Comment pubs.acs.org/JPCC
Comment on “Particle Size and Structural Control of ZnWO4 Nanocrystals via Sn2+ Doping for Tunable Optical and Visible Photocatalytic Properties” V. V. Atuchin Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia
J. Phys. Chem. C 2012, 116 (34), 18508−18517. DOI: 10.1021/jp3052505 From Figure 5b,d of ref 1 the values of ΔBEZn = BE (O 1s) − BE(Zn 2p3/2) = 532.14 − 1023.9 = −491.8 eV and 530.25 − 1021.9 = −491.7 eV can be estimated for Zn2+ in pure ZnWO4 and Zn0.549Sn0.451WO4 samples, respectively.1 As for W6+ ions from Figure 5c,d, values ΔBEW = BE(O 1s) − BE(W 4f7/2) = 532.14 − 37.2 = 494.9 eV and 530.25 − 35.1 = 495.1 eV can be found for pure ZnWO4 and Zn0.549Sn0.451WO4 samples. Thus, the values of ΔBEZn and ΔBEW are, practically, persistent in ZnWO4 and Zn0.549Sn0.451WO4 oxides within the possible error range of ∼0.1 eV of BE measurements using modern XPS devices. So, the effect of the large BE shifts induced by Sndoping, as was measured in ref 1, cannot be related to Zn−O or W−O bond ionicity variation on the basis of XPS results shown in Figure 5. Different causes of these large red BE shifts may be considered, but a jump in BE(C 1s) seems to be the most probable because the shift magnitudes of ∼2 eV are nearly the same for the Zn 2p3/2, O 1s, and W 4f7/2 lines. There is no noticeable difference in ΔBEZn and ΔBEW found in pure and Sn-doped ZnWO4. Above this, the values obtained from pure ZnWO4 are well related to ΔBEZn = −491.35 and ΔBEW = 495.0 eV previously evaluated for the cleaved ZnWO4(010) crystal surface.9 Moreover, the value ΔBEW = 495.1 eV measured from the Zn0.549Sn0.451WO4 sample relates well to that in many other tungstates with different chemical compositions and crystal structures.9 This result obtained in ref 1 verifies the general relationship that W−O bond ionicity is practically insensitive to the W6+ coordination in oxides and the element composition of tungstates as was found in ref 9.
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n a recent study, ZnWO4 and ZnWO4:Sn nanocrystals have been prepared by hydrothermal synthesis and evaluated by XRD, TEM, XPS, and other techniques to see the effects of Sndoping.1 As was found by XRD analysis of the final powder products, a solid solution Zn1−xSnxWO4 is formed over the range of 0 < x ≤ 0.5 in the initial charge. This concentration range was also verified by the chemical analysis performed by the ICP-AES technique. Thus, the existence of wide range solid solution Zn1−xSnxWO4 is real. As a result of XPS measurements, drastic binding energy (BE) shifts to lower BE values were detected for Zn 2p3/2, O 1s, and W 4f7/2 lines recorded from the sample with composition Zn0.549Sn0.451WO4 in reference to BE positions of the lines in pure ZnWO4. It should be pointed that the magnitude of the shifts of all these lines is nearly the same, by ∼2 eV, as is evident from the detailed spectra shown in Figure 5 of ref 1. On this basis, the authors assert that the slight variation of ionicity of Zn−O and W−O bonds is induced by incorporation of Sn2+ ions into the ZnWO4 crystal lattice. In our opinion, such interpretation of the XPS results is not correct. When metal interacts with oxygen, a shift of valence electrons from a metal atom to oxygen occurs and that results in some variations of the electronic structure of inner shells of the cation and anion. In the metal ion, the effective displacement of the valence electron density away from the atomic nucleus results in the reduction of electrical screening of the inner shells with an increase of the inner electrons' binding energies. In the oxygen ion, the electrical screening of the inner shells becomes higher due to the capture of valence electrons from the metal, and this results in a decrease of BE values of the inner electrons. So, the formation of oxide bonds can be detected as a variation in core level BE values in XPS spectra, but the energy shifts are of opposite signs for metal and oxygen ions, as demonstrated by numerous examples.2 However, the use of absolute BE values for the identification of an element's chemical state in dielectric materials frequently leads to contradictory results because of pronounced surface charging effects. For this reason, the absolute BE values measured for the same oxide compound in different XPS devices can be a little different due to the variation of BE value of the C 1s level typically used for the energy scale calibration. In this situation, the application of the BE difference ΔBE = BE(O 1s) − BE(M), where M indicates the suitable core level of the metal ion, seems to be optimal because this parameter in insensitive to the BE(C 1s) value.3−8 Evidently, higher ionicity of M−O bonds leads to lower ΔBE values. © 2012 American Chemical Society
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
REFERENCES
(1) Su, Y.; Zhu, B.; Guan, K.; Gao, S.; Lv, L.; Du, C.; Peng, L.; Hou, L.; Wang, X. J. Phys. Chem. C 2012, 116, 18508−18517. (2) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corp.: Phys. Elect. Div. 6509 Flying Cloud Drive, Eden Prairie, MN 55344, USA, 1992. (3) Fukuda, Y.; Nagoshi, M.; Suzuki, T.; Namba, Y.; Syono, Y.; Tachiki, M. Phys. Rev. B 1989, 39, 11494−11497. (4) Atuchin, V. V.; Kalabin, I. E.; Kesler, V. G.; Pervukhina, N. V. J. Electron Spectrosc. Relat. Phenom. 2005, 142, 129−134. Received: October 20, 2012 Revised: November 15, 2012 Published: November 20, 2012 26106
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The Journal of Physical Chemistry C
Comment
(5) Atuchin, V. V.; Kesler, V. G.; Pervukhina, N. V.; Zhang, Z. J. Electron Spectrosc. Relat. Phenom. 2006, 152, 18−24. (6) Atuchin, V. V.; Kesler, V. G.; Pervukhina, N. V. Surf. Rev. Lett. 2008, 15, 391−399. (7) Tarasova, A. Yu.; Isaenko, L. I.; Kesler, V. G.; Pashkov, V. M.; Yelisseyev, A. P.; Denysyuk, N. M.; Khyzhun, O. Yu. J. Phys. Chem. Solids 2012, 73, 674−682. (8) Atuchin, V. V.; Molokeev, M. S.; Yurkin, G. Yu.; Gavrilova, T. A.; Kesler, V. G.; Laptash, N. M.; Flerov, I. N.; Patrin, G. S. J. Phys. Chem. C 2012, 116, 10162−10170. (9) Atuchin, V. V.; Galashov, E. N.; Khyzhun, O. Yu.; Kozhukhov, A. S.; Pokrovsky, L. D.; Shlegel, V. N. Cryst. Growth Des. 2011, 11, 2479− 2484.
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