Article pubs.acs.org/JPCC
Preparation and Visible Light Photocatalytic Activity of a GraphiteLike Carbonaceous Surface Modified TiO2 Photocatalyst Lijing Chen, Feng Chen,* Yanfen Shi, and Jinlong Zhang Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ABSTRACT: By using water-soluble linear phenolic resin (PF) as C-source, a graphite-like carbonaceous surface modified TiO2 photocatalyst (PF/TiO2) was successfully synthesized through a hydrothermal process. The as-formed PF/TiO2 exhibited significantly higher photocatalytic activity than bare TiO2 toward the degradation of methyl orange (MO) under visible irradiation, while a tetrahydrofuran (THF) postrinsing operation can further enhance the photocatalytic activity of PF/ TiO2. To exploit the mystery of surface carbonaceous covering, a series of characterizations such as XPS, TG, TEM, and FTIR, as well as 1H NMR were employed. It is proposed that PF experienced a cross-linking and a carbonization process that are catalyzed by TiO2 at the surface of TiO2. The as-formed organics covering on the PF/TiO2 seems composed of four portions from the surface of TiO2 to the outside as oxidized graphite-like carbon (OGC), graphite-like carbon (GC), cross-linked PF (cl-PF), and PF. OGC and GC substances endow the catalysts visible absorption and photocatalytic activity, which are insoluble in the THF; meanwhile, PF and cl-PF are adverse to the visible light photocatalysis and can be rinsed out with THF. The visible light photocatalytic activity unrinsed PF/TiO2 reaches its optimal condition at a PF dosage of 0.7 g (for 5.0 g titanium sulfate) while the THF rinsed PF/TiO2 presents the highest photocatalytic activity with the maximum PF dosage in this work and gives a MO degradation rate of 100% in 4 h.
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INTRODUCTION Semiconductor photocatalysis, as one of the most promising technologies, has main applications in providing clean hydrogen energy and environmental applications.1−3 Titanium dioxide, due to its cheapness, nontoxicity, effectiveness, and photostability,4,5 is attracting more attention among various semiconductor materials. Unfortunately, because of its large band gap of 3.2 eV, it can utilize only a very small UV fraction (about 3−5%) of solar light arriving at the earth surface.6−8 Hence, it has aroused great interest for extending the photoresponse of TiO2 to the visible light region for better solar light utilization. In the past decade, many efforts have been made to achieve the utilization of visible light for TiO2 material, e.g., transition metal doping,9−11 nonmetal doping,6,8 and dye sensitizing.12,13 The dopants can produce new hybrid states in the band gap, which narrow the band gap and shift the optical response of TiO2 to the visible light region. Particularly, doping TiO2 with nonmetal elements has received much attention, especially for carbon element. Nevertheless, the mechanism of how nonmetal elements (e.g., carbon) enter the TiO2 lattice is still kept unclear. Recently, another explanation, suggested by Kisch et al.14 on basis of their recent work, has gradually been recognized, which involves surface modification and interfacial sensitization. Our group also prepared a carbon deposited TiO2 (C@TiO2)15 with visible light photocatalytic activity by a hydrothermal method using glucose as the carbon source in which we found the C element did not enter the TiO2 lattice but deposited on the surface of TiO2 grains. © 2012 American Chemical Society
Polymers were also begun to be utilized to modify the photocatalysts in visible range. Polymers with conjugated structures and polar groups have shown more competitive applications in preparing visible-response TiO2 photocatalysts because of their much broader absorption spectra and large dipole moment. Zhang et al.16 prepared a fluorene−thiophene copolymer sensitized TiO2 and verified the electronic transfer between polymer and TiO2 under visible irradiation. Li et al.17 employed polyaniline to modify the TiO2, and then, the photocatalytic activity for degradation of phenol was evidently enhanced. As a functional polymer, water-soluble phenolic resin (PF) has been frequently studied. It owns a linear chain, which makes itself aqueous soluble and nonvolatilizable. PF would be pyrolyzed and carbonized in situ under high temperature treatment (300−2400 °C).18−20 Considering that the watersoluble PF could be chemically adsorbed to the surface of TiO2 efficiently due to its phenolic hydroxyl groups and that TiO2 can promote inversely the dehydration and carbonization of PF under thermal/hydrothermal treatment, a hybrid photocatalyst, PF/TiO2 nanomaterial, was prepared in this work. The carbonization of PF in the hybrid photocatalyst was operated via hydrothermal treatment. A typical azo-dye, methyl orange (MO), was employed to investigate the photocatalytic reactivity of PF/TiO2 under visible irradiation. To exploit the specified Received: December 14, 2011 Revised: March 16, 2012 Published: March 28, 2012 8579
