Facile Vacuum Annealing-Induced Modification of TiO₂ with an Enhanced Photocatalytic Performance

Abstract

In this work, the photocatalytic performance enhancement of hydrothermally prepared TiO2 was achieved by facile vacuum annealing treatment. Calcination of TiO2 powder in air (CA-TiO2) maintained its white color, while gray powder was obtained when the annealing was performed under vacuum (CV-TiO2). Fourier transform infrared, total organic carbon, X-ray photoelectron spectroscopy, and electron paramagnetic resonance analyses proved that vacuum annealing transformed ethanol adsorbed on the surface of TiO2 into carbon-related species accompanied by the formation of surface oxygen vacancies (Vo). The residual carbon-related species on the surface of CV-TiO2 favored its adsorption of organic dyes. Compared with TiO2 and CA-TiO2, CV-TiO2 exhibited an improved charge carrier separation with surface Vo as trapping sites for electrons. Vacuum annealing-induced improvement of crystallinity, enhancement of adsorption capacity, and formation of surface Vo contributed to the excellent photocatalytic activity of CV-TiO2, which was superior to that of commercial TiO2 (P25, Degussa). Obviously, vacuum annealing-triggered decomposition of ethanol played an important role in the modification of TiO2. In the presence of ethanol, vacuum annealing was also suitable for the introduction of Vo into P25. Therefore, the current work offers an easy approach for the modification of TiO2 to enhance its photocatalytic performance by facile vacuum annealing in the presence of ethanol.

Facile Vacuum Annealing of TiO2 with Ethanol-Induced Enhancement of Its Photocatalytic Performance under Visible Light

“…As proved in our previous work, hydrothermally prepared TiO 2 adsorbs a small amount of ethanol and vacuum annealing of TiO 2 in a sealed glass tube (500 °C, 2 h) forms dark gray products as a result of the carbonation of ethanol. 38 Thus, vacuum annealing of TiO 2 in high vacuum at 300 °C in the presence of ethanol may form yellow products on the surface of TiO 2 which are responsible for the color changes. 41 This assumption is supported by the increased visible light absorption when the vacuum annealing of TiO 2 in high vacuum at 300 °C is conducted with additional ethanol (Figure 1a).…”

“…Compared with hydrothermally prepared TiO 2 , CA-TiO 2 has improved crystallinity due to the calcination process (Figure S4). 38 As a result, vacuum annealing of CA-TiO 2 at 300 °C with ethanol in high vacuum not only induces the visible absorption but also the formation of Vo in well-crystallized CA-TiO 2 . Both CA-TiO 2 -Ethanol-300 and TiO 2 -Ethanol-400 show comparable signal intensities of Vo (Figures 1b and 2b), while the visible light absorption intensity of CA-TiO 2 -Ethanol-300 (Figure 2a) is higher than that of TiO 2 -Ethanol-400 (Figure 1a).…”

“…The g factor was calculated to confirm the formation of oxygen vacancy (Vo). 38 The active species formed in the ■ RESULTS AND DISCUSSION Characterizations of TiO 2 Photocatalysts. From Figure 1a, it is clear that TiO 2 shows clear visible light absorption after vacuum annealing (300 °C, 24 h) (TiO 2 -300), and the absorbance increases when TiO 2 is annealed in vacuum with ethanol (TiO 2 -Ethanol-300).…”

“…Our previous work has proved that vacuum annealing of TiO 2 in a vacuumed glass tube (500 °C, 2 h) creates Vo (CV-TiO 2 ) without visible light absorption. 38 Although many works propose that Vo is responsible for visible light absorption, 31,37,43 the relationship between Vo and visible light absorption still needs further analysis. Compared with TiO 2 -300, TiO 2 -Ethanol-300 exhibits enhanced visible light absorption (Figure 1a) suggesting that the pyrolysis of ethanol at 300 °C in high vacuum may be responsible for this phenomenon.…”

“…35−37 However, our previous work shows that vacuum annealing of hydrothermally prepared TiO 2 leads to the formation of Vo without visible light absorption. 38 Thus, the reason for vacuum annealing of TiO 2 induced visible light absorption and the role of Vo need to be further investigated.…”