Safe and facile hydrogenation of commercial Degussa P25 at room temperature with enhanced photocatalytic activity

Abstract

In this paper, we developed a new safe and facile route to prepare black titania at room temperature for visible light photocatalysis. The commercial Degussa P25 was used as the starting material, and it was hydrogenated at 35 bar hydrogen and room temperature for up to 20 days. The resulting hydrogenated P25 was characterized by XRD, FT-IR, Raman, UV-Vis, TEM and photocatalysis tests under visible light in methanol solution. It was found that P25 powders under hydrogen treated for more than 15 days have a dark appearance, a crystalline-disordered core–shell structure, unique phase structure and good photocatalytic performance. The H₂ evolution rates are 3.14, 3.56 and 3.94 mmol g⁻¹ h⁻¹ in 20% methanol solution for hydrogen treated P25 at 15, 17 and 20 days, respectively, which were largely higher than that with hydrogen treatment time less than 11 days. This work will provide a practical, green and facile method for the large scale synthesis of black titania at room temperature.

---

Introduction

Titanium dioxide (TiO₂) has been heavily investigated as a prime photocatalyst due to its abundance, low toxicity, relatively high efficiency and excellent photostability¹ for breaking water into oxygen and hydrogen under ultraviolet light irradiation.² TiO₂ has a large band gap energy (3.0 eV for rutile and 3.2 eV for anatase) and thus it does not absorb visible light. The photoconversion efficiency of TiO₂ is much lower than the acceptable solar-to-hydrogen efficiency (10%) for benchmark applications.^3,4 A lot of work has been carried out to improve its efficiency over the past decades. The popular approach is to dope nonmetal ions^5,6 or metal ions⁷ into TiO₂, which could extend the working spectrum of TiO₂ to the visible- or infrared-light region. For example, nitrogen-doped TiO₂ exhibits the greatest optical response to solar radiation with the color change from white to yellow or even light gray, but its absorption in the visible and infrared remains insufficient.⁸ Recently, Chen et al. designed a novel approach to generate disordered nanophase TiO₂ and simultaneously incorporate hydrogen into TiO₂ through the hydrogenation of TiO₂ nanocrystals at 20 bar hydrogen and 200 °C. The hydrogenated TiO₂ turned to black, and was shown to improve the photo-absorption and solar-driven photocatalytic activity for the splitting of water to H₂.^9,10 A large amounts of lattice disorder could yield mid-gap states in the surface of TiO₂ that makes the efficient solar-driven photocatalysis.⁹ However, most surface modification methods require highly crystalline anatase TiO₂ as the starting material, and involve hydrogen treatment at atmospheric pressure or high pressure under high temperature (e.g., 200 °C)^9,11–13 to get surface disorders and point defects, such as oxygen vacancies and Ti interstitial.¹⁴ Recently, Leshuk et al. found a higher temperature (>250 °C) hydrogenation of TiO₂ had worse effects to the photocatalytic activity due to the propensity to form bulk vacancy defects.¹⁵

Photocatalytic hydrogen production through water splitting has been extensively studied but they are still not suitable for a large scale hydrogen production. On one hand, most of light-driven water splitting to produce hydrogen is based on sacrificial reaction that greatly increases the system cost. Some efforts have been made to increase the efficiency and economy of the total system, such as the simultaneous degradation of organic pollutants and production of high value chemicals.^16–18 On the other hand, the above mentioned surface modification methods, such as hydrogenation at high temperature, are complicated and expensive. And hydrogen is extremely active and highly explosive at high temperature but quite stable at room temperature. It is desirable to find an economical and safe way to generate the hydrogenation of TiO₂.

The commercial Degussa P25 is composed of anatase and rutile. It is widely studied and well known to have good photocatalytic activity. Actually, it is not easy to find a photocatalyst showing activity higher than that of P25, and P25 has therefore been used as a de-facto standard titania photocatalyst.¹⁹ In this work, P25 was directly hydrogenated at room temperature to create disordered surface with color change to black for the solar-driven photocatalysis. This is the first time report of the hydrogenation of TiO₂ at room temperature, and it is a practical method to prepare hydrogenated TiO₂ using cheap and widely available P25 TiO₂.

Experimental section

Hydrogenation of P25

Commercial P25 TiO₂ (Degussa) with an anatase/rutile ratio of 80/20 was used as the starting material. 0.5 g P25 TiO₂ powders were first maintained in a vacuum for 24 h and then hydrogenated at 35 bar hydrogen atmosphere and room temperature (about 15 °C) in a home-made stainless steel cell (500 mL). The black P25 (P25-black) was obtained after 20 days of hydrogenation. In order to understand the hydrogenation process and photocatalytic activity at different times, P25 powders were also treated under 35 bar hydrogen at room temperature for 3–17 days.

