International Journal of PhotoenergyVolume 2012 (2012), Article ID 548647, 13 pagesdoi:10.1155/2012/548647

Research Article

Photocatalytic Activity and Characterization of Carbon-Modified Titania for Visible-Light-Active Photodegradation of Nitrogen Oxides

Chun-Hung Huang,¹ Yu-Ming Lin,² I-Kai Wang,³ and Chun-Mei Lu⁴

¹Product Development Division, Daxin Material Co., No. 15, Keyuan 1st Road, Central Taiwan Science Park, Taichung 40763, Taiwan²ITRI South, Industrial Technology Research Institute, Room 602, Building 3, 31 Gongye 2nd Road, Annan District, Tainan 70955, Taiwan³Department of Chemical Engineering, National Tsing-Hua University, Hsinchu 30043, Taiwan⁴Department of Chemical and Materials Engineering, National Chin-Yi University of Technology, Taichung 41101, Taiwan

Received 15 July 2011; Revised 24 October 2011; Accepted 24 October 2011

Academic Editor: Jinlong Zhang

Copyright © 2012 Chun-Hung Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

A variety of carbon-modified titania powders were prepared by impregnation method using a commercial available titania powder, Hombikat UV100, as matrix material while a range of alcohols from propanol to hexanol were used as precursors of carbon sources. Rising the carbon number of alcoholic precursor molecule, the modified titania showed increasing visible activities of photodegradation. The catalyst modified with cyclohexanol exhibited the best activities of 62%, 62%, 59%, and 54% for the total removal under UV, blue, green, and red light irradiation, respectively. The high activity with long wavelength irradiation suggested a good capability of photocatalysis in full visible light spectrum. Analysis of UV-visible spectrum indicated that carbon modification promoted visible light absorption and red shift in band gap. XPS spectroscopic analysis identified the existence of carbonate species (C=O), which increased with the increasing carbon number of precursor molecule. Photoluminescence spectra demonstrated that the carbonate species suppressed the recombination rate of electron-hole pair. As a result, a mechanism of visible-light-active photocatalyst was proposed according to the formation of carbonate species on carbon-modified TiO₂.

1. Introduction

In the field of environmental protection, photocatalysis is believed to have an extraordinary potential to treatment of contaminating compounds in water purification and air pollutants control, such as degradation of nitrogen oxides (), sulfur oxides (), and volatile organic compounds (VOCs) [1–5]. Among the semiconductors, titania has been regarded as the most promising material for photocatalysis due to its several advantages, including complete mineralization without other derivative chemicals, solar-light-induced operation at room temperature, non-toxicity, and low cost [6, 7]. Generally, titania may exist in three different forms of crystalline phases: rutile, brookite, and anatase phase [8]. Rutile is thermodynamically stable and formed after calcination over 600°C with dense structure, resulting in low photocatalytic efficiency [9, 10]. Brookite and anatase are structures of metastable states, whereas the former is not stable enough to exist alone. Anatase is recognized as the most suitable phase for photocatalysis [5]. However, with the band gap of 3.2 eV, anatase phase TiO₂ can be photo-activated under UV light irradiation (wavelength shorter than 388 nm), which only accounts for 4% of the solar spectrum. To extend the utilization over the main part (45%) of visible region in solar spectrum, several attempts have been made to reduce the band gap.

One strategy is preparing metal-modified titania by doping or loading transitional metals, such as V, Mn, Fe, Cr, Cu, Ni, W, and others [11–15]. The noble metals were reported to produce the highest Schottky barrier and to facilitate electron capture [16]. Therefore, Ag, Ru, and Pt [17–21] were employed to modify TiO₂ for enhancing UV photocatalytic efficiency and/or visible light activities.

