Investigating the Composition and Electrochemical Activity of Ti/SnO2-Sb2O5 Anodes Fabricated by Thermal Decomposition

Introduction

The Ti/SnO₂-Sb₂O₅ anode was found to be suitable for the electro-oxidation of organics in municipal wastewater, due to its low cost, high efficiency and strong oxidation power¹. Similar to other anodes of metal-oxides on titanium, it can be prepared by the thermal decomposition of metal chlorides on a titanium substrate, but this method suffers the loss of oxides by the evaporation of the chloride salts², which is particularly serious with the Ti/SnO₂-Sb₂O₅ anode.

The present work focused on investigating this problem, using XPS to explore the surface and in-depth profiles of the coating film.

Experimental Methods

The Ti/SnO₂-Sb₂O₅ anodes were prepared by methods modified from literature^1,3. An isopropanol solution of SbCl₃ and SnCl₄·5H₂O mixed at a target composition (Sb:Sn) was brush coated onto a polished Ti surface. The coated anodes were then heated slowly to 500°C in 2 hours to finish the conversion from chlorides to oxides.

XPS analysis was conducted on the fabricated anodes. The spectra were analyzed and quantified, using powdered SnO₂, Sb₂O₃ and Sb₂O₅ as external standards. The Sn3d_5/2, Sb3d_3/2 and O1s peaks were used for quantification. The O1s peak was processed by subtracting the overlapping Sb3d_5/2 signal and then deconvoluting into 3 sub-peaks. Oxygen in the metallic oxide lattice had the lowest binding energy and also the highest intensity, and it was used for quantification. In-depth profiling was performed with angle resolved XPS and with XPS under simultaneous Ar⁺ ion etching.

Results and Discussions

Fig 1(a) summarizes the measured composition from XPS versus the target composition expected from the coating solution in some samples, with black dots representing the electrodes prepared by the present method. Fig 1(b) shows that the majority of the Sb was in the 5+ state.

The measured Sb/Sn ratio was much lower than the target and was randomly distributed. The loss of Sb was suspected to be caused by the vaporization of SbCl₃ from the coated anodes. Unlike SnCl₄·5H₂O which only decomposes under the temperatures used, SbCl₃ can vaporize at 220°C. Therefore during the heating, the element of Sb may undergo both evaporation and oxidation. Evaporation potentially can occur at a lower temperature than oxidation and result in significant loss of Sb prior to the oxide film formation. To test this hypothesis, another piece of Ti/SnO₂-Sb₂O₅ was prepared and heated rapidly to 500°C. Rapid heating should reduce evaporation, and indeed the resulting film was found to retain more Sb (Fig 1(a) red square marker).

In-depth profiles of Sb/Sn were analyzed investigate the distribution of Sb in the film. Angle-resolved XPS (Fig1(c)) showed an increase in the Sb/Sn ratio with tilted viewing angle towards the surface. XPS with Ar⁺ ion etching also showed that Sb was detected only within the top 5nm of the material. It can be deduced that the antimony was concentrated at the surface. This could be of concern as the Sb may not effectively dope the SnO₂ to lower its conductivity.

Conclusions and Future Work

XPS revealed the loss and surface concentration of Sb in Ti/SnO₂-Sb₂O₅ anodes. These were probably associated with the vaporization of SbCl₃ during the electrode fabrication which is undesirable in the pyrolytic preparation of this electrode. Future work will be on the electro-deposition method as a substitute to produce adhering films of controllable Sb/Sn compositions.

References

1. L. Lipp and D. Pletcher, Electrochim. Act. 42 1091 (1997)
2. Ch.Comninellis and G.P.Vercesi, J. App. Electrochem. 21 136 (1991)
3. Q. Ni, D. W. Kirk, S. J. Thorpe, ECS Trans. 50(19) 87 (2013)