J. Phys. Chem. B 2006, 110, 7079-7080
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COMMENTS Comment on “Photoelectron Spectroscopic Investigation of Nitrogen-Doped Titania Nanoparticles” Chinnakonda S. Gopinath* Catalysis DiVision, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India ReceiVed: August 11, 2005; In Final Form: January 30, 2006 Recently there have been a few reports1-3 on the introduction of foreign atoms, such as C, N, F, S, and P through substitutional doping into the anatase TiO2 lattice in the place of oxygen, mainly to decrease the TiO2 band gap (3.2 eV) and hence to improve the associated photocatalytic activity. Considerable success has already been achieved in increasing the photocatalytic activity by bringing down the band gap with N-doped TiO2 (N-TiO2),1-11 especially with nano N-TiO2 materials, due to either mixing of N 2p states with O 2p states on the top of the valence band or by creation of an N-induced mid-gap level. The mechanism of band-gap reduction is not clear, due to the problems in locating the exact N position in the TiO2 lattice. Nonetheless, the success in producing N-TiO2 provides good opportunities to employ the same in photocatalytic oxidation of CO, ethanol, acetaldehyde, etc. at room temperature as well as in the decomposition of dyes such as methylene blue in the presence of solar radiation and visible light source (>380 nm) available within the buildings. However, there seems to be no consensus among the reports2,5-11 about the state of doped nitrogen in the N-TiO2 lattice. X-ray photoelectron spectroscopy (XPS) analysis of N-TiO2 claimed the state of nitrogen to be nitrogen anion (N-),8 atomic β N atoms2,6,7 as explained in the oxidation of TiN,12 due to the low binding energy (BE) observed at 396-397 eV for N 1s core level. Additional N 1s peaks with high intensity were observed at 400 and 402 eV and attributed to chemisorbed N2 or adsorbed organic compounds.2,7 Further, no anionic-like nitrogen species around 396 eV was observed by Sakthivel and Kisch,5 rather a N 1s peak at 404 eV that corresponds to hyponitrite type nitrogen. Although Valentin et al.10 recognized the above controversy in the assignment of the N 1s XPS result, they also observed the N 1s core level at BE ) 400 eV and hinted at a lower valent state for N. Recently, Chen and Burda,13 presented a detailed XPS investigations of nano N-TiO2 particles and compared that sample with other relevant materials such as commercial TiO2 (Degussa P25), N-Degussa, and nano TiO2 particles. It has been suggested from the XPS results that there is N-Ti-O bond formation due to nitrogen doping, and no oxidized nitrogen is present;13 further, the above XPS results are compared to the literature reports, especially on NO, NO2, NO3 adsorbed ZnO surfaces,14 oxidation of TiN,12 and nitridation of Ti15 to arrive at the above conclusions. Nonetheless, the conclusions derived * E-mail:
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in ref 13 are not consistent with their own XPS observations. Three main points deserve our comments and are discussed in detail in this communication. In the present comment we would like to point out that there might be a possible surface contamination of the nano N-TiO2 material analyzed, likely due to atmospheric degradation. The analysis of the XPS results by the authors in ref 13, with the assumption of uncontaminated pristine surface, therefore, lead to the below mentioned issues. Recent XPS measurements obtained on in situ scraped N-TiO2 surfaces16 clearly show the status of nitrogen to be anion-like and support our comments raised in this communication. First of all, the XPS results in ref 13 (Figure 1B in ref 13) for the N 1s core level in N-TiO2 at a BE 401.3 eV is attributed to N in N-Ti-O linkages. Further, it is suggested that there is no oxidized nitrogen, such as Ti-N-O, present on N-TiO2 and that the local electron density on N is lower for more positive formal charges.13 A careful look at