Nonmetal doping is the other approach to achieve the goal of TiO₂ modification. Sato [22] discovered the red-shift phenomena of N-promoted TiO₂. Afterward Asahi et al. [23] reported that a small amount of substitutional N atoms into the TiO₂ structure by the sputtering method will induce the Ti³⁺ sites on the TiO₂ surface and significantly narrow the band gap. Nakamura et al. [24] found that TiO₂ treated with hydrogen plasma can generate oxygen vacancies and create subband gap to promote visible light response, despite the expensive appliance. Organic dyes as the light sensitization agents were also considered to be added on the TiO₂ surface to improve the light efficiency [25, 26]. However, the consumption of dye molecules during the reaction process will limit the application. Recently, carbon was widely used for visible modification of TiO₂ [27–31]. It was reported that carbon modification is more efficient, even higher than nitrogen modification [28]. The carbon doped TiO₂ could be prepared by simple method such as mechanochemical operation by grinding TiO₂ with ethanol in air [32]. On a comprehensive survey, carbon is one of the practicable materials, for the consideration of economic and practical mass production. Therefore, in this study, we tried to use carbon precursors to modify TiO₂ powder by impregnation method, enabling the significant visible photoactivity. Moreover, a possible photocatalytic mechanism was proposed to speculate the role of carbonaceous species functioning with TiO₂, based on the evidences observed from various characterization techniques.

2. Experimental

2.1. Preparation of Carbon-Modified TiO₂ Photocatalysts

All chemicals in this study are reagent-grade without further purification. A commercially available TiO₂ powder (Hombikat UV100, Sachtleben Chemie, 100 anatase), symbolized as UV100, was used as the matrix material for modification. Seven kinds of alcohols were adopted as the carbon sources: 2-propanol, n-propanol, 2-butanol, i-butanol, 4-methyl-2-pentanol, n-hexanol, and cyclohexanol, symbolized as 2-C3, C3, 2-C4, iC4, 2-C6, C6, and cC6, respectively, according to the carbon number of molecule and the isomer structure. Impregnation method was used to prepare carbon-modified TiO₂. The synthesis procedures are depicted in Figure 1. Firstly, 5 g UV100 powder was added into an alcoholic solution containing 30 mL alcohol and 70 mL D.I. water. After ultrasonic bath for several minutes, the slurry was then magnetically stirred for 30 min with sufficient mixing. Subsequently, the modified sample was evaporated in an oven at 110°C for 4 h and then milled to powder, followed by calcination at a heating rate of 1°C /min up to 200°C for 5 h in a furnace. Finally, seven kinds of carbon-modified TiO₂ were obtained and symbolized as 2-C3/UV100, C3/UV100, 2-C4/UV100, iC4/UV100, 2-C6/UV100, C6/UV100, and cC6/UV100, ascribed to their alcoholic precursors.

Figure 1: Schematic procedure for synthesis of carbon-modified TiO₂ powder by impregnation method.

2.2. Experimental Apparatus and Procedure

Photocatalytic degradation of nitrogen oxides (De-) was carried out at room temperature by the setup consisting of a continuous flow reactor (a round-shaped Pyrex glass vessel) connected to the gas suppliers and the analytic system, as shown in Figure 2. To simulate atmospheric environment and reach the optimal value, the relative humidity was maintained at 50% by adjusting a humidified air stream with a dry air stream [4]. Then, a nitric oxide (NO) gas stream, provided by a cylinder containing 100 ppmv NO (San Fu Chemical Co., N₂ balance), was added to the air stream to reach a desired NO concentration of 1 ppmv and a total volumetric flow rate of 1 L/min as the feedstock. The photocatalyst with amount of 0.5 g was dispersed into an appropriate amount of D.I. water contained in a 50 mL beaker, followed by well stirring. Then the mixed slurry was uniformly spread on a disk-like glass plate (diameter 12 cm). After drying at 110°C for 30 min, dehydrated powder attached to the plate was obtained. Subsequently, the glass plate coated with the photocatalyst was set in the center of reactor for use. A UV light lamp over the reactor delivered the ultraviolet illumination in the wavelength range of 330–420 nm. For visible light reaction, three light-emitting diodes (LEDs) were employed, including blue LEDs (BLED, 430–530 nm), green LEDs (GLED, 470–570 nm), and red LEDs (RLED, 590–680 nm). The related photon energy distribution profiles are shown in the inset of Figure 2. The intensity of the light source was controlled at 1 mW/cm² and was measured by a spectrophotometer (Ocean Optics, USB2000).

Figure 2: Schematic diagram of De- system. The inset shows photon energy distribution of UV lamp and LEDs.

