6546
J. Phys. Chem. C 2008, 112, 6546-6550
Highly Active TiO2-xNx Visible Photocatalyst Prepared by N-Doping in Et3N/EtOH Fluid under Supercritical Conditions Yuning Huo, Zhengfeng Bian, Xinyu Zhang, Yi Jin, Jian Zhu, and Hexing Li* Department of Chemistry, Shanghai Normal UniVersity, Shanghai 200234, China ReceiVed: December 20, 2007; In Final Form: February 4, 2008
TiO2-xNx nanocrystallines were prepared by treating the TiO2 precursor in triethylamine/ethanol (Et3N/EtOH) fluid under supercritical conditions. During photodegradation of p-chlorophenol under irradiation with visible lights, this catalyst exhibits much higher activity than the N-doped TiO2 obtained via direct calcination. The promoting effect of the supercritical treatment could be attributed to the high surface area, the highly crystallized anatase, and the strong interaction of N-dopants with TiO2, leading to the enhanced light harvesting and reactant adsorption as well as quantum efficiency of photocatalysis via inhibiting the recombination between photoinduced electrons and holes. Supercritical treatment using Et3N/EtOH fluid results in higher N-content in the TiO2-xNx than that using NH3/EtOH fluid. Even with the same N-content, the TiO2-xNx obtained in Et3N/EtOH is still much more active than that obtained in NH3/EtOH fluid, possibly owing to the enhanced surface hydrophobicity which could promote the adsorption of reactant molecules for degradation.
1. Introduction Photocatalysis has caused much attention owing to its potential applications in environmental cleaning and H-energy production. The TiO2 photocatalyst is most frequently employed owing to its cheapness, nontoxicity, and structural stability.1 To date, a great number of attempts have been made to promote the practical applications of photocatalysis by extending the spectral response from the UV area to the visible region and enhancing quantum efficiency.2-6 One of the promising routes is to dope TiO2 with nitrogen.7 Up to now, nearly all the N-doped TiO2 samples (TiO2-xNx) are prepared by treating the TiO2 precursors under N2 and/or NH3 atmosphere at high temperature.8-14 Besides the energy waste, the treatment under high temperature usually induces particle agglomeration and collapse of pore structure, leading to surface area decrease. Preparation of TiO2 photocatalysts under supercritical conditions has the advantage of obtaining high surface area and crystallization degree of anatase.15-17 On the basis of this method, a TiO2-xNx has been synthesized by treating the TiO2 precursor in NH3/ethanol fluid, which shows high activity under UV light irradiation (characteristic wavelength ) 365 nm).18 However, we found that only very low content of N-dopants could be incorporated into the TiO2 network, which limited the promoting effect from N-doping. Herein, we report a new approach to prepare TiO2-xNx visible photocatalysts with high N-content and enhanced activity by using triethylamine/ethanol (Et3N/EtOH) fluid during supercritical treatment. 2. Experimental Section 2.1. Catalyst Preparation. A solution containing 2.5 mL of 1.0 M HNO3 and 10 mL of EtOH was added dropwise into another solution containing 10 mL of Ti(OC4H9)4 and 40 mL of EtOH at 298 K under vigorous stirring and kept stirring until the formation of TiO2 gel. After being aged for 48 h at 313 K, * Corresponding author. E-mail:
[email protected], tel: (86-21) 64322272, fax: (86-21) 64322272.