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Catalyst Characterization. X-ray diffraction (XRD) patterns of all samples were carried out in the range of 10− 80° (2θ) using a Rigaku D/MAX 2550 diffractometer (Cu Kα radiation, λ = 0.15406 nm), operated at 40 kV and 100 mA. The crystallite size was estimated by applying the Scherrer equation to the full width at half-maximum (fwhm) of the (101) peak of anatase. The surface morphologies and particle sizes were observed by high-resolution transmission electron microscopy (HRTEM, JEM-2011), using an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was recorded with a PerkinElmer PHI 5000C ESCA System with Al Kα radiation operated at 250 W. The shift of binding energy was corrected using the C1s level at 284.6 eV as an internal standard. The BET specific surface area (SBET) was determined by nitrogen adsorption at 77.3 K (Micromeritics ASAP 2020). Samples were degassed at 473 K for 5 h prior to the measurement. Thermogravimetric and differential thermal analysis (TG-DTA) curves were recorded on a Rigaku TG8120 instrument at a heating rate of 10 °C min−1 under air using RAl2O3 as the standard material. IR spectra of the photocatalysts, as KBr pellets, were recorded using a Nicolet 380 FT-IR spectrometer. To analyze the light absorption of the photocatalysts, the UV−vis diffused reflectance spectra (DRS) were obtained from the dry-pressed disk samples using a Scan UV− vis spectrophotometer (Shimadzu, UV-2450) equipped with an integrating sphere assembly, using BaSO4 as the reflectance sample. Photocatalytic Activities. The photocatalytic activity of the photocatalysts was evaluated by measuring the decomposition of MO (20 mg/L). For a typical photocatalytic experiment, 0.07 g of catalyst powders was added into 70 mL of the above MO solution in a 100 mL glass tube. A 1000 W halogen lamp equipped with a UV cutoff filter (λ > 420 nm) was used as a visible light source in a homemade photoreactor, cooled with flowing water in a quartz cylindrical jacket around the lamp. Prior to the irradiation, the suspension was magnetically stirred in the dark for 30 min in order to reach an adsorption−desorption equilibrium. At given time intervals, the analytical samples were taken from the suspension and centrifuged to remove the photocatalysts. The residual MO level was then analyzed by recording variations in the UV−vis absorption of MO using a UV-2450 UV−visible spectrometer.
relationship between the grafted organics and visible photoactivity, a postrinsing process by tetrahydrofuran (THF) to the photocatalyst was further carried out. Intensive comparisons of degradation of MO with respect to the different characterization technique of catalysts were fully illustrated, which presented some key information to elucidate the role of various organic substances on the visible photocatalysis in graphite-like substance surface modified TiO2 such as PF/TiO2.
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EXPERIMENTAL METHODS Preparation of PF. In a typical procedure,21 47.0 g of phenol, 52.5 g of 37% formaldehyde solution, and 29.0 g of sodium hydroxide were added into the flask. Subsequently, the suspension solution was heated to 65 °C with stirring and kept for 30 min. Then, the temperature of the solution was raised up to 95 °C and kept for 45 min. The water-soluble PF (Scheme 1) was thus obtained after cooling down. Scheme 1. Molecular Structure of PF
Preparation of PF/TiO2 Catalysts. PF-modified TiO2 nanoparticles were synthesized by a hydrothermal method: A certain amount of PF was dissolved in 70 mL of double distilled water to form a clear solution. Five grams of titanium sulfate was then added into the PF solution. After vigorous stirring for 1 h, the suspensions were transferred into an autoclave (100 mL) and kept at 120 °C for 8 h. The precipitate as-prepared in the autoclave was then washed with water several times, dried, and ground to obtain PF-modified TiO2 nanoparticles. According to the different quantity of PF used (0.0, 0.5, 0.7, 1.5, 3.5, and 5.0 g), the corresponding product PFTx series (PFT0, PFT0.5, PFT0.7, PFT1.5 PFT3.5, and PFT5.0) PF/ TiO2 were obtained. Postrinsing of PF/TiO2 Catalysts. PF/TiO2 catalysts (0.5 g) were suspended in 50 mL of tetrahydrofuran (THF). Thereafter, the suspension was kept at 60 °C for 5 h under vigorous stirring. The powder was then obtained by filtration, washing with THF solvent, and drying in vacuum oven of 60 °C for 10 h, the as-obtained photocatalyst was denoted as TPF/TiO2.