In a typical experiment, the feedstock was introduced to the reactor with sample plate for several minutes in the dark. Light was turned on as the system reached the adsorption equilibrium (i.e., NO concentration recovering to 1 ppmv). Meanwhile, the effluent concentrations of NO and nitrogen dioxide (NO₂) from reactor were measured by an on-line chemiluminescent analyzer (Eco Physics, CLD 700AL). In addition, blank test was performed under the same condition except photocatalyst existence. The result certainly guaranteed all the materials of the system inactive on photocatalytic De-.

NO is oxidized in a serial steps (NONO₂NO₃ ⁻) over the photocatalyst under illumination [4]. The final product, NO₃ ⁻ ion, was adsorbed on the photocatalyst surface and could be easily washed out by water for photocatalyst in reuse [17]. In data analysis, the total concentration ([]), NO conversion (NOC), NO₂ selectivity (NO₂S), and total removal (R) were defined as follows:

The photocatalyst performance could be evaluated by the total removal, or the NO conversion conjunctive with the NO₂ selectivity.

2.3. Characterization and Analysis

The particle sizes and the morphology of photocatalysts were examined by field-emission scanning electron microscopy (FE-SEM, Hitachi S4800-1) and transmission electron microscopy (TEM, Philips Tecnai 20). Each powder sample was dispersed in anhydrous alcohol and then transferred to a copper grid (with carbon support film, 300-mesh). After drying at 50°C in an oven, the pretreatment was completed for use.

The specific surface areas (SSAs) were measured by nitrogen adsorption at 77 K using Micrometrics equipment (Model ASAP 2000). Samples were outgassed to a vacuum lower than 10⁻⁴ Torr at 200°C for 2 h to remove adsorbed impurities prior to each measurement. Specific surface areas were evaluated by Barrett-Emmet-Teller (BET) method [33].

The crystal structures were analyzed by an X-ray powder diffractometry (XRD, Rigaku RU-H3R) in the reflection mode with Cu Kα radiation. The angular domain was 20–80°  . The surface structures were identified by a Micro-Raman spectrometer (Renishaw 1000B), conducted with a low-power green laser (100 mW) at 532 nm. Weak laser of 1% to 10% power was used to examine the surface structure of the samples.

The UV-visible absorption spectra of all samples were performed by a powder UV-visible spectrophotometer (UV-VIS, Shimadzu UV-2450) equipped with an integrating sphere accessory for diffuse reflectance spectra over a range of 200–800 nm by using BaSO₄ as the reference. The obtained reflectance data () was converted to the absorbance value, , based on the Kubelka-Munk theory, as follows:

The surface composition and the chemical states of catalysts were investigated by an X-ray photoelectron spectroscopy (XPS, Physical Electronics ESCA PHI 1600) with Mg radiation. The charging effects were corrected by adjusting the C 1s peak to a position of 284.5 eV.

The decomposition behavior of products was determined by thermogravimetric analysis (TGA) using a thermogravimeter mass spectrometer (Seiko instruments, TG/DT 6200) at a heating rate of 10°C/min between 200 to 800°C in flowing nitrogen. Samples (about 3 mg) were set and heated to 200°C for 30 min prior to each experiment.

The photoluminescence (PL) emission spectra were measured by using a Xe lamp with wavelength of 325 nm as an excitation light source. The reflectance spectra of the samples are over a range of 200–800 nm.

3. Results and Discussion

3.1. Photocatalytic Ativity of Carbon-Modified TiO₂

The NO conversion (NOC), NO₂ selectivity (NO₂S), and total removal (R) on unmodified UV100 and carbon-modified UV100 under various light irradiations are illustrated in Figure 3. In a serial oxidation steps (NONO₂NO₃ ⁻), NOC, NO₂S, and R represent the initial oxidation of NO converted, the undesired byproduct ratio, and the total oxidation ability, respectively. Accordingly, a well-active photocatalyst should basically provide a high NOC as well as a low NO₂S, resulting in a high R. With the same NO conversion, the one with lower NO₂ selectivity catalyzed the oxidation of NO₂ to NO₃ ⁻ more completely.

Figure 3: De- activities including NO conversion (A), NO₂ selectivity (B), and total removal (C) on UV100 (a), 2-C3/UV100 (b), C3/UV100 (c), 2-C4/UV100 (d), iC4/UV100 (e), 2-C6/UV100 (f), C6/UV100 (g), and cC6/UV100 (h).