the TiO2 xerogel was transferred into a 500 mL autoclave containing 250 mL of EtOH and a certain amount of triethylamine (Et3N), followed by treating under supercritical conditions (553 K and 15 MPa) for 2 h. Then, the vapor inside was released slowly and the system was allowed to cool to room temperature in the N2 flow. The as-received solid was then calcined for 8 h to remove the residual organic species. Preliminary experiments confirmed that the optimum calcination temperature was 623 K. The as-prepared samples were denoted as TiO2-xNx(SC)-m, where SC refers to supercritical conditions and m refers to percent N/Ti molar ratio which could be adjusted by changing the Et3N concentration in EtOH solution. Similar sample was also synthesized by using NH3/EtOH instead of Et3N/EtOH as a supercritical fluid. For comparison, the N-doped TiO2 was also prepared by stirring the TiO2 precursor for 6 h in 250 mL of NH3/EtOH solution, followed by calcination at 623 K for 8 h. The sample was denoted as TiO2-xNx(DC)-m, where DC represents direct calcination. 2.2. Catalyst Characterization. The structure and phase transformation were examined by X-ray diffraction (XRD, Rigacu Dmax-3C, Cu KR radiation), Fourier transform infrared spectra (FTIR, Nexus 470), and Raman spectra (Super LabRam). Differential scanning analysis (DSC, Perkin-Elmer DTA 7) was employed to evaluate thermal stability. The light response was determined by UV-visible diffuse reflectance spectra (UVDRS, MC-2530) and photoluminescence spectra (PLS, Varian Cary-Eclipse 500). N2 adsorption-desorption isotherms were measured at 77 K on a NOVA 4000, from which the surface area (SBET) and the pore volume (VP) were calculated. Surface morphologies were observed through a transmission electronic micrograph (TEM, JEM-2010). The surface electronic states were investigated by X-ray photoelectron spectroscopy (XPS, Perkin-Elmer PHI 5000C). All the binding energies were calibrated by using the contaminant carbon (C1S ) 284.6 eV) as a reference. The molar ratio of N/Ti in the TiO2-xNx was determined by using 0.477 and 2.001 as the PHI sensitivity factors corresponding to the principal XPS peaks in N1S and Ti2P3/2 levels, respectively.19
10.1021/jp711966c CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008
TiO2-xNx Visible Photocatalyst
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6547 SCHEME 1: Illustration of the Formation of the N-Ti-O Bonding and the Electron Transfer in TiO2-xNx(SC)
TABLE 1: Structural Parameters and Activities of the Undoped and the N-Doped TiO2a photocatalyst Figure 1. FTIR spectra of the TiO2-xNx(SC)-3.4 calcined at 623 K.
P 25 TiO2(DC) TiO2(SC) TiO2-xNx(DC)-1.3 TiO2-xNx(SC)-1.2 TiO2-xNx(SC)-1.7 TiO2-xNx(SC)-2.4 TiO2-xNx(SC)-3.4 TiO2-xNx(SC)(NH3)-1.7
Vp degradation supercritical SBET fluid (m2/g) (cm3/g) yield (%) EtOH Et3N/EtOH Et3N/EtOH Et3N/EtOH Et3N/EtOH NH3/EtOH
45 23 53 29 72 81 85 102 90
0.25 0.03 0.18 0.06 0.20 0.26 0.26 0.30 0.60
10 8 12 22 48 58 65 76 41
a Reaction conditions: 0.050 g of catalyst, 50 mL of 0.010 g/L p-chlorophenol aqueous solution, 500 W xenon lamp with wavelength >420 nm located at 18 cm above the reaction solution, reaction temperature ) 298 K, stirring rate >800 rpm, reaction period ) 4 h.
Figure 2. XPS spectra of the TiO2-xNx(SC)-3.4 calcined at 623 K.
2.3. Activity Test. The photocatalyzed reaction was carried out at 298 K in a self-designed 100 mL glassy reactor containing 0.050 g catalyst and 50 mL of 0.010 g/L p-chlorophenol aqueous solution. The reaction system was stirred vigorously (>800 rpm) to eliminate the diffusion effect. After reaching adsorption equilibrium, the solution was irradiated with a 500 W xenon lamp (CHF-XM500, light intensity ) 600 mW/cm2). All the UV lights with the wavelength lower than 420 nm were removed by a glass filter. Each run of the reactions lasted for 4 h, and the conversion (i.e., degradation yield) of p-chlorophenol was determined by a UV spectrophotometer (UV 7504/PC) at characteristic wavelength of 224 nm. Preliminary tests confirmed a good linear relationship between the light absorbance and the p-chlorophenol concentration. Meanwhile, blank experiments demonstrated that only less than 2.0% of p-chlorophenol decomposed after reaction for 4 h in the absence of either the photocatalyst or the light irradiation and thus could be neglected in comparison with the degradation yield resulting from photocatalysis. The reproducibility of the results was checked by repeating the results at least three times and was found to be within acceptable limits ((5%). 3. Results and Discussion The FTIR spectrum (Figure 1) revealed that, besides the absorbance peaks at 1630 and 3400 cm-1 indicative of surface O-H groups20 and a weak peak around 1450 cm-1 characteristic of residual CH3CH2-N groups,21 the TiO2-xNx(SC) contained two kinds of N species. The absorbance peaks at 1387, 1104, and 1060 cm-1 were assigned to the N species adsorbed on the TiO2 surface, while the peaks at 1220 cm-1 could be attributed to the nitrogen species incorporated in the TiO2 network.10,22 The XPS spectra in Figure 2 further confirmed that, besides a trace of adsorbed N species observed at the binding energy (BE)