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RESULTS AND DISCUSSION PF is a long chain organic polymer with hydroxyl groups (Scheme 2). Hence, a chemical adsorption can possibly be achieved by a condensation reaction between a phenolic −OH group of PF and the surface Ti−OH group of TiO2, which can
Scheme 2. Radial Distance-Dependent Organic Covering of PF/TiO2
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yield a R−O−Ti structure.22,23 The formation of surface complexes will theoretically lead to a bathochromic shift of the photocatalyst and reversely a promoted condensation and carbonization of PF. Crystal and Morphology Properties of PF/TiO 2 Catalysts. The XRD patterns of various PF/TiO2 photocatalysts in Figure 1 show that hydrothermal treatment process
interspace of TiO2 grains with PF modification, which, however, can be removed by THF rinsing. This proposal can be confirmed with an ethanol soaking treatment. Figure 2D,E shows the TEM pictures of PFT5.0 and T-PFT5.0 that have been soaked with absolute ethanol for a while. The organics covering on the PFT5.0 catalyst surface is significantly swelled with the ethanol solvent; and hence, a large gray area began to appear around the catalyst granules. The gray area disappears after PFT5.0 catalyst was washed by THF (Figure 2E), which firmly identifies the occurrence and the rinse out of organic materials on the surface of TiO2. Spectral Characteristic of PF/TiO2. The spectrum of PFT0 shows an absorption edge of 389 nm, which is coincident with the literature value of anatase of ca. 387 nm.25 Surface modification of TiO2 with PF leads to a significant absorption in the visible region. In addition to the absorption edge around 389 nm, a new absorption edge appears beyond the 600 nm (608−650 nm as shown in Figure 3 for various PF/TiO2). The
Figure 1. Wide-angle XRD patterns of the various PF/TiO2: (a) PFT0, (b) PFT0.5, (c) PFT1.5, (d) PFT5.0, and (e) T- PFT5.0.
during the PF/TiO2 preparation does not change the crystal phase, crystallinity, nor the grain size of TiO2. The dominant phase in all samples is anatase, which should be attributed to the effect of sulfate ion.24 The average grain sizes of TiO2 estimated from the Scherrer equation give a value of 8.0 ± 0.2 nm, which are hardly changed for all the samples. In a word, the presence of PF during the hydrothermal process and the rinse operation seem not vary much the crystal properties of TiO2. The TEM images of PFT0 (that is TiO2), PFT5.0, and TPFT5.0 were presented in Figure 2. After the modification of PF, the boundary of granules in PFT5.0 became a little bit ambiguous, while those in PFT0 and T-PFT5.0 are very sharp. It should be due to the organics covering on the surface and
Figure 3. UV−vis DRS spectra of different PF/TiO2 samples: (a) PFT0, (b) PFT0.5, (c) PFT1.5, (d) PFT5.0, and (e) T-PFT5.0.