In UV light reaction, unmodified UV100 exhibited a high NOC attributed to its pure anatase phase structure and high specific surface area [34]. However, the high NO₂S revealed the insufficiency in NO₂ suppressive ability and led to the reduction of R . Compared with UV100, carbon-modified UV100 showed a similar or slight-lower NOC. It was referred that carbonaceous residues on TiO₂ surface partly obstructed UV absorption, as evidenced in UV-VIS analysis (discussed later). Even so, carbon species probably functioned with TiO₂ and substantially lowered the NO₂S. As a result, all carbon-modified TiO₂ displayed higher R than UV100.

In visible light reaction, apparently UV100 provided poor activities, especially under GLED and RLED irradiation with the R of 3% and 1%, respectively. In contrast, carbon-modified UV100 showed progressive activity under whichever visible light irradiation. It suggested that the visible light response may be offered by the conjugation of carbonaceous residues and TiO₂. Additionally, like the performance in UV reaction, NO₂S was also successfully suppressed on carbon-modified UV100 under visible light irradiation. Furthermore, the visible light activity increased with the increase of carbon number in alcoholic precursors. Visible-photocatalytic activities were in the order of UV100 < 2-C3/UV100 < C3/UV100 < 2-C4/UV100 < iC4/UV100 < 2-C6/UV100 < C6/UV100 < cC6/UV100. Among the products, cC6/UV100, prepared by modification of the highest carbon number in alcoholic precursor, exhibited the highest visible activity of 62%, 59%, and 54% for BLED, GLED, and RLED reaction, respectively. Compared with its UV activity , cC6/UV100 was stable enough to extend its photoactivity to visible region, even under red light irradiation. As can be seen in Tables 1–3, specific surface area, carbon amount, and C–C and C=O ratio obviously increase with increasing carbon number of alcoholic precursor. In addition, higher visible light absorbance and lower band gap were observed as the carbon number of alcoholic precursor increased (Figure 7). All these positive factors may contribute to the highest activity of cC6/UV100 catalyst.

Table 1: Characterization of the band gap, specific surface area, and weight loss in TGA.

Sample
Band gapb (eV)
Specific surface area  (m²/g)
Weight lossa (wt., between 200 and 800°C)

Total
200–380°C
380–600°C
600–800°C

UV100
3.26
249
4.23
2.33
1.01
0.90

2-C3
3.24
252
3.53
1.71
0.95
0.87

C3
3.24
272
4.84
2.10
2.18
0.57

2-C4
3.24
279
4.88
2.13
2.18
0.58

iC4
3.23
298
5.73
2.86
2.01
0.89

2-C6
3.18
290
5.71
2.44
2.69
0.58

C6
3.07
294
7.73
3.70
3.18
0.86

cC6
3.04
297
6.82
2.91
3.27
0.64

aAll samples were pretreated at 200°C before test.

bDetermined by the intercept in Tauc Plot.

Table 2: The elemental surface composition and atomic ratio of O/Ti resulted from XPS analysis.

Sample
Ti (%)
O (%)
N (%)
C (%)
O/Ti

UV100
15.7
58.9
1.4
24.0
3.8

2-C3
20.6
52.7
—
26.7
2.6

C3
18.8
49.9
—
31.3
2.7

2-C4
16.7
49.3
—
34.0
3.0

iC4
11.4
49.9
—
38.7
4.3

2-C6
16.0
52.6
—
31.4
3.3

C6
12.1
45.2
—
42.7
3.7

cC6
14.0
42.3
—
43.7
3.0

Table 3: Deconvolution results of C 1s and O 1s spectra from XPS analysis.

Sample
Deconvolution of C 1s spectraa
Deconvolution of O 1s spectrab

C–C ratio (%)
C=O ratio (%)
Ti–O ratio (%)
O–H ratio (%)
C=O ratio (%)

UV100
22.6
1.4
47.2
10.7
1.0

2-C3
19.9
6.8
44.9
2.9
4.9

C3
22.8
8.5
42.5
2.7
4.7

2-C4
25.4
8.6
40.1
2.7
6.5

iC4
26.1
12.6
38.3
3.2
8.3

2-C6
18.8
12.6
39.3
3.7
9.6

C6
29.2
13.4
33.4
2.6
9.1

cC6
31.4
12.3
28.1
3.1
11.1

a Peaks fitted at 284.5–284.7 eV and 287.2–287.6 eV for C–C and C=O bonding.

bPeaks fitted at 529.3–529.9 eV, 530.9–531.5 eV, and 531.9–532.8 eV for Ti–O, O–H, and Ti=O bonding, respectively.

