J. Phys. Chem. B 2001, 105, 595-596
Comment on “Electron Source in Photoinduced Hydrogen Production on Pt-Supported TiO2 Particles” Craig L. Perkins,* Michael A. Henderson, David E. McCready, and Greg S. Herman EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, MS K8-93, Richland, Washington 99352 ReceiVed: May 15, 2000; In Final Form: October 26, 2000 In a recent paper, Abe et al.1 reported on photocatalytic water splitting on TiO2/Pt. On the strength of photochemical reactions in which hydrogen production was observed, X-ray diffraction data (XRD), and ex situ X-ray photoelectron spectroscopy (XPS) data, the authors proposed that a photochemical reaction oxidized Ti4+ in the photocatalyst to Ti5+. It is our opinion that the extraordinary claim of a photochemically produced Ti5+ oxide is not supported by the authors’ data, and that a recent retraction of their XRD data2 as a result of private communications between our laboratories does not go far enough in correcting the literature. Abe et al. illuminated an aqueous suspension of TiO2/Pt with a Hg arc lamp and observed H2 production in the absence of O2 production. An unspecified elemental analysis excluded carbonaceous contaminants, and the source of the electron donor was concluded to be Ti4+ cations. We can suggest at least three more likely (than Ti4+) sources for their mysterious electron donor: (1) an overlooked inorganic species, (2) Ti3+, and/or (3) organic impurities. The authors excluded the latter in their paper based on an ex situ test described only as an “elemental analysis”.1 We view the conclusion that no carbonaceous contaminants were present in the reaction mixture as suspect, given the lack of information on the detection limits of the carbon assay, the possibility of contamination between the carbon assay and the actual reaction, and the fact that the choice of argon (thermal conductivity ) 41.33 × 10-6 cal cm-2 s-1 (°C cm-1)-1)3 as a carrier gas in their thermal conductivity detector4 probably precluded the in-situ observation of CO2 (thermal conductivity ) 37.61 × 10-6 cal cm-2 s-1 (°C cm-1)-1)3, a common product of photochemical oxidations of organic species. No specific mention is made of an analysis for inorganic impurities, our first proposed electron source. With regard to our second proposed source, it is widely accepted that nanometer scale TiO2 particles can have significant fractions of undercoordinated Ti3+ sites that can be easily oxidized to Ti4+, the highest valence of Ti observed in condensed phases. Other workers have demonstrated that hydrogen generation from water/TiO2 mixtures can be the result of photoassisted reoxidation of the partially reduced oxide rather than true photocatalytic water splitting.5,6 Here we point out that oxidation of Ti3+ to Ti4+ is much more physically realistic than oxidation of Ti4+ to Ti5+. Removal of a fifth electron from titanium requires ∼56 eV.7 Ti5+, if it were to be generated in a solid * Corresponding author. Current address: National Renewable Energy Laboratory, 1617 Cole Blvd., MS 3215, Golden, CO 80401-3393. E-mail:
[email protected].
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oxide, would extract an electron from the predominately O 2s and O 2p valence band rather than remain in the +5 valence state. Abe et al. refer in ref 1 to a “structural change of the TiO2”, a claim based on XRD data which has been retracted.2 Casual inspection of these data (Figure 2 of ref 1) reveals that the “before photoreaction” and “after photoreaction” XRD patterns are virtually identical if either is offset by ∼10° and that this large linear offset across a wide range of 2θ is not consistent with Bragg’s law. With the retraction of the XRD data of ref 1, the only remaining piece of information claimed to support the presence of Ti5+ cations are ex situ XPS spectra. No reference is given to support the authors’ statement “The peak at 530.4 eV is ascribable to O 1s of the catalyst with Ti5+ valent state”.1 No mention is made of how the sample was transferred from solution to the XPS system, of the sample mounting method, of surface cleaning procedures, or of the procedure used to calibrate the spectrometer binding energy scale. The shifts in the Ti 2p3/2 and O 1s binding energies said to mark the change from Ti4+ to Ti5+ are small (0.7 eV), and the shifting of both core levels by this same amount is suggestive of band bending. For the sake of discussion of band bending, we disregard the significanty smaller (∼1.3 eV) Ti 2p3/2 binding energies in Ti3+ relative to that of Ti4+.8 Assuming that band bending was indeed the cause of the Ti 2p3/2 and O 1s shifts noted in ref 1 and that the bending resulted only from the photoassisted oxidation of Ti3+ to Ti4+, then the measured binding energies of the postreaction catalyst should have been lower than that of the prereaction catalyst. The opposite effect is observed in Figure 3 and Figure 4 of ref 1, and therefore this specific case of band bending can be ruled out. However, by the authors’ own admission and as seen in the XPS scans of the O 1s region (Figure 4 of ref 1), the postreaction catalyst was contaminated with SiOx from the reaction vessel walls. Silicate contamination is a possible source of downward band bending in the postreaction catalyst and of electrical conductivity changes in the sample or sample mount, all of which are possible explanations for the small 0.7 eV shift observed in the Ti 2p3/2 and O 1s peaks. Whether or not band bending was the cause of the shifts noted in ref 1 would have been partially answered if the authors had also published XPS spectra covering the Pt 4f region. There are two further points regarding the XPS data of ref 1. Inspection of photoemission studies on rutile TiO2 will demonstrate that a 0.7 eV “shift” is within the scatter of Ti 2p3/2 binding energies obtained even from well-defined single crystals.8 In addition, the intensity of a “shoulder” purportedly from unreacted Ti4+ in the XPS scan of the Ti 2p3/2 region of the postphotoreaction catalyst is on par with the noise level in the experiment (Figure 3b of ref 1). Finally, we wish to point out that ref 10 of ref 1 (an assay of surface OH concentration) and ref 11 of ref 1 (a reference Ti 2p3/2 binding energy) are missing from the article’s “References and Notes” section. In summary, the conclusions reached in ref 1, that hydrogen production in the authors’ photochemical reactor was the result of oxidation of Ti4+ to a solid oxide containing Ti5+, are not physically realistic and are not supported by the authors’ published data.
10.1021/jp001793e CCC: $20.00 © 2001 American Chemical Society Published on Web 12/05/2000
596 J. Phys. Chem. B, Vol. 105, No. 2, 2001 References and Notes (1) Abe, T.; Suzuki, E.; Nagoshi, K.; Miyashita, K.; Kaneko, M. J. Phys. Chem. B 1999, 103, 1119. (2) Abe, T.; Suzuki, E.; Nagoshi, K.; Miyashita, K.; Kaneko, M. J. Phys. Chem. B 2000, 104, 3766. (3) CRC Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1984; p E-2. (4) The authors do not mention the type of detector on their gas chromatograph (Shimadzu GCPT-4C). Contact with Shimadzu technical
Comments support revealed that this GC was usually equipped with a thermal conductivity detector. (5) Sato, W.; White, J. H. J. Am. Chem. Soc. 1980, 102, 7206. (6) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. ReV. 1995, 95, 735. (7) CRC Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1984; p E-63. (8) Mayer, J. T.; Diebold, U.; Madey, T. E.; Garfunkel, E. J. Electron Spectrosc. Relat. Phenom. 1995, 73, 1.
