Comment on “A ‘one pot’ gel combustion strategy towards Ti³⁺ self-doped ‘black’ anatase TiO_2−x solar photocatalyst,” by S. G. Ullattil and P. Periyat, J. Mater. Chem. A, 2016, 4, 5854

Abstract

In this work, it is shown that the high concentration of Ti³⁺ in “black” anatase TiO_2−x, as claimed by Sanjay Gopal Ullattil et al. [Sanjay Gopal Ullattil et al., J. Mater. Chem. A, 2016, 4, 5854–5858], is inappropriate. Using the binding energy difference parameter ΔBE = BE(O 1s) − BE(Ti 2p_3/2) = 528.7 − 457.6 = 71.1 eV, the domination of the Ti⁴⁺ state in the considered compound is verified.

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In the considered study, interesting experimental results were obtained for the so-called “black” TiO_2−x powder prepared by gel combustion synthesis.¹ Conventional experimental methods were carried out for the structural and spectroscopic characterization of the final product. As reported, the color of the TiO_2−x powder was black and this clearly verifies the presence of oxygen vacancies. The XRD analysis indicates the pure anatase phase in the sample. After this, X-ray photoelectron spectroscopy (XPS) analysis was performed to explore the chemical state of titanium and oxygen ions in the TiO_2−x powder. The XPS spectra of the representative titanium doublet Ti 2p, O 1s core level and valence band were recorded. At the top of the valence band, a new band appearance has been detected and the states were attributed to the defect formation. The binding energy (BE) values of the Ti 2p_3/2 and O 1s lines were found to be BE(Ti 2p_3/2) = 457.6 and BE(O 1s) = 528.7 eV, respectively. On this basis, the presence of “a high Ti³⁺ concentration” has been stated.¹

The quality of the XPS measurements carried out in ref. 1 seems to be good and the interpretation of the XPS results given in ref. 1 can be developed using the BE difference method.^2–8 The calculated value ΔBE = BE(O 1s) − BE(Ti 2p_3/2) = 528.7 − 457.6 = 71.1 eV is in close proximity to that of anatase (∼71.4 eV)⁴ and it is very far from the ΔBE range characteristic of Ti³⁺-containing oxides (72.9–73.1 eV).⁴ If one has to take the value BE(O 1s) = 528.7 eV as a reference and account for the ΔBE = 72.9–73.1 eV estimated in ref. 4 for Ti³⁺-containing oxide crystals, the assumed energy position of the Ti³⁺ component should be at BE(Ti 2p_3/2) = 455.6–455.8 eV. However, as is evident from Fig. 3A,¹ there is no noticeable component or shoulder in this energy range. Thus, the XPS spectrum shown in Fig. 3A¹ indicates clearly the domination of the Ti⁴⁺ state in the “black” TiO_2−x powder synthesized in ref. 1. The interpretation of the experimental results obtained in ref. 1 should be rewritten taking into account the possible Ti³⁺ content at the level not above ∼1–2%, following the conventional sensitivity of XPS methods.⁹

References

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