Photocatalytic measurements

Photocatalytic experiments were performed using a closed system with an inner-irradiation-type Pyrex reactor. A 300 W Xe arc lamp was plunged into a quartz immersion well cooled by water. All the photocatalytic tests were carried out using 0.5 g of photocatalyst and 200 mL of 20 vol% methanol aqueous solution. An appropriate amount (1% Pt) of chloroplatinic acid, H₂PtCl₆·6H₂O, was added to the solution. The Pt is used because the Fermi level of Pt is lower than that of TiO₂, where photo-excited electrons can be transferred from conduction band to Pt particles deposited on the TiO₂ surface, and photo-generated valence band holes remain on the TiO₂. The reaction temperature was maintained at 10 °C. The amount of H₂ evolved was determined by using a gas chromatograph (GC).

Characterizations of photocatalysts

The phase structure of the TiO₂-based photocatalyst was examined by X-ray diffraction (XRD) using Rigaku MiniFlex II with Cu Kα radiation (λ = 0.1542 nm) at 40 kV. The TEM image was obtained on a JEM-2010 electron microscope operated at 200 kV. The sample was prepared by suspension in ethanol and drop-cast onto a carbon-coated copper TEM grid. The UV-Vis absorption spectra were recorded on a Perkin-Elmer Lambda 750 UV/Vis/NIR Spectrometer. Raman measurements were performed on a Jobin-Yvon HR-800 Raman system, using the 514 nm line of an Ar laser as the excitation source. FT-IR spectra were obtained on a Nicolet 500 spectrometer. The samples were prepared by a KBr pellet technique.

Results and discussion

Fig. 1a and b show the photographic images of P25 and the hydrogenated P25 (P25-black). P25 nanocrystals turn into black after 20 days hydrogenation at room temperature. Like the reported anatase TiO₂ nanocrystals,^9,10 nanowires^11,20 and nanotubes,^21,22 the color change of P25 after hydrogenation should arise from the introduction of disorder and dopant at their surface. There is no doubt that the result is extremely encouraging. It has been generally accepted that the Degussa P25 has been a standard, cheap, and widely available photocatalyst, which consists of mixed phases of anatase and rutile in the ratio of ca. 4 : 1.¹⁹ The P25 photocatalyst shows high activity for many kinds of photocatalytic reactions including the photocatalytic water splitting, and the mixed phase of P25 shows a possible synergetic effect on the photocalysis.²³ The synergetic effect has been basically clarified as the formation of heterojunctions.²⁴ The easy and direct hydrogenation based on Degussa P25 makes it possible to realize the large-scale application of photocatalytic water splitting. As expected, P25-black has a crystalline-disordered core–shell structure. The distance between the adjacent lattice planes is 0.35 nm in the core that is corresponding to the anatase phase (Fig. 1c).

Fig. 1 Photographic images of commercial P25 (a) and the hydrogenated P25-black (b) prepared at 35 bar hydrogen atmosphere at room temperature for 20 days, and its high resolution TEM image (c).

XRD patterns of P25 and P25-black indicate both of them are composed of anatase (2θ = 25.5°, 37.9°, 48.2°, 53.8°, and 55.0°) and rutile phase (2θ = 27.6°, 36.1°, 41.2°, and 54.3°) (Fig. 2a). P25-black shows relatively weak peaks that should arise from the presence of some disordered structure due to the hydrogenation. It is highly interesting to indicate there are two obvious XRD diffraction peaks at 2θ = 30.22° and 35.54° for the P25-black. Such XRD pattern is different to the previous study, where the hydrogenation was carried out at a low concentration of hydrogen but at high temperature. The strong peak at 35.54° is well matched to the phase of Ti₂O₃ (JCPDS Card # 894746). It suggests the self-doped Ti³⁺ species with hydrogen atoms on its surface are formed during hydrogenation at 35 bar hydrogen. Such results have been confirmed by X-ray photoemission spectroscopy analysis and electron paramagnetic resonance measurement in literature.¹² FT-IR spectra of P25 and P25-black shown in Fig. 2b display the same trend but more stronger as the literature.²⁰ The FT-IR spectrum of P25 shows the OH stretching bands at ca. 3400 cm⁻¹ that are superimposed with a broad absorption band. For P25-black, the intensity of the OH peak in the FT-IR spectrum is much lower. Raman spectrum of P25 shows the anatase phase bands at 144, 197, 395, 515, and 638 cm⁻¹, which can be attributed to the five Raman-active modes with the symmetries of E_g, E_g, A1_g, B1_g, and E_g, respectively.¹⁷ P25-black shows the same Raman spectrum as P25 with broadened peaks that arise most likely from the disorder or destruction of the crystal lattice (Fig. 2c).^9,20 However, the rutile phase in Raman spectra is not observable for both samples. This is mainly because Raman compared to XRD is slightly insensitive to crystal structure of materials. UV-Vis spectra reveal that the band gap of the P25 nanocrystals is about 3.2 eV, and the absorbance in the visible even to near infrared range for P25-black is enhanced compared to the P25.¹³ The onset of optical absorption of the P25-black is lowered to about 1.0 eV. An abrupt change in the absorbance spectrum at about 1.82 eV (680 nm)²⁵ suggests that the optical gap of the P25-black is substantially narrowed by intraband transitions (Fig. 2d). It is widely accepted that spectral absorbance could reveal the band gap of the photocatalysts.⁹ No color change was observed for the P25-black nanocrystals over 9 months after they were synthesized.