In summary, via impregnation with alcoholic precursors followed by low temperature calcination at 200°C, carbonaceous residues acted as sensitizing agent with TiO₂ to lower NO₂ production efficiently, facilitated more complete reaction, and improved the visible photoactivity. Visible activities increased with the rise of carbon number in precursor molecule. And with the same carbon number (e.g., ), photocatalytic levels were in the order of 2-Cn < Cn < cCn, depending on the isomer structure. Therefore, the carbonaceous species resulted from different molecular structure of alcoholic precursors may play an important role on visible light activity.

3.2. Characterization of Carbon-Modified TiO₂

3.2.1. Morphology

Figure 4 shows the FE-SEM and TEM images of various photocatalysts. FE-SEM photographs indicated that the primary particle size was unchanged (7–14 nm) after carbon-modification, while the secondary particle was more agglomerate. From TEM photographs, it could be found that amorphous carbonaceous species exist on the interfaces or surface of TiO₂. It may be the reason for high agglomeration of carbon-modified TiO₂ catalysts.

Figure 4: FE-SEM images (A) of UV100 (a), C3/UV100 (b), C6/UV100 (c), and cC6/UV100 (d) and TEM image of cC6/UV100 (B).

3.2.2. Specific Surface Area

The specific surface areas of all samples were in range of 249–298 m²/g (Column 3 in Table 1). All the modified samples showed larger specific surface areas than UV100 (249 m²/g). The higher the carbon number in precursor molecule was, the higher the specific surface area of the catalyst was obtained. Zhang et al. [35] investigated carbon-modified TiO₂ by using glucose as precursor and mentioned that carbon residue did not make TiO₂ growth. Same thing was observed in this study. Therefore, the carbonaceous species, existing on the TiO₂ surface, may create more specific surface area and favor the photocatalytic efficiency.

3.2.3. Crystal Structure

The XRD patterns are shown in Figure 5. All the crystal structures of samples were almost anatase, attributed to several main peaks ( with [101], 37.8 with [004], 48.1 with [200], 54.0 with [105], and 62.7 with [204]). There was no significant difference between samples, which meant that carbon modification did not obviously change the crystal phase through low-temperature calcination. According to Scherrer’s equation, the crystal size is larger with sharper main peak. All the crystal sizes of samples were in range of 9–12 nm, consistent with those observed in TEM graphs.

Figure 5: XRD patterns of the photocatalysts.

Raman spectrum is surface-sensitive for surface characterization. The Raman spectra of all samples are shown in Figure 6. Firstly, a weak laser power (1% of 100 mW) was introduced as the source to detect Raman signals. As shown in Figure 6(a), all samples showed anatase peaks mainly. However, a shoulder peak at 480 cm⁻¹ grew up with the increasing carbon number. It was attributed to brookite, regarded as a visible-light-responsive phase [36]. When the laser power was enhanced to 10% of 100 mW, the brookite peak disappeared (Figure 6(b)) due to its characteristics of unstable phase. This phenomenon confirmed that the unstable phase was brookite. Moreover, 10% of 100 mW power was employed to treat the same sample for several times. The main crystal phase would transform from anatase to rutile (Figure 6(c)) over repeated treatment because the strong incident power might cause transformation to the ultimately stable phase, that is, rutile. Basically, UV100 was almost of anatase phase. After carbon impregnation, small portion of brookite phase was observed on UV100 surface. And visible light activities might be enhanced with the brookite phase.

(a)

(b)

(c)

Figure 6: Raman spectra with 1% (a) and 10% (b) power for all samples; 10% power for iC4/UV100 over 6 times of test (c).

(a)

(b)

Figure 7: UV-visible absorption spectra (a) and the plot of transformed Kubelka-Munk function versus the energy band gap (b).