the literature indicates that zero-valent N in NH3 and primary alkyl/aromatic amines and anionic nitride (N3-) in TiN appear at BE 398399 eV and 396-397 eV, respectively.12,15,17-18 Adsorbed NO and NO2 on ZnO are reported to occur at 401 and 405 eV, respectively.14 Sato et al.19 also observed the N 1s core level at BE ) 400 eV for the N-doped TiO2 prepared by a wet method and attributed the above observation to N as in NO and hence impurity sensitization. The above-reported BE values for nitrogen with different charge densities (and/or oxidation states) indicate that the BE 401.3 eV in N-TiO2 is likely due to oxidized nitrogen, such as N-O-Ti-O or O-N-Ti-O. The high BE 401.3 eV from N-TiO2 also points to at least some partial positive charge associated with nitrogen. An implication of partial cationic character of nitrogen might rather indicate the N-O-Ti induced midgap band that forms between valence and conduction bands,1,2,10,11,19 and that might reflect in the UVvis absorption spectra. However, the UV-vis absorption spectrum and photocatalytic acitivity recorded for N-TiO2 in one of the earlier works of Burda et al.9 is comparable to that of other groups2,7,11 suggests that there might be other problems, such as surface contamination, of these materials employed in ref 13. It is to be mentioned here that the anionic nitride type nitrogen was observed only after surface cleaning with sputtering,7,8,18 or in situ scraping16 and that additional N 1s peaks were shown before sputtering.7 Chen and Burda13 further suggested that the oxygen would reduce the electron density on nitrogen to give a higher BE for N in N-TiO2 than in TiN. A comparison of the BE of N atoms in TiN (396-397 eV)2,12 and N-TiO2 (401.3 eV)13 and NO on ZnO (401 eV)14 demonstrates a large negative and some positive charge density on the former and latter cases, respectively, making the above suggestion invalid. Indeed, the N 1s peak around 400-401 eV is attributed to O-N-Ti species in the nitridation of Ti15 and adsorbed NO on ZnO.14 Second, the oxygen 1s core level results reported (Figure 3 in ref 13) for N-TiO2 show a main peak at 530 eV and a broad peak at 532 eV that correspond to oxygen in O-Ti-O and N-Ti-O structural units, respectively. However, it is also mentioned that Ti-N-O (oxynitride) structure leads to an O 1s feature at 532 eV from depth profiling studies of TiN by
10.1021/jp054495w CCC: $33.50 © 2006 American Chemical Society Published on Web 03/15/2006
7080 J. Phys. Chem. B, Vol. 110, No. 13, 2006 Gyorgy et al.15 and cited in ref 13. The above Ti-N-O feature will also lead to a reduction in the BE of Ti 2p compared to TiO2.15 Apart from that, there seems to be no surface cleaning procedure, such as Ar+ sputtering,7,8,15 or in situ scraping16 applied to make sure that the nanoparticle surface is clean. Generally, the O 1s feature around 532 eV might be attributed to hydroxide as well as to possible adsorbed carbon oxide contamination.7,16,19,20 Other TiO2 materials showed a relatively sharp O 1s feature13 compared to N-TiO2. This suggests that the N-TiO2 surface might have been contaminated or degraded by atmospheric moisture and CO2. In general, there is an increase in hydrophilic character of the TiO2 surface after the N-doping2 and that might lead to an enhanced atmospheric degradation. The third issue is on the calculated surface N content. A 4-8% N substitution derived from the XPS seems too high, especially from the N 1s and O 1s results of N-TiO2.13 A very broad peak assigned to oxygen in O-Ti-N feature (with an approximate signal-to-noise ratio of 100) and a low intensity N 1s peak (with a signal-to-noise ratio of 6-8) do not seem to correlate well with the estimated N content of 4-8%, after taking into consideration the atomic photoionization cross sections of O 1s (0.04 Mb) and N 1s (0.024 Mb).21 An approximate comparison of the areas (calculated from the multiplication of full width at half-maximum and height) of both the oxygen species for N-TiO2 