new absorption at the visible region should be ascribed to the large conjugated structure in the organic covering in PF/TiO2. The more PF materials used for the preparation of PF/TiO2, the higher the visible absorbance of PF/TiO2 photocatalyst. Resultantly, PFT5.0 gives the maximum visible absorption as shown in Figure 3. THF rinsing does not change much of the spectral performance of PFT5.0. The DRS spectrum of T-PFT5.0 presents a very similar absorbance as that of PFT5.0. It seems that the substance that causes the visible absorption of PF/ TiO2 should be insoluble to the THF or chemically combine to the surface of TiO2 and thus cannot be rinsed away from TiO2 with THF. Chemical Analysis toward the Organic Covering in PF/TiO2. XPS analysis was operated to analyze the chemical state of all elements in the hybrid catalysts. Literature works show that organic carbon has three signals around the 284.6, 286.4, and 288.6 eV,15,26 which correspond to C−C chain structure (and the carbon contaminants from the ambience), C−OH (and C−O), and Ti−O−C, respectively, while the lattice-doped carbon (C−Ti) shows a binding energy around 282 eV.27,28 Figure 4 shows the C1s fine XPS spectra of PFT0, PFT5.0, and T-PFT5.0. The XPS signals for PFT0 sample should be attributed to the carbon contaminants from the ambience, as no organic carbon species had been used throughout its preparation. The intensity of C1s fine XPS spectrum is obviously enhanced for PFT5.0, which suggests the relative content of carbon element (that is organic matter)
Figure 2. TEM images of (A) PFT0, (B) PFT5.0, and (C) T-PFT5.0, and (D) PFT5.0 and (E) T-PFT5.0 soaked with absolute ethanol. 8581
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sections:29,30 (1) The mass loss under 260 °C corresponds to the desorption of physically and chemically adsorbed water and condensation of hydroxyl. (2) The mass loss between 260 and 530 °C corresponds to the burning of organic matters, which gives a value of 26.06% for PFT5.0. Comparing with that of 2.83% for PFT0, an increment of 23.23% refers to the additional organic matters in PFT5.0. (3) At a temperature above 530 °C, where all the organics have burnt up, a further mass loss should be attributed to the decomposition of chemically adsorbed SO42−. As the Ti precursor used in this work was titanium sulfate, it is reasonable to chemically adsorb some SO42− at the interface of TiO2. The PFT0 gives a mass loss of 3.60% for the SO42− decomposition, while that for PFT5.0 only shows a mass loss of 1.96%. Surely, the chemical adsorption of PF competes with the adsorption of SO42− on the surface of TiO2. The mass loss of PFT5.0 in the second section decreases evidently with THF rinsing, and hence, T-PFT5.0 only gives a mass loss of 13.47% as shown in Figure 5. Figure 6 presents the O1s fine XPS spectra of PFT0, PFT5.0, and T-PFT5.0. Some literature works suggested that the O1s Figure 4. C1s fine XPS spectra of (a) PFT0, (b) PFT5.0, and (c) TPFT5.0.
increases in PFT5.0, and is consistent with the TEM observation. THF rinsing lowers the XPS peak intensity of C1s, which suggests the removal of organics. The signal intensity of C1s for T-PFT5.0, however, is relatively higher than that of PFT0, which suggests an incomplete removal of surface organics. Correspondingly, the C/Ti ratios of PFT0, PFT5.0, and T-PFT5.0 obtained from their XPS survey spectra give values of 3.04, 5.18, and 4.35, respectively. No carbon species with binding energy around 282 eV was observed, which denies the lattice doping of C element. The relative contents of the deconvolution ascribed to Ti−O−C (288.6 eV) is relatively higher for PFT5.0 than PFT0, which may be due to the condensation reaction between the phenolic −OH group and the Ti−OH group during the chemical adsorbing of PF on TiO2. The content variation of organic matters in PF/TiO2 can be further verified with TG measurement. Figure 5 presents the mass loss of PF/TiO 2 in TG measurement and the corresponding calculated weight percentage of organic species in each sample. The TG curve of PFT5.0 shows once more that the carbon element here is not likely to be doped into the TiO2 lattice, as the burning of the carbon species burst at only ca. 260 °C. The TG curve in Figure 5 can be divided into three
Figure 6. O1s fine XPS spectra of (a) PFT0, (b) PFT5.0, and (c) TPFT5.0.
fine XPS signal contains 4 deconvolutions,15 which are 529.6 eV (lattice O), 531.3 eV (CO), 532.0 eV (Ti−OH), and 533.5 eV (C−OH), while others reported O1s binding energy of lattice oxygen around 529.9/530.0 eV31,32 and that of C−O, CO, and COO bonds around 531.3/531.5/531.6 eV.33 The O1s fine XPS spectra in Figure 6 can be fitted into 3 deconvolutions around 529.9 eV (lattice O), 531.9 eV (C−O, CO as well as Ti−OH, as they are too close to be effectively deconvoluted independently), and 533.8 eV (C−OH). The binding energy variations between three deconvolutions are fixed at 2.0 and 3.9 eV. The signals of 531.9 and 533.8 eV obviously increase for PFT5.0 compared with PFT0. However, after THF rinsing, the signals of C−O, CO, and C−OH decrease seriously, which clarified that there are some C−O, CO, and C−OH present in the soluble organic covering. The Ti2p binding energies of all of the samples are located at ca. 458.4 eV (Ti 2p3/2) and 464.4 eV (Ti 2p1/2) without any shift as shown in Figure 7, which are in good agreement with
Figure 5. TG plots of (a) PFT5.0, (b) T-PFT5.0, and (c) PFT0. 8582
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enhanced markedly, which can be obviously observed even after the THF rinsing. Surely, the THF insoluble organics have a significant π-conjugation structure and some chemical bonding to the TiO2, which is in good agreement with the XPS observation. The organics rinsed out from the PF/TiO2 hybrid photocatalysts with THF can be further identified with 1H NMR spectroscopy. Figure 9 illustrates the 1H NMR spectra of PF
Figure 7. Ti2p fine XPS spectra of (a) PFT0, (b) PFT5.0, and (c) TPFT5.0.
