Carbon-doped TiO2 photocatalyst synthesized without using an external carbon precursor and the visible light activity

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Abstract

Carbon-doped TiO2 (C-TiO2) was successfully prepared from a conventional sol–gel synthesis without using external carbon precursors whereas all the previous reports on the synthesis of C-TiO2 utilized them. The carbons contained in titanium alkoxide precursor could be incorporated into the lattice of TiO2 with creating mid-bandgap electronic states through the controlled calcination. The level of carbon doping was changed sensitively depending on the calcination temperature, which was verified by UV–visible diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The charge pair generation/recombination in C-TiO2 was compared under visible and UV light through a time-resolved diffuse reflectance spectroscopy study, which suggested the presence of midgap energy levels induced by the incorporation of carbon dopants. The carbon doping was maximal when the calcination temperature was around 200–250 °C and hindered at higher temperatures. The visible light activities of the prepared TiO2 samples for the conversion of 4-chlorophenol and iodide were also strongly dependent on the calcination temperature and maximized at around 250 °C. This study implies that the carbon doping in TiO2 can be obtained even unintentionally in the conventional sol–gel synthesis.

Introduction

Titanium dioxide has been the most popular photocatalyst due to high photo-oxidation power, excellent stability, non-toxicity, and low material cost. The photocatalytic reactions on TiO2 are initiated by bandgap excitation and the subsequent generation and transfer of electron (ecb) and hole (hvb+) pairs [1]. In general, TiO2 with a large bandgap (∼3.2 eV) is activated only under UV light irradiation, which is a major drawback. Therefore, it has been modified by various ways such as dye sensitization [2], [3] and transition metal doping [4], [5], [6], enabling the photocatalyst to operate under visible light irradiation. Recently, researchers have reported that doping TiO2 with non-metals such as N, S or C extends the absorption wavelengths from UV to visible region, which has been ascribed to the introduction of localized electronic states in the bandgap [7], [8], [9], [10].

The intentional and controlled incorporation of carbon impurities into TiO2 lattice is an efficient method to induce visible light photocatalytic activity [11], [12], [13], [14], [15], [16], [17], [18]. TiO2-mounted activated carbon [19], [20] and carbon-coated TiO2 [21], [22] have been also investigated for their photocatalytic activity but the role of carbon in these cases is related mainly with enhancing the adsorption of organic substrates. The carbon-doped TiO2 (C-TiO2) has been prepared using different synthetic methods and the state of impurity carbons in the TiO2 lattice has been interpreted differently. In the literature, the carbon dopant has been described either as an anion that replaces oxygen substitutionally in the lattice [11], [12], [13], [14] or as a cation that occupies an interstitial lattice site [15], [16], [17], [18]. The formal oxidation state of carbon dopants ranges from −4 (as carbides with Ti–C bond) to +4 (as carbonates with C–O bond) [9]. Ti–C bond was formed at conditions like flame pyrolysis of Ti metal sheet, annealing of TiC powders [11], [12], [13], and ion-assisted electron beam evaporation. On the other hand, C–O bond (carbonate) was often observed at conditions like sol–gel processes with carbon precursors and high temperature reactions of TiO2 with carbon precursors [15], [16], [17], [18]. The existence of the former or the latter state seems to be strongly dependent on the preparation method and condition. Both carbon states may be even co-present depending on the preparation condition [23]. Although carbon is a ubiquitous impurity, the addition of external carbon precursors (e.g., alkylammonium, urea, glucose) was essentially needed to make C-TiO2 in all reported cases.

In this study, we note that the common precursors of TiO2, titanium alkoxides, contain organic carbons which might be incorporated into the oxide lattice as an impurity dopant during synthesis even in the absence of extra carbon precursors. We synthesized C-TiO2 through a sol–gel route (using titanium butoxide as a precursor of both Ti and C) and an incomplete calcination at low temperature. Although the post-heat treatment (calcination) usually removes all carbon residues, a controlled calcination at temperature ranging from 200 to 300 °C could leave some carbon species as a dopant within the TiO2 lattice. Various experimental characterizations of the material properties and photocatalytic activities confirmed that the TiO2 prepared in the absence of external carbon precursors was indeed C-TiO2.

Section snippets

Catalyst preparation and characterization

C-TiO2 was prepared according to a typical sol–gel method. 32 mmol of titanium butoxide (Ti[O(CH2)3CH3]4, Aldrich) was dissolved in 50 ml of 2-propanol, and then HClO4 solution (7 mmol) was added dropwise under vigorous stirring in an ice bath. The resulting gel was aged at room temperature for 1 day and dried in an oven at 80 °C and subsequently heat-treated in a furnace at various temperatures with a ramping rate of 5 °C/min for 3 h.

The phase identification of C-TiO2 was carried out by powder X-ray

Evidences of carbon dopants in TiO2

Fig. 1a shows the XRD patterns of the synthesized TiO2 powder that was treated at various calcination temperatures. The TiO2 powder obtained through the sol–gel process had primarily the anatase phase, and the rutile phase appeared when the temperature increased between 300 and 400 °C and grew substantially high at 500 °C. The crystalline particle size and the BET surface area of the TiO2 sample powders calcined at various temperatures were summarized in Table 1. With increasing the temperature

Conclusions

This study demonstrates that the carbon-doped TiO2 could be successfully obtained from a sol–gel synthesis process even without using external carbon precursors such as urea and alkylammonium that have been frequently employed. The carbons contained in titanium alkoxide precursor can be incorporated into the lattice of TiO2 through the controlled calcination. It should be emphasized that the visible light activity induced by the carbon doping closely depends on how it is prepared. The visible

Acknowledgements

This work was supported by a KOSEF grant (No. R0A-2008-000-20068-0), the KOSEF EPB center (Grant No. R11-2008-052-02002), and BK 21 program. T.M. and T.T. thank to a Grant-in-Aid for Scientific Research (Project 17105005, 19750115) from MEXT (Japan).

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