal centered at 529.8 eV and 531.3 eV are the typical signals of Ti–O–Ti and surface OH species, respectively.¹⁶ The O 1s spectra illustrate that TiO₂–H has much more surface OH species, which are produced by the O and H interaction, than TiO₂. Comparing to TiO₂, the F atoms in TiO₂–H are almost removed (Fig. S4, ESI†). The F atoms may escape from the surface and inner structure of TiO₂–H because of the similar chemical properties of O and F. Thus, more oxygen vacancies are formed and the surface Ti–F bonds are broken.

Fig. 2 Raman (a, b), Ti 2p XPS (c), and EPR (d) spectra of TiO₂ and TiO₂–H.

Fig. 3a compares the photocatalytic activity of TiO₂, TiO₂–H, and P25 in decomposing MB under UV-visible light irradiation. The photocatalysis efficiency of TiO₂–H is much higher than that of TiO₂ and P25. The photocatalytic activity (Fig. S5, ESI†) of TiO₂–H decreased when the photocatalysts were irradiated only by visible light, which may be ascribed to the decrease of the light intensity. More importantly, the visible light photocatalytic activity of TiO₂ is higher than that of P25. As discussed before, this phenomenon may result from the existence of the Ti³⁺ center created by F substitution. We also tested the photocatalytic activity of the photocatalysts under UV (Fig. S6, ESI†) and visible light (Fig. S7, ESI†) irradiation in decomposing phenol. The results are very similar to those of MB degradation. The formation of ·OH (Fig. 3b), which is considered to be very favorable for photocatalysis reactions, in the photocatalytic reaction system was also examined upon UV light irradiation. The PL intensity of 2-hydroxyterephthalic acid, which was formed by the interaction of terephthalic acid with the generated ·OH, in the three reaction systems are very different. TiO₂–H gives a much stronger photo-oxidation capability than P25, while TiO₂ shows the weakest photo-oxidation capability. This depicts that TiO₂–H is a very effective photocatalyst which can be used in a wide range of applications.

Fig. 3 Photocatalytic decomposition of MB (a) and ·OH generation measurement (b) of TiO₂ and TiO₂–H under UV-visible light irradiation. Schematic illustration (c) of the hydrogenation effect on the structural change in TiO₂ and TiO₂–H.

To get a deep insight on the hydrogenation effect on the structure change of TiO₂, the schematic illustration is shown in Fig. 3c. The {001}-facets-dominated TiO₂ is predominantly exposed with the unsaturated Ti_5c and O_2c atoms. As calculated, the H atoms will chemically bond with the O atoms, resulting in the formation of Ti³⁺ centers.¹⁷ On the other hand, the hydrogenation effect removes almost all of the F atoms on the surface of the TiO₂. The Ti_5c atoms, which were originally bonded with the F atoms, are now unsaturated again. After the H atoms reached an interaction equilibrium with the O atoms, the Ti_5c atoms are very favorable for H absorption.¹⁶ As a result, the (001) surface of TiO₂–H is now packed with Ti_6c atoms, which may be the reason for the retention of the {001} facets even though the F atoms are removed from the surface of TiO₂–H. On the other hand, the removal of the O and bulk F atoms induces the formation of oxygen vacancies and Ti³⁺, resulting in the band gap narrowing and the surface O⁻ adsorption of TiO₂–H.

In summary, we have hydrogenated the F-modified, both surface adsorption and inner substitution, anatase TiO₂ nanosheets with a high percentage of the {001} facets. The hydrogenated {001} facets dominated TiO₂–H shows higher photocatalytic activity than P25 and pristine TiO₂ under UV-visible and visible light irradiation. Based on a systematic analysis, the surface of TiO₂ is fully covered with the Ti–H and O–H chemical bonds. There are also a lot of Ti³⁺ and oxygen vacancies that induce the adsorption of O⁻ species on the surface of TiO₂–H, resulting in a significant band-gap narrowing. The Ti_5c atoms on the {001} faces are fully coordinated by the formation of Ti–H chemical bonds. As a result, a high percentage of {001} facets is kept after the hydrogenation treatment. We believe the high-performance TiO₂–H with exposed {001} facets can be used in other fields, such as self-cleaning and solar cells. This work gives new insight into the development of TiO₂ photocatalysts with a high percentage of highly reactive facets and enhanced light absorption ability.

Acknowledgements

This work was supported by the Innovation Foundation for Graduate Students of Jiangsu Province China (CXLX11_0346) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Footnote

† Electronic Supplementary Information (ESI) available: details of the synthesis and characterization of the photocatalysts, powder XRD data of the (101) peak, valence band XPS spectra, O 1s and F 1s XPS spectra, photocatalytic activity measurement of TiO₂ and TiO₂–H. See DOI: 10.1039/c2ra21049e/

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