pure TiO234,35 and C1s XPS results in Figure 4. This indicates carbonaceous species here should be mainly deposited and bonded on the surface of TiO2, as neither Ti2p binding energy shift34,35 nor C1s binding energy around 282 eV (C−Ti)27,28 was observed in this work. Figure 8 presents the FT-IR spectra of PFT0 and various PF/ TiO2 samples before and after THF rinsing. The IR signals around 3200 cm−1 (broad) and 1640 cm−1 correspond to the stretching and deformation bands of surface-adsorbed water and hydroxyl groups.36,37 The absorption around 534 cm−1 is assigned to Ti−O vibration in TiO2, while the peaks emerge around 1135 cm−1 and 1050 cm−1 might be attributed to the stretching vibration of SO originating from the precursor of Ti(SO4)2.38,39 Several new IR signals appear for PF modified TiO2. The peaks around 2925 cm−1, 2850 cm−1, and 1470 cm−1 are due to the stretching vibration of the methylene group,40 while the peak at 1508 cm−1 is the typical stretching vibration of CC in aromatic ring. The peak with moderate intensity around 1240 cm−1 is due to the vibration of Ti−O−C41,42 and C−O−C43 (aromatic). Additionally, the peaks in 1275−1200 cm−1 represented the vibration of Ar−OH. It seems that the organics here have a π-conjugation structure and chemically bonds to the surface of TiO2 in PF/TiO2 hybrid photocatalysts. Rinsing with THF does not move the FT-IR signals but lowers their intensity, which suggests a partial removal of organics in PF/TiO2 without an obviously chemical change, which illustrates there are still some organics residual on the surface of TiO2 after THF rinsing. It is consistent with C1s fine XPS spectra in which the intensity of C1s for T-PFT5.0 is relatively higher than that of PFT0. With the increase of the PF dosage, the peak intensity of 1470 cm−1, 1508 cm−1, and 1240 cm−1 are
Figure 9. 1H NMR spectra of (a) PF and the organics rinsed out from (b) PFT0.5 and (c) PFT5.0 with THF.
and organics rinsed out from the PF/TiO2. The signals with a chemical shift of 7.2 ppm is from the CDCl3 solvent. The integral area ratio of the peak at 6.8−7.1 ppm (aromatic H) to that around 4.7 ppm (−CH2−)40 is 3:2 for PF and well consistent with the stoichiometric ratio of linear PF. The peaks near 8.0 ppm, 9.8 ppm, 2.2 ppm, and around 3.5−4.0 ppm might be due to the H in the phenolic hydroxyl group, formaldehyde, the methyl at the end point of the linear PF, and the methylene in low aggregation state PF, respectively.44 The organics rinsed out from the PFT0.5 and PFT5.0 with THF show a chemical shift from 4.7 ppm to 5.0 ppm, which corresponds to the chemical change from a methylene group (−CH2−Ar) to a methenyl group (−CH−Ar) resulting from the cross-linking between the linear chains of PF. The integral area ratio for the H atoms from the methylene (and methenyl)
Figure 8. FT-IR spectra of various PF/TiO2 samples: (a) PFT0, (b) PFT0.5, (c) PFT1.5, and (d) PFT5.0, (A) before and (B) after THF rinsing. 8583
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to that from the aromatic is basically consistent with PF. However, chemical shift of the alkyl H changes a lot as shown in Figure 9. PF gives a main peak around 1.6 ppm and a weak signal at 1.2 ppm as well as two peaks around 2.2 and 2.3 ppm for the methyl group. However, PFT 5.0 shows a dominant peak at 2.3 ppm, and PFT0.5 even has an additional peak at 2.8 ppm for the methyl group. Meanwhile, the peak intensity of 1.6 ppm (−CH3) weakens gradually and appears at 1.4 ppm for PFT5.0 and even 1.2 ppm (as well as 1.4 ppm) for PFT0.5. The chemical state of PF seems to have changed more in PFT0.5 than PFT5.0, which may be ascribed to that the catalytic ability of TiO2 toward the condensation and carbonization of PF drops off rapidly with distance. The 1H NMR in Figure 9 indicated that PF undergoes cross-linking to produce the crosslinked PF (cl-PF) as well as claisen rearrangement reaction, which generates a number of alkyl H at the side of the phenyl ring during the hydrothermal treatment. Combined 1H NMR data with