in ref 13 suggests that oxygen content on N-Ti-O units is slightly higher than that of O-Ti-O units on the surface. The amount of nitrogen calculated above is too high to be accommodated into the TiO2 lattice. Earlier reports on N-TiO2 suggest an N substitution up to 2%.2,5-7 Further, such high doping levels (even 4-8% N, as calculated in ref 13) would change the crystal structure considerably and could have been observed by X-ray diffraction.9 The above points suggest that the N-TiO2 sample used in ref 13 could have been considerably contaminated. However, the N-TiO2 preparation procedure used in ref 13 might have also led to the above observations in XPS, which needs to be clarified with standard cleaning procedures. A minor comment also is made on giving the same reference8a with consecutive reference numbers, 29 and 30 in ref 13, but described two different works. Reference 8b is the correct reference. In summary, we would like to comment that the results observed in ref 13 and the interpretations given are not consistent. Contrarily, the observed result of N 1s BE at 401.3 eV corresponds to the oxidized nitrogen and indicates a possible Ti-N-O linkage on the surface of N-TiO2 nanoparticles. The
Comments possibility of significant atmospheric degradation of the N-TiO2 nanoparticle surface is likely, and the same has been demonstrated from the O 1s results. Large nitrogen content in N-TiO2 is also unlikely. A very careful XPS analysis of well-characterized N-TiO2 could throw more light on the key issue of the state of the nitrogen atoms in N-TiO2. Indeed our recent results on nano N-TiO2 show the substitutional nitrogen doping and the status of N to be anion (N-) like along with the band-gap reduction mechanism.16 It is also generally suggested that better care shall be employed in any surface analysis of nanomaterials, as they are prone to surface degradation. Acknowledgment. The author is grateful to Prof. R. P. Viswanath, IIT-Madras, Chennai, for giving an overview of N-TiO2 material and active cooperation. References and Notes (1) Sato, S. Chem. Phys. Lett. 1986, 123 126. (2) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (3) Gra¨tzel, M. Nature 2001, 414, 338. (4) Yu, J. C.; Yu, J. G.; Ho, W. K.; Jiang, Z. T.; Zhang, L. G. Chem. Mater. 2002, 14, 3808. (5) Sakthivel, S.; Kisch, H. Chem. Phys. Chem. 2003, 4, 487. (6) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483. (7) Sano, T.; Negishi, N.; Koike, K.; Takeuchi, K.; Matsuzawa, S. J. Mater. Chem. 2004, 14, 380. (8) (a) Diwald, O.; Thompson, T. L.; Goralski, E. G.; Walck, S. D.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 52. (b) Diwald, O.; Thompson, T. L.; Zubkov, T.; Goralski, E. G.; Walck, S. D.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 6004. (9) Gole, J. L.; Stout, J.; Burda, C.; Lou, Y.; Chen. X. J. Phys. Chem. B 2004, 108, 1230. (10) Valentin, C. D.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Giamello, E. J. Phys. Chem. B 2005, 109, 11414. (11) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2004, 108, 10617. (12) Saha, N. C.; Tomkins, H. C. J. Appl. Phys. 1992, 72, 3072. (13) Chen, X.; Burda, C. J. Phys. Chem. B 2004, 108, 15446. (14) Rodriguez, J. A.; Jirsak, T.; Dvorak, J.; Sambasivan, S.; Fischer, D. J. Phys. Chem. B 2000, 104, 319. (15) Gyorgy, E.; Perez del Pino, A.; Serra, P.; Morenza, J. L. Surf. Coat. Technol. 2003, 173, 265. (16) Satish, M.; Viswanathan, B.; Viswanath, R. P.; Gopinath, C. S. Chem. Mater. 2005, 17, 6349. (17) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray photoelectron spectroscopy: PerkinElmer Corporation, Eden Prairie, Minnesota, 1979. (18) http://srdata.nist.gov/xps/ (19) Sato, S.; Nakamura, R.; Abe, S. Appl. Catal. A 2005, 284, 131. (20) Mathew, T.; Shiju, N. R.; Sreekumar, K.; Rao, B. S.; Gopinath, C. S. J. Catal. 2002, 210, 405. Joly, V. L. J.; Joy, P. A.; Date, S. K.; Gopinath, C. S. Phys. ReV. B 2002, 65, 184416/1-11. (21) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables, 1985, 32, 1.