around 395.4 eV, most of N species were incorporated into TiO2 network, corresponding to the BE of 399.6 eV.23 In comparison with the pure N-Ti-N,24 the BE of nitrogen in the TiO2-xNx(SC) shifted positively by 0.30 eV. Meanwhile, the BE of both the Ti and the O in the TiO2-xNx(SC) shifted negatively by 0.30 eV in comparison with the pure O-Ti-O. These results demonstrated the formation of the O-Ti-N bonding rather than the N-Ti-N bonding. The formation of the O-Ti-N bonding was simply described in Scheme 1, which could also account for the electron transfer from N to Ti and then to O, taking into account the higher electronegativity of the O atom. According to the XPS spectra, the N-contents in various TiO2-xNx samples were calculated. As shown in Table 1, the maximum N/Ti molar ratio in the TiO2-xNx(SC) obtained under supercritical conditions in Et3N/EtOH fluid could reach 3.4%, while the maximum N/Ti molar ratio in the TiO2-xNx(SC)(NH3) obtained in NH3/EtOH fluid under supercritical conditions could not exceed 1.7% regardless of the change of NH3-concentration.18 The higher maximum N-content in the TiO2-xNx(SC) than that in the TiO2-xNx(SC)(NH3) could be attributed to the easier condensation between the Et3N and the Ti-OH bond (see Scheme 1). The TiO2-xNx(DC) obtained via direct calcination showed an even lower maximum N/Ti molar ratio (1.3%). The TiO2-xNx(SC) contained higher N-content than the TiO2-xNx(DC), obviously due to the stronger interaction between the N-dopants and the TiO2, which ensured more N-dopants to be incorporated in the TiO2 network. Such N species are stable and could not be easily removed during calcination at high temperature, while the TiO2-xNx(SC) contained a large portion of N species incorporated into the TiO2 network besides the surface-adsorbed N species. It was also found that both the TiO2(SC) and TiO2-xNx(SC) exhibited much higher SBET and VP than the corresponding TiO2(DC) and TiO2-xNx(DC) since the porous structure in the precursor could be reserved under supercritical conditions owing to the lack of surface tension while the direct calcination usually leads to collapse of porous structure.15,17 The
6548 J. Phys. Chem. C, Vol. 112, No. 16, 2008
Figure 3. Raman spectrum of the TiO2-xNx(SC)-3.4 calcined at 623 K.
Huo et al.
Figure 5. DSC curve of the TiO2-xNx(SC)-3.4 calcined at 623 K.
Figure 6. TEM morphologies of (a) the TiO2(DC), (b) the TiO2(SC), and (c) the TiO2-xNx(SC)-3.4 samples calcined at 623 K. The attached is the SAED and HRTEM images of the TiO2-xNx(SC)-3.4.
Figure 4. XRD patterns of (a) the TiO2(DC) calcined at 623 K, (b) the TiO2(SC) calcined at 623 K, (c-g) the TiO2-xNx(SC)-3.4 calcined at 623, 823, 1023, 1223, and 1323 K, respectively.
N-doping further enhanced SBET and VP owing to the stabilizing effect on both the gel pore structure and the colloidal particles, which inhibited the pore collapse and particle agglomeration.25 As shown in Figure 3, the Raman spectrum confirmed that the TiO2-xNx(SC) was present in pure anatase phase, corresponding to three principal peaks around 398 cm-1(B1g), 514 cm-1(A1g), and 638 cm-1(Eg), respectively.26 The XRD patterns in Figure 4 demonstrated that the TiO2(DC) was present in an amorphous state while both the TiO2(SC) and the TiO2-xNx(SC) were present in highly crystallized anatase. The supercritical treatment could enhance crystallization degree owing to the supersaturation and self-assembly induced under supercritical conditions (553 K and 15 MPa),27 leading to nucleation, crystallization, and crystal growth. The phase transformation from anatase to rutile occurred when the TiO2-xNx(SC) was calcined at 1323 K for 8 h, which was in good accordance with the DSC analysis (Figure 5). The excellent thermal stability of the TiO2-xNx(SC) could be attributed to the porous structure and the N-dopants incorporated in the TiO2 network which may inhibit the gathering and rearrangement of the TiO2 particles due to the strong straining force.28 As shown in Figure 6, the TEM morphologies revealed that the TiO2(DC) exhibited almost shapeless particles while both the TiO2(SC) and the TiO2-xNx(SC) exhibited uniform cubic particles. The SAED image and the attached HRTEM picture clearly displayed well-resolved diffractional cycles and crystal lattice (0.35 nm) characteristic of well-crystallized anatase,29-31 which was in good accordance with the XRD patterns. Thus,
Figure 7. Structural model of the anatase crystal cell.