the XPS and FT-IR results, a stepwise chemical change of PF thus be proposed. The hydrothermal treatment during the preparation of PF/TiO2 hybrids first induce the further condensation of PF, that is, cross-linking of linear PF by attacking the phenol ring with the methylene groups. Considering the catalytic role of Ti4+, the condensation (cross-linking) process is more seriously in the positions directly connected to the surface of TiO 2 . Subsequently, carbonization (as well as oxidation) of cl-PF occurs at the surface of TiO2, which is also catalyzed by Ti4+ and drops off rapidly with distance to the surface of TiO2. Consequently, a radial distance-dependent organic covering is formed on TiO2, which should be oxidized graphite-like carbon (OGC), graphite-like carbon (GC),18 cl-PF, and PF in sequence from the surface of TiO2 as shown in Scheme 2. THF rinsing can wash off PF and most cl-PF while OGC, GC, and some highly cross-linked cl-PF are insoluble to THF. Visible Light Photocatalytic Performance of PF/TiO2. Figure 10 shows the degradation of MO with various PF/TiO2
and their THF rinsed samples. PFT0.7 displays the highest photocatalytic activity among all the PF/TiO2 with a MO degradation rate of ca. 80%. Considering that the commercial P25 TiO2 and the unmodified TiO2 (PFT0) only give MO degradation rates of 25.5% and 26%, respectively; PF modification undoubtedly gives TiO2 significant visible light photocatalytic reactivity. As the PF and cl-PF cannot absorb visible light, the visible light photocatalytic activity of PF/TiO2 should be ascribed to the carbonized components, OGC and GC. The visible light photoactivity of PF/TiO2 changes much after THF rinsing. All PF/TiO2 samples except PFT0.7 and PFT1.5 exhibit obvious activity enhancement after THF rinsing. T-PFT5.0 gives the highest MO degradation rate of 100%. It suggests that the PF and cl-PF are adverse to the photocatalytic activity of PF/TiO2, and the photocatalytic activity of PF/TiO2 can be thus enhanced by washing off PF and most cl-PF. In order to have a better understanding on the adsorption capacity and the photocatalytic performance of PF/TiO2, the specific surface areas and the corresponding MO adsorption rates of various PF/TiO2 photocatalysts are presented in Table 1. The specific surface area of PF/TiO2 obviously decreases at high PF dosages, which can, however, be recovered with THF rinsing. As shown in Table 1, the specific surface area of TPFTs changes little throughout this work. However, the surface organic materials benefit the adsorption of MO. Although the adsorbed amount of MO increases little from PFT0 to PFT0.7, it is significantly enhanced for PFT1.5, PFT3.5, and PFT5.0. THF rinsing further benefits the adsorption capacity of photocatalyst at low PF dosages (T-PFT0.1−T-PFT0.7) and changes little the adsorption capacity of photocatalyst at high PF dosages content (T-PFT1.5−T-PFT5.0). As illustrated in Scheme 2, organic covering makes up OGC, GC, cl-PF and PF in sequence from the surface of TiO2 to the outer. The OGC is chemically connected to the surface of TiO2 via a Ti−O−C bond. GC is also a well carbonized portion but avoids being oxidized by an OGC block layer. Both OGC and GC can absorb visible light and have contribution to the visible photocatalytic activity of PF/TiO2. In the case of low PF dosages, increasing the amount of PF used in the hydrothermal preparation process induces the formation of more OGC and GC substances on the surface of TiO2 and hence increases the visible light photocatalytic activity before and after THF rinsing. For the PFT0.7, the amount of OGC and GC in the organic covering reaches one preferable value, thus PFT0.7 exhibits the highest visible light photocatalytic activity before THF rinsing. Further increasing the dosage of PF in the hydrothermal preparation process would significantly increase the portions of cl-PF and PF in the organic covering, which significantly enhance the adsorption capacity of PF/TiO2 but are adverse to its photocatalytic activity.