3.2.4. UV-Visible Absorption

The apparent colors are whitish for UV100, pale-yellowish for 2-C3/UV100, C3/UV100 and 2-C4/UV100, yellowish for iC4/UV100 and 2-C6/UV100, and brownish for C6/UV100 and cC6/UV100. The colors shift from white to brown indicated the extent of visible light absorption influenced by carbon modification, depending on the carbon number of precursor molecule. The UV-visible absorbance spectra for various TiO₂ samples can be seen in Figure 7(a). Obviously, UV100 displayed strong absorbance in UV region, corresponding to a sharp absorption edge near 380 nm. The band gap of UV100 was well known as 3.2 eV based on the pure anatase phase structure. In the presence of carbon residue, the visible absorbance increased with the increasing carbon number of precursor molecule for carbon-modified TiO₂. Despite a little expense of UV absorbance, the modified samples still provided sufficient UV activities in De- reaction. In addition, the optical absorption edges of carbon-modified TiO₂ showed obvious red shifts and long absorption tailing over 700 nm in visible region, especially in samples of C6/UV100 and cC6/UV100. This phenomenon implied certain change in band gap structure occurred due to the existence of carbon residue.

For semiconductor material, the direct band gap can be derived by establishing Tauc Plot of transformed Kubelka-Munk function versus the absorbed light energy [37]. As can be seen in Figure 7(b), the so-called Tauc optical band gap is obtained at the intercept between the extension line of slop and the base line. For example, the band gap of UV100 is calculated as 3.26 eV from the Tauc Plot, corresponding to the wavelength of 380 nm. The direct band gaps were thus calculated and listed in Table 1 (Column 2). The band gap reduced with increasing carbon number of precursor molecule, which evidenced that the alcoholic modification certainly made a red shift and thus the absorption of visible light. The red shift essentially provided the activity of catalyst under the visible light illumination.

For carbon-modified TiO₂, there exist two types of structure, that is, oxygen substitution to form Ti-C bonding and carbon residue deposition in the form of carbonate species (C=O). In former case, a subband gap may be created to show both main peak and shoulder peak in UV-Vis spectrum [27, 38]. In the latter case, the carbon residues are usually deposited on the interfaces or surface of TiO₂, making a red shift in UV-VIS spectrum [39]. In this study, obvious red shift was found for carbon-modified TiO₂ samples. Therefore, the formation of carbonate species was expected.

3.2.5. XPS Analysis

Analysis of Chemical ElementsXPS analysis revealed the elemental surface composition of the samples, including Ti, O, N, and C, as summarized in Table 2. According to the synthesis procedure, element N was not detectable in the carbon-modified TiO₂ sample as expected, while trace N was detected in UV100 catalyst probably introduced with stray ions in the commercial manufacturing process. The composition of elements Ti and O and its atomic ratio of O/Ti are demonstrated in Table 2. The atomic ratios of O/Ti were in range of 2.6–4.3, greater than theoretical value of 2, probably due to the surface phydrophilicity. Carbon-modified TiO₂ showed lower O/Ti ratio than that of UV100 (3.8), indicating the decrease of hydrophilicity caused by the existence of carbon residue. The carbon content showed relatively high values in range of 24.0–. The carbon might come from two sources: adventitious element carbon from the impurity of equipment chamber [36], and carbon residues from the impregnation of alcoholic precursor. The carbon content detected in UV100 could be regarded as the baseline amount of adventitious element carbon. The content higher than that baseline could be determined as the amount of carbon residues from impregnated alcoholic precursor. Obviously, the higher the carbon number of precursor molecule was, the larger amount of carbon residue was obtained.

