Daylight Photocatalysis by Carbon‐Modified Titanium Dioxide

Abstract

Green titana: Carbon-doped titanium dioxide, supported onto filter paper, photocatalyzes the gas-phase degradation of the atmospheric pollutants benzene (a), acetaldehyde (b) and carbon monoxide (c) in diffuse indoor visible light (see picture). Semiconductor photocatalysis is an efficient method for the chemical utilization of solar energy. It is based on the surface trapping of light-generated charges which induce interfacial electron-transfer reactions with a great variety of substrates. Titanium dioxide, the most promising photocatalyst is already used in various practical applications, such as self-cleaning paints and window panes.1 However, because of its large band gap of 3.20 eV only the small UV fraction of solar light, about 2–3 %, can be utilized. Therefore many attempts have been made to sensitize titanium dioxide for the much larger visible fraction. In previous work we have shown that incorporation of a few weight percent of a transition-metal chloride either in the bulk or at the surface of titanium dioxide leads to visible-light photocatalysis.2 Especially the materials modified by chemisorbed platinum(IV) chloride were very active and sustained photomineralization of the ubiquitous pollutant 4-chlorophenol by visible light (λ≥455 nm) for several days.2e Different from modification by metal complexes, we recently found that a coke-containing titanium dioxide was active also with visible light. This material was prepared by a sol–gel method using various titanium alkoxide precursors.3 Since its activity, especially at low light intensity was much smaller than the platinum(IV) chloride modified titanium dioxide, we searched for improvement. This was achieved during work on nitrogen-doped titanium dioxide4a in which we hydrolyzed titanium tetrachloride with nitrogen bases, such as tetrabutylammonium hydroxide. At the subsequent calcination step we observed that prolonged heating at 400 °C afforded an anatase material which contained carbon instead of nitrogen. This preparation method has an excellent reproducibility and greater applicability than the oxidation of a titanium metal sheet to a carbon-doped rutile layer in a natural gas flame.5, 6 In the degradation of 4-chlorophenol by artificial light (λ≥455 nm) these new, blackish-brown powders are five times more active than nitrogen-doped titanium dioxide. Herein we report the preparation, photoelectrochemical, and photocatalytic properties of carbon doped TiO2 under direct artificial and diffuse natural light. The samples TiO2-C1a, TiO2-C1b, and TiO2-C2 containing 2.98, 0.42, and 0.03 % carbon were prepared by the hydrolysis of titanium tetrachloride with tetrabutylammonium hydroxide followed by calcination at 400 °C for 0.25 h (TiO2-C1a) and 1 h (TiO2-C1b), and at 550 °C for 4 h (TiO2-C2). Unmodified TiO2 was prepared by the same procedure but replacing the nitrogen base by sodium hydroxide. Except for TiO2-C1a, which contained traces of nitrogen, the three other samples were nitrogen-free. According to X-ray diffraction (XRD) measurements all the materials were in the anatase modification except TiO2-C2 which contained trace quantities of the rutile phase. X-ray photoelectron spectroscopy (XPS) measurements of the C1 binding energy indicated the complete absence of carbon in the undoped sample, whereas for TiO2-C1b, peaks at 285.6, 287.5, and 288.5 eV were found.7 The first value arises from adventitious elemental carbon,8, 9 the latter two values suggest the presence of a carbonate species.8 This assumption is supported by the IR spectrum of TiO2-C1b which exhibits low-intensity peaks at 1738, 1096, and 798 cm−1 which are indicative for the carbonate ion.10, 11 In contrast to these results, it was proposed that in the rutile layer, prepared by flame oxidation, carbon partially substitutes the oxide ion.6 Diffuse reflectance spectra reveal that the new absorption at 400–700 nm is related to the carbon content. It is stronger