Figure 10. Visible light photocatalytic degradation of MO for 5 h with P25 TiO2, PF/TiO2, and T-PF/TiO2 photocatalysts (λ > 420 nm).
Table 1. Specific Surface Areas and the Corresponding MO Adsorption Rates of Various Photocatalysts PFT0
PFT0.1
SBET (m2/g) adsorption rate photocatalyst
photocatalyst
146.0 11.4% T-PFT0.1
139.5 11.2%, T-PFT0.5
SBET (m2/g) adsorption rate
145.2 15.8%
23.1%
PFT0.5
PFT0.7
PFT1.5
PFT3.5
PFT5.0
14.4%
156.1 13.8%
101.6 24.5% T-PFT1.5
40.9% T-PFT3.5
69.7 55.1% T-PFT5.0
T-PFT0.7 138.5 24.0% 8584
142.0 26.1%
43.9%
146.7 52.0%
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The Journal of Physical Chemistry C Nevertheless, PF and most of cl-PF can be washed off with THF; therefore, the photocatalytic activity of T-PF/TiO2 hybrid photocatalysts follows another order. As shown in Figure 10, the photocatalytic activity of T-PF/TiO2 increases along with the increase of PF dosage. However, some activity decrement can be observed from T-PFT0.5 to T-PFT0.7 and T-PFT1.5. The unexpected activity decrement from T-PFT0.5 to T-PFT1.5 may be due to the amount and chemical state changes of OGC and GC. Several literature works have found that graphite-like substances show more advantageous to photocatalytic activity than its corresponding oxidized form when connected to the TiO2 photocatalyst.45,46 One very possible hypothesis is that the suddenly increased amount of organic materials on the surface of TiO2 is undesired for oxidation as well as carbonization of organic covering by hindering the mass transfer of oxidants such as O2, and thus reduces the amount of OGC a lot. Although it is less effective than GC, OGC is one of the main substances that induce the photocatalytic activity of PF/TiO2. Consequently, T-PFT1.5 exhibits a less photocatalytic activity than that of T-PFT0.5. At higher PF dosages, the further increased amount of organic covering increased the amount of GC on the surface of TiO2, which is formed from carbonization of cl-PF and avoids overoxidization by the shielding of the outer layer. As shown in Figure 3, T-PFT5.0 as well as PFT5.0 shows much stronger absorption in the visible region and therefore owns the highest visible light photocatalytic activity among all T-PF/TiO2 hybrid photocatalysts. However, the highly enhanced adsorption capacity of T-PFT5.0 would also attribute a little to its superior visible light photocatalytic activity.
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CONCLUSIONS
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AUTHOR INFORMATION
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ACKNOWLEDGMENTS
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REFERENCES
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This work was supported by the National Nature Science Foundations of China (21177039) and the Fundamental Research Funds for the Central Universities.
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A graphite-like carbonaceous surface modified TiO2 photocatalyst, PF/TiO2, was prepared with a hydrothermal method by employing water-soluble linear PF as C sources and Ti (SO4)2 as Ti precursor. PF/TiO2 exhibits good activity in the visible light photocatalytic degradation of MO. Particularly, its photocatalytic activity can be further improved by a THF postrinsing operation. PF experiences a cross-linking, carbonization and oxidization course at the surface of TiO2. The visible absorption and photocatalytic activity of PF/TiO2 are due to the OGC and GC substances in the organic covering, which is insoluble in the THF. The visible light insensitive portions in PF/TiO2 are made up of PF and cl-PF, which is adverse to the photocatalytic activity and can be rinsed out with THF. PFT0.7 exhibits the highest visible light activity in the unrinsed PF/TiO2 catalysts, which gives a MO degradation rate of 80% in 5 h. As for the THF rinsed PF/TiO2, T-PFT5.0 that was prepared with the maximum PF dosage, presents the highest photocatalytic activity and gives a MO degradation rate of 100% in 4 h. Our results show that PF modified TiO2 might be a promising visible photocatalyst for environmental applications.
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dx.doi.org/10.1021/jp2120862 | J. Phys. Chem. C 2012, 116, 8579−8586
The Journal of Physical Chemistry C
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