Analysis of Chemical StatesTi 2p, C 1s, and O 1s spectra were conducted for analysis of chemical states. No difference was found in Ti 2p spectrum (not shown here) for various samples, indicating no change of surface Ti bonding occurred because of the impregnation of carbon precursor. For C 1s spectrum (Figure 8(a)), two peaks at 284.5 eV and 287-288 eV were observed for all the carbon-modified samples, except for the original UV100 sample. The peak at 284.5 eV denoted C–C bonding caused by adventitious element carbon. The peak at 287-288 eV denoted C=O bonding represented by carbonate species. All the carbon-modified samples showed higher intensity of C=O peak than that of UV100, indicating the effect of carbon residues. Also, the intensity increased with increasing carbon number of precursor molecule. For O 1s spectrum (Figure 8(b)), three kinds of peaks, that is, Ti–O (529-530 eV), O–H (531 eV), and C=O (532 eV), could be observed. Ti–O bonding is the main peak of TiO₂ and can be seen in all the samples. O–H bonding might be introduced by the hydrophilicity of water vapor to form the surface-bound hydroxyl group [1]. The third one was C=O bonding which represented the occurrence of carbonate species due to the carbon modification. Therefore, the intensity of C=O bonding increased with increasing carbon number of precursor molecule, which was consistent with the results from C 1s spectrum.To further investigate the contribution of individual bonding, the ratio of two peaks in C 1s and three peaks in O 1s spectrum would be calculated separately. Deconvolution method was thus conducted to calculate the ratio of each bonding. Taking examples of UV100 and cC6/UV100, the peaks of C=O and C–C bonding in C 1s spectrum could be separated and the ratio could be calculated by the integration area, as shown in Figure 9(a). The same procedure could be conducted for O 1s spectrum (Figure 9(b)). Using deconvolution method, the ratio of each bonding in all samples could be calculated and listed in Table 3. For C 1s analysis, adventitious element carbon (represented by C–C bonding) possessed the ratio of 18.8– and no obvious trend among the samples was observed due to the characteristics of adventive impurity itself. In contrast, carbonate species (represented by C=O bonding) showed obvious trend with the carbon number of precursor molecule. In UV100 sample, only trace carbonate species were observed. In carbon-modified samples, the amount of carbonate species clearly increased with increasing carbon number of precursor molecule.For O 1s analysis, Ti–O bonding possessed major portion as expected in TiO₂ matrix. The ratio of O–H bonding showed great difference between UV100 and carbon-modified TiO₂ samples. High ratio of O–H bonding exhibited high hydrophilicity of UV100. However, low ratio of O–H bonding (2.6–) implied low hydrophilicity of carbon-modified TiO₂, as described in literature [40, 41]. Carbonate species (represented by C=O bonding) showed obvious trend with the carbon number of precursor molecule, consistent with the result from C 1s analysis. In UV100 sample, only trace carbonate species were observed. In carbon-modified samples, the amount of carbonate species clearly increased with increasing carbon number of precursor molecule. In summary, the trend of carbonate species is in good correlation with the visible light absorption and photoactivity of nitrogen oxides degradation under visible light illumination. Therefore, it can be concluded that the carbonate species may make major contributions for visible-light-active photocatalysis.

(a)

(b)

Figure 8: C 1s (a) and O 1s (b) XPS spectra for all samples.

(a)

(b)

Figure 9: Deconvolution of C 1s (a) and O 1s (b) XPS spectra for UV100 and cC6/UV100.

3.2.6. Thermogravimetric Analysis (TGA)

Figure 10(a) shows TGA spectra of samples over the temperature range of 200–800°C under nitrogen atmosphere. All the samples were preheated at 200°C to remove the physical adsorbed water [42, 43]. Figure 10(b) shows DTG spectra of samples, resulted from first derivation of TGA curves, to indicate the rate of weight loss. Three weight loss zones could be categorized as follows: Zone I: 200–380°C, weight loss from chemical adsorbed water and some low-temperature volatile carbon residues; Zone II: 380–600°C, weight loss from the high-temperature pyrolytic carbon species, that is, carbonate species mainly; and Zone III: 600–800°C, weight loss probably accompanied with collapse of interagglomerate and escape of trace moiety during the phase transformation from anatase to rutile [43–45]. The weight loss data was summarized in Table 1 (Column 4–7). The total weight loss of each sample was in range of 3.53–7.73%. The distributions in Zone I and Zone III could be found in each sample, ascribed to the common properties of all the samples. However, the weight loss in Zone II showed different trend and obviously increased with increasing carbon number of precursor molecule. As a result, large amount of carbon residues were existed on TiO₂ sample modified by precursor with large carbon number. And carbonate species should be the main composition according to the XPS analysis as discussed earlier.

(a)

(b)

Figure 10: TGA (a) and DTG (b) curves for all samples obtained in flowing N₂.