Fig. 2 XRD patters (a), FT-IR spectra (b), Raman spectra (c) and UV-Vis spectra (d) of P25 and P25-black.

In order to observe the color change of P25 after hydrogenation, the original P25 was hydrogenated at different period by trapping the intermediate. The photographic images of hydrogen treated P25 for 0–20 days are shown in Fig. 3. After hydrogenation for 3 days, the raw P25, originally pure white in color, begins to turn into pale yellow. With the prolongation of the reaction duration, P25 becomes much darker with a longer hydrogenation time. After treatment for more than 15 days, the samples turn into gray and black. The dark samples would exhibit obvious and strong solar light absorbance.

Fig. 3 Photographic images of P25 treated under hydrogen for 0–20 days at 35 bar hydrogen atmosphere at room temperature.

XRD analyses (Fig. 4) were conducted in order to understand the structure evolution of P25 at different hydrogenation time. All the samples show the same XRD peaks ascribed to anatase and rutile phases. After 15 days of hydrogen treatment, however, the diffraction peaks at 2θ = 30.22°, 35.54° increasingly appear and become stronger with the hydrogenation duration, indicating there is a phase change under hydrogenation time of over 15 days at room temperature (see the arrows in Fig. 4).

Fig. 4 XRD patterns of P25 treated under hydrogen for 0–20 days at room temperature.

UV-Vis absorption spectra indicate that P25 shows enhanced solar light absorption with the hydrogenation duration (Fig. 5). For the UV-Vis spectra, however, the curves could be divided into two groups: one is with the hydrogenation time of over 15 days; and the other is with the hydrogenation time of less than 11 days. These results are well match with the color change and phase change of the hydrogenated P25.

Fig. 5 UV-Vis spectra of P25 treated under hydrogen for 0–20 days at room temperature.

The photocatalytic activity of the samples for water splitting under a Xe lamp (300 W) was studied in 20 vol% methanol (as a sacrificial agent) aqueous solution (Fig. 6). For the original P25, the rate of H₂ production is only 0.19 mmol g⁻¹ h⁻¹, indicating P25 almost has no photocatalytic activity under visible light. P25-black displays the highest photocatalytic activity, and the rate of H₂ production is about 3.94 mmol g⁻¹ h⁻¹. For hydrogen treated P25 within 11 days, the photocatalytic activity gradually increases with the increase of the hydrogenation time. However, for P25-11, the highest H₂ generation is only 1.02 mmol g⁻¹ h⁻¹, which is only one forth of that of P25-black. For the hydrogenation time of over 15 days, the rates of H₂ production greatly increase, which are 3.14 and 3.56 mmol g⁻¹ h⁻¹ for P25-15 and P25-17, respectively. It could be concluded that the color of the photocatalysts (Fig. 3) are correlated with the photocatalytic activity: the more dark of the photocatalysts will have the higher photocatalytic performance under visible light. The photocatalytic activity is also well matched with the phase change (Fig. 4) and UV-Vis absorption spectra change (Fig. 5). The hydrogenation process for P25 is facile and safe at room temperature but only time consuming. However, the reaction time will be shortened by increasing the H₂ pressure in the practical reaction, and this will make the large scale synthesis of P25-black possible.

Fig. 6 H₂ generation of P25 treated for different time in methanol aqueous solution.

Conclusions

In conclusion, commercial P25 TiO₂ was successfully hydrogenated at 35 bar hydrogen atmosphere and room temperature with enhanced photocatalytic activity under visible light. P25 turns into a dark color after 15 days of hydrogenation, and the 20-day hydrogen treated sample (P25-black) is black. P25-black has a crystalline-disordered core–shell structure, and exhibits a high H₂ generation rate of 3.94 mmol g⁻¹ h⁻¹ in photocatalytic reaction, while the original P25 only has a H₂ generation rate of 0.19 mmol g⁻¹ h⁻¹. For the P25 with the hydrogen treated over 15 days, the photocatalytic activity is much higher than that with hydrogen treatment time less than 11 days. This work provides a facile and green method to prepare black titanium oxide for visible light photocatalysis.