for TiO2-C1a than for TiO2-C1b and completely absent in TiO2-C2 (Figure 1, Table 1), which was calcined at 550 °C. Assuming the materials to be indirect semiconductors, like TiO2, a plot of the modified Kubelka–Munk function versus the energy of exciting light12, 13 affords band-gap energies of 3.16, 3.02, 3.11, and 3.17 eV for TiO2, TiO2-C1a, TiO2-C1b, and TiO2-C2, respectively (Figure 1, Table 1).14 The maximum band-gap narrowing of 0.14 eV is comparable with the value of 0.05 eV recently observed for nitrogen-doped TiO2.4a,4c A) Diffuse reflectance spectra of modified and pure TiO2; B) Plot of transformed Kubelka–Munk function versus the energy of the light absorbed; a) TiO2, b) TiO2-C1a, c) TiO2-C1b, d) TiO2-C2. Catalyst C [%] pH0[a] Ebg [eV][b] Ufb (NHE)[a] [V][c] ri [10−8 mol L−1 s−1][d] TiO2 0.00 5.80 3.16 −0.52 0.10 TiO2-C1a 2.98 7.90 3.02 −0.39 3.75 TiO2-C1b 0.42 6.40 3.11 −0.48 7.65 TiO2-C2 0.03 5.44 3.17 −0.54 0.13 To understand whether a shift of the valence or conduction band edge is responsible for this decrease of the band-gap energy, the position of the flat-band potential (Ufb) was determined by the “slurry method”15, 16 through measuring the photovoltage16 as a function of the suspension pH value (Figure 2). The inflection point corresponds to the pH0 value from which the flat-band potential can be calculated (Table 1). The value of −0.52 V measured for TiO2 at pH 7 is in good agreement with the −0.58 reported for an anatase single crystal and measured by the Mott–Schottky method17 and −0.47 V for anatase powder also measured by the slurry method.18 As observed for the band gap, the change in flat-band potential is also related to the carbon content. Whereas the potential of −0.54 V, found for the low-carbon sample TiO2-C2, does not differ significantly from that of unmodified TiO2 (−0.52 V) it is shifted to −0.39 and −0.48 V for TiO2-C1a and TiO2-C1b, respectively. The anodic shifts of 130 and 40 mV may be compared with the value of 50 mV found for nitrogen-doped TiO2.4a Assuming that the distance between flat-band potential and the conduction band edge is vanishing for these probably highly doped n-type semiconductor materials, the position of the valence-band edge can be located from the corresponding band-gap energies as +2.63 V for all samples. Variation of photovoltage with pH value for the suspension of catalyst in the presence of methyl viologen a) TiO2, b) TiO2-C1a, c) TiO2-C1b, d) TiO2-C2. Since the carbon-modified samples start absorbing at about 735 nm (1.70 eV; Figure 1) a variety of surface states must exist. To determine their approximate energy, the wavelength dependence of OH-radical formation by TiO2-C1b was investigated in the presence of two different electron acceptors and benzoic acid as OH scavenger. In the presence of oxygen, salicylic acid was produced irrespective of exciting TiO2-C1b at λ≥320 (3.88 eV), 455 (2.73 eV), and 495 nm (2.51 eV). This can be rationalized by recalling that the reduction potential of the O2/O2− couple at pH 7 is −0.16 V.19 If light absorption at the longest wavelength would occur from the valence band to a surface state close to the conduction band, the resulting potential of the trapped electron should be located at about +0.12 V and reduction of oxygen und subsequent OH-radical formation is not feasible. Contrary, this process becomes possible when absorption occurs from a surface state located near the valence band to the conduction band, since now the trapped electron potential is −0.39 V. In accord with this explanation, upon replacement of oxygen by tetranitromethane as an electron acceptor, salicylic acid formation proceeded only for the short wavelength irradiation. Under these band-to-band absorption conditions the OH-radical formation occurs directly through hole-oxidation of water or surface hydroxy groups (Figure 3). This significant difference suggests the presence of surface states close to the valence-band edge, as was also concluded from theoretical calculations.4c Formation