the TiO2(SC) and the TiO2-xNx(SC) were present in uniform cubic particles owing to the high crystallization of anatase, corresponding to the cubic crystal cell of anatase (see Figure 7) while the TiO2(DC) was composed of shapeless particles due to the poor crystallization. Figure 8 shows the PLS spectra of the TiO2(DC), TiO2(SC), and TiO2-xNx(SC) samples which displayed two peaks around 382 and 560 nm, corresponding to the emission peak from band edge free excitation and the dual-frequency peak, respectively.32,33 The TiO2-xNx(SC) exhibited a much stronger peak around 382 nm, suggesting a lower recombination probability between photoinduced electrons and holes. On one hand, the higher crystallization degree may facilitate the transfer of photoelectrons from the bulk to the surface and thus reduce their recombination with photoinduced holes. On the other hand, the N-dopants incorporated in the TiO2 network were electrondeficient which could capture photoelectrons and thus also inhibit the recombination. The TiO2-xNx(SC) displayed a weaker PLS peak around 560 nm, indicating the higher light harvesting33 which could be mainly attributed to its higher SBET. As shown in Figure 9, the UV-visible DRS demonstrated that both the
TiO2-xNx Visible Photocatalyst
J. Phys. Chem. C, Vol. 112, No. 16, 2008 6549 under supercritical conditions. One possible reason was the enhanced hydrophobicity of the photocatalyst, owing to presence of surface CH3CH2-groups (as confirmed by FTIR) which was favorable for the adsorption of p-chlorophenol molecules for degradation. 4. Conclusion
Figure 8. PLS spectra of (a) the TiO2(DC), (b) the TiO2(SC), and (c) the TiO2-xNx(SC)-3.4 calcined at 623 K.
In summary, the present work developed a new approach to prepare TiO2-xNx(SC) with high N-content by treating the TiO2 precursor in Et3N/EtOH fluid under supercritical conditions. During photodegradation of p-chlorophenol under visible light irradiation, such a catalyst exhibited higher activity than the corresponding TiO2-xNx(DC) prepared by direct calcinations owing to the larger SBET, the higher crystallization degree, and the stronger incorporation of the N-dopants in the TiO2 network, leading to the enhanced absorbance for visible light and quantum efficiency. Doping TiO2 with nitrogen in Et3N/EtOH fluid was superior to that in NH3/EtOH fluid owing to the higher content of N-dopants in the TiO2 network and the enhanced surface hydrophobicity, leading to the higher photocatalytic activity. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20377031), Shanghai Leading Academic Discipline Project (T0402), the Science and Technology Ministry of China (2005CCA01100) and Shanghai Municipal Education Commission (06DZ013). References and Notes
Figure 9. UV-vis DRS spectra of (a) the TiO2(DC), (b) the TiO2(SC) and (c) the TiO2-xNx(SC)-3.4 calcined at 623 K.
TiO2(DC) and the TiO2(SC) displayed no spectral response in the visible region owing to the bigger energy gap (3.2 eV). However, the TiO2-xNx(SC) exhibited stronger absorbance for visible light owing to the formation of O-Ti-N bonds which might build up intermediate energy levels between the valance and conduction bands, leading to a narrower energy band gap.7,11,34-36 Table 1 summarizes activities of various photocatalysts during p-chlorophenol photodegradation under visible irradiation. The TiO2(SC), TiO2(DC), and P 25 TiO2 were almost inactive since they could not be activated by visible light. For the TiO2-xNx (SC)-m series, the activity increased with an increase in N-content, obviously owing to the enhanced absorbance for visible light. Meanwhile, the increased SBET might facilitate the adsorption of reactant molecules while the presence of more N-dopants could inhibit the recombination between photoelectrons and holes by capturing photoelectrons, leading to the enhanced quantum efficiency. Accordingly, the TiO2-xNx(SC)3.4 exhibited higher activity than either the TiO2-xNx(DC)-1.3 or the TiO2-xNx(SC)(NH3)-1.7, owing to the higher N-content. However, even the TiO2-xNx(SC)-1.2 still exhibited much higher activity than the TiO2-xNx(DC)-1.3, showing that the N-doping under supercritical conditions was more powerful for enhancing the photocatalytic activity. Besides the higher SBET and the enhanced crystallization degree of anatase,15,17 the supercritical treatment resulted in stronger incorporation of the N-dopants into the TiO2 network, leading to the enhanced absorbance for visible light, owing to the narrower energy band gap. It is interesting to note that the TiO2-xNx(SC)-1.7 was more active than the TiO2-xNx(SC)(NH3)-1.7, showing the superiority of the Et3N/EtOH fluid over the NH3/EtOH fluid for the N-doping
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