Acknowledgements

This research was financially supported by New Staff Start-up Research Fund from School of Chemical and Biological Engineering, Taiyuan University of Science and Technology. J.Y. thanks Monash University for the Monash Fellowship.

References

1. S. Hoang, S. P. Berglund, N. T. Hahn, A. J. Bard and C. B. Mullins, J. Am. Chem. Soc., 2012, 134, 3659–3662.
2. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
3. A. J. Bard and M. A. Fox, Acc. Chem. Res., 1995, 28, 141–145.
4. Y. H. Hu, Angew. Chem., Int. Ed., 2012, 51, 12410–12412.
5. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Science, 2001, 293, 269–271.
6. S. U. Khan, M. Al-Shahry and W. B. Ingler, Science, 2002, 297, 2243–2245.
7. C. Y. Wang, C. Böttcher, D. W. Bahnemann and J. K. Dohrmann, J. Mater. Chem., 2003, 13, 2322–2329.
8. X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959.
9. X. B. Chen, L. Liu, P. Y. Yu and S. S. Mao, Science, 2011, 331, 746–750.
10. X. B. Chen, L. Liu, Z. Liu, M. A. Marcus, W. C. Wang, N. A. Oyler, M. E. Grass, B. H. Mao, P. A. Glans, P. Y. Yu, J. H. Guo and S. S. Mao, Sci. Rep., 2013, 3, 1510.
11. G. M. Wang, H. Y. Wang, Y. C. Ling, Y. C. Tang, X. Y. Yang, R. C. Fitzmorris, C. C. Wang, J. Z. Zhang and Y. Li, Nano Lett., 2011, 11, 3026–3033.
12. R. H. Li, H. Kobayashi, J. F. Guo and J. Fan, Chem. Commun., 2011, 47, 8584–8586.
13. F. Zuo, L. Wang, T. Wu, Z. Y. Zhang, D. Borchardt and P. Y. Feng, J. Am. Chem. Soc., 2010, 132, 11856–11857.
14. A. Naldoni, M. Allieta, S. Santangelo, M. Marelli, F. Fabbri, S. Cappelli, C. L. Bianchi, R. Psaro and V. Dal Santo, J. Am. Chem. Soc., 2012, 134, 7600–7603.
15. T. Leshuk, R. Parviz, P. Everett, H. Krishnakumar, R. A. Varin and F. Gu, ACS Appl. Mater. Interfaces, 2013, 5, 1892–1895.
16. J. Kim and W. Choi, Energy Environ. Sci., 2010, 3, 1042–1045.
17. H. Q. Lu, J. H. Zhao, L. Li, L. M. Gong, J. F. Zheng, L. X. Zhang, Z. J. Wang, J. Zhang and Z. P. Zhu, Energy Environ. Sci., 2011, 4, 3384–3388.
18. H. Q. Lu, B. B. Zhao, D. Zhang, Y. L. Lv, B. P. Shi, X. C. Shi, J. Wen, J. F. Yao and Z. P. Zhu, J. Photochem. Photobiol., A, 2013, 272, 1–5.
19. B. Ohtani, O. O. Prieto-Mahaney, D. Li and R. Abe, J. Photochem. Photobiol., A, 2010, 216, 179–182.
20. Z. K. Zheng, B. B. Huang, J. B. Lu, Z. Y. Wang, X. Y. Qin, X. Y. Zhang, Y. Dai and M. H. Whangbo, Chem. Commun., 2012, 48, 5733–5735.
21. X. H. Lu, G. M. Wang, T. Zhai, M. H. Yu, J. Y. Gan, Y. X. Tong and Y. Li, Nano Lett., 2012, 12, 1690–1696.
22. A. Danon, K. Bhattacharyya, B. K. Vijayan, J. L. Lu, D. J. Sauter, K. A. Gray, P. C. Stair and E. Weitz, ACS Catal., 2012, 2, 45–49.
23. K. Komaguchi, H. Nakano, A. Araki and Y. Harima, Chem. Phys. Lett., 2006, 428, 338–342.
24. J. Zhang, Q. Xu, Z. Feng, M. Li and C. Li, Angew. Chem., Int. Ed., 2008, 47, 1766–1769.
25. A. Hagfeldt and M. Gratzel, Chem. Rev., 1995, 95, 49–68.

---