of salicylic acid (SA) upon irradiation of TiO2-C1b in the presence of benzoic acid; a) λ≥495 nm, under O2, b,c) λ≥495 nm, under Ar and in the presence of tetranitromethane c) λ≥320 nm, under Ar and in the presence of tetranitromethane. To explore the photocatalytic activity of the new materials, the degradation of some typical pollutants in water and air by artificial and natural light was investigated. Figure 4 A illustrates the photomineralization of the ubiquitous water pollutant 4-chlorophenol with visible light (λ=455 nm) in the presence of the various photocatalysts. TiO2-C1a and TiO2-C1b induced complete mineralization after 210 and 180 min, respectively (curves b, c). In contrast, TiO2-C2 and unmodified TiO2 did not significantly change the total organic carbon value (curves a, d). However, they induced a small reduction in the concentration of 4-chlorophenol as measured by UV absorption spectroscopy. The corresponding rates found for TiO2-C1a and TiO2-C1b were about 30 and 60 times larger (Table 1). The twofold rate enhancement induced by TiO2-C1b relative to TiO2-C1b although the latter contains more carbon may be due to the higher crystallinity of TiO2-C1a as suggested by XRD spectra. A) Photomineralization of 4-chlorophenol with artificial visible light (λ=455 nm; TOC0 and TOCt=total organic carbon content at times 0 and t); a) TiO2, b) TiO2-C1a, c) TiO2-C1b, d) TiO2-C2. B) Diffuse indoor daylight degradation of 4-chlorophenol (4-CP) and remazol red in the presence of pure TiO2 and TiO2-C1b; a) TiO2/4-CP, b) TiO2/remazol red, c) TiO2-C1b/4-CP, d) TiO2-C1b/remazol red. C) Gas-phase photodegradation of acetaldehyde (a and c1, 5 vol %), benzene (c2, 5 vol %), and carbon monoxide (c3, 5 vol %) in air through diffuse indoor light in the presence of pure TiO2 (a) and TiO2-C1b (c). The superior photocatalytic activity of the carbon-doped materials is demonstrated by illumination experiments in diffuse indoor daylight. In solution 4-chlorophenol and the azo dye remazol red are efficiently mineralized only by TiO2-C1b (Figure 4 B, curves c, d) whereas unmodified TiO2 is almost inactive (curves a, b). Furthermore TiO2-C1b supported on filter paper, photocatalyzes the oxidation of gaseous acetaldehyde, benzene, and carbon monoxide (Figure 4 C). Preparation of photocatalysts: A solution of tetrabutylammonium hydroxide (0.25 mol L−1) was added dropwise to 0.25 M TiCl4 (200 mL) at 0 °C until a pH value of 5.5 was reached. After ageing the suspension for 24 h at room temperature, the precipitate was collected by filtration and dried under air at 70 °C. The residue was crushed into a fine powder and calcined in a muffle furnace at 400 °C for 0.25 and 1 h, and at 550 °C for 4 h. Calcination temperatures were attained at a heating rate of about 5 °C min−1. Photochemical experiments: Photomineralizations in the aqueous phase were carried out with catalyst powder (15 mg) suspended in a water-cooled cylindrical 15-mL quartz cuvette; the suspension was stirred magnetically. Appropriate cut-off filters were inserted before irradiating with an Osram XBO 150 W xenon arc lamp. Salicylic acid was measured through its intense fluorescence at 400–420 nm as described elsewhere.2d Daylight experiments were performed in the laboratory by exposing the reaction system to the diffuse radiation arriving through the window (4–10 W m−2 at 400–1200 nm). Solution experiments were conducted in an Erlenmeyer flask containing TiO2-C1b (50 mg) and solutions of 4-chlorophenol (50 mL; 2.5×10−4 mol L−1) or solutions of remazol red (50 mL; 1×10−4 mol L−1). Gas-phase experiments were conducted in a 1-L round bottom flask containing a filter paper of 11-cm diameter onto which the catalyst (12 mg) was supported; concentration change and CO2 formation were measured by IR spectroscopy. Publication delayed at authors request In memory of Dieter Sellmann Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2003/z51577_s.pdf or from the author. 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