3.3. Role of Carbonate Species in Mechanism of Photocatalytic Reaction

3.3.1. Visible Light Absorption of Carbonate Species

In this paper, we already proved that carbonate species could be formed onto interfaces or surface of TiO₂ by impregnating alcoholic precursor and calcining at 200°C. The prepared carbon-modified TiO₂ illustrated obvious visible light absorption. To verify the absorption property mainly originated from the carbonate species, we prepared the sample with carbon residue on fumed silica by using cyclohexanol as precursor. Two calcination temperatures (110°C and 200°C) were employed. The sample calcined at 110°C was denoted as cC6/SiO₂_110°C and the other at 200°C as cC6/SiO₂_200°C. The UV-Vis spectra of carbon deposited SiO₂ are shown in Figure 11. SiO₂ sample exhibited no absorbance of light. The light absorbance of carbon deposited SiO₂ increased with rising calcination temperature, where absorbance was higher at 200°C than that at 110°C. The enhancement of absorbance may be attributed to the existence of conjugate structure in the carbon species [30, 34, 46]. Long alkyl chain or cyclic configuration of alcoholic precursor might favor the formation of conjugate structure in carbonate species. Therefore, the precursor with high carbon number of molecule could make better visible light absorption and then the photocatalytic activity.

Figure 11: UV-visible absorption spectra for SiO₂, cC6/SiO₂_110°C, and cC6/SiO₂_200°C.

3.3.2. Photoluminescence(PL) Emission Spectra

Photoluminescence emission occurs when a short wavelength (e.g., 32 nm) photoenergy is absorbed to excite an electron from valence band and then a longer wavelength luminescence is emitted via recombination of electron-hole pair. The higher the intensity of luminescence is, the faster is the recombination of electron and hole. Therefore, high intensity of luminescence may imply low photocatalytic activity [47]. As shown in Figure 12, UV100 had the highest intensity of luminescence which meant the lowest photocatalytic activity as discussed in previous paragraph. The intensity of modified sample decreased with increasing carbon number of precursor molecule. The sample modified with cyclohexanol, cC6/UV100, showed the much lower intensity of luminescence and led to higher photocatalytic activity, especially in visible region (Figure 3). This suppression of luminescence might be due to the existence of carbonate species, which acted as the sensitizing agent of TiO₂ and prohibitor of electron-hole recombination.

Figure 12: PL spectra for UV100, C3/UV100, iC4/UV100, 2-C6/UV100, C6/UV100, and cC6/UV100. Excitation light source: Xe lamp with wavelength of 325 nm.

3.3.3. Photocatalytic Mechanism on Carbon-Modified TiO₂

In summary, a mechanism of photocatalytic degradation of nitrogen oxides over carbon-modified TiO₂ might be proposed in Figure 13. According to the mechanism, the carbonate species in carbon residue on TiO₂ surface were regarded as the sensitizer to absorb visible light and induce the electron-hole pair of TiO₂ so that hydroxyl radicals () or superoxide ions ( ⁻), which may transform to with in humid environment, could be generated as active species for oxidation reaction [29]. The existence of carbonate species could also suppressed the recombination rate of electron-hole pair to enhance the photocatalytic activity. In addition, the carbonate species acted as promoter for deep oxidation in the degradation of nitrogen oxides and lower the selectivity of NO₂. Therefore, highly active visible light photocatalyst could be achieved by carbonmodified TiO₂.

Figure 13: Schematic illustration of photocatalytic mechanism on carbon-modified TiO₂.

4. Conclusions

An active visible light photocatalyst was prepared by impregnation method using a range of alcohols, that is, from propanol to cyclohexanol, as precursor to modify TiO₂ powder. The prepared carbon-modified TiO₂ showed better photoactivity of nitrogen oxides degradation than that of unmodified TiO₂. The activity increased with increasing carbon number of alcoholic precursor. The form of carbonate species on carbon-modified TiO₂ was identified by XPS and TGA analysis. A photocatalytic mechanism was thus proposed for photodegradation of nitrogen oxides. According to the mechanism, the carbonate species in carbon residue onto TiO₂ interfaces or surface acted as sensitizer for visible light response, prohibitor for electron-hole recombination, and promoter for oxidation of nitrogen oxides. High carbon number of alcoholic precursor would make high amount of carbonate species so that better performance of photocatalysis could be achieved.

Acknowledgment

The authors gratefully express thanks for the assistance of Micro-Raman spectrometer provided by Professor Chia-Liang Cheng (Department of Physics, National Dong Hwa University).

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