Nanoparticle Surface Enhanced Raman Spectra of Nb-Doped TiO2 Single Crystal Electrodes
Authors: Kobayashi Mizuki , Nakamura Masashi , Hoshi Nagahiro 
Published: December 2024

A fuel cell is a generator that produces electric power by the hydrogen oxidation reaction at a fuel electrode and the oxygen reduction reaction (ORR) at an air electrode. A fuel cell is a clean energy conversion system that produces only water without emission of carbon dioxide. However, Pt (an expensive precious metal) is used as a catalyst of a fuel cell. Reduction of Pt loading in the catalyst or development of alternative materials of Pt is important for the widespread of fuel cells as a primary energy source in the future. Finding out materials that have higher ORR activity is a major target of the development of fuel cells.
TiO2, a more abundant resource compared to Pt, is one of the alternative materials of Pt. The ORR activity of TiO2 can be enhanced by metal doping [1]. Theoretical calculation predicts that doping of Pd and Rh reduces the overpotential of the ORR to zero [2]. The ORR activity of Nb doped TiO2 electrodes depends on the crystal orientation as follows: TiO2(100) < TiO2(111) < TiO2(110) [3]. However, the real surface structure and adsorbed species for enhancing the ORR activity have not been determined on TiO2 single crystal electrodes in electrochemical environments.
In this study, we have studied the adsorbates on the low index planes of TiO2 single crystals in 0.1 M HClO4 using nanoparticle surface-enhanced Raman spectroscopy (NPSERS). We assigned adsorbed species using DFT calculation.
1%Nb-doped rutile TiO2(100), TiO2(110) and TiO2(111) were annealed and chemically polished with a 30w% HF solution. Au nanoparticles of about 50 nm were dispersed on the single crystal surfaces. Raman spectra were measured by stepping the potential positively with the interval of 0.1 V. The electrolyte solution was 0.1 M HClO4. All the potentials were referred to RHE.
NPSERS bands were found at 580 cm-1 and 360 cm-1 in Ar saturated HClO4. The onset potentials of the bands were 0.4, 0.6, and 0.7 V(RHE) on TiO2(100), TiO2(110) and TiO2(111), respectively. The Au-O stretching vibration of Au nanoparticles appears from 1.4 V(RHE) at 590 cm-1 [4]. Thus the observed bands originate from surface adsorbates of TiO2. The band at 580 cm-1 was shifted 15 cm-1 to lower frequency in D2O solution, showing that the band is an adsorbed species containing hydrogen. On the other hand, the band at 360 cm-1 is an oxygen species such as Ti-O because no deuterium isotope effect was observed in D2O solution.
DFT calculation showed that symmetrical stretching vibrations of Ti-OH-Ti (bridged OH) and Ti-OD-Ti locate at 577 and 564 cm-1, respectively. Ti-O-H out-of-plane bending vibration and in-plane bending vibration also locate at 350 cm-1 and 680 cm-1, respectively, which are close to the above-mentioned 577 cm-1 according to the DFT calculation using the bridged OH model. However, these two vibrations shift more than 100 cm-1 by the substitution with deuterium, which contradicts the experimental result. Other candidates of the adsorbed species containing hydrogen are OH and OOH on 5-coordinated titanium. However, TiO2(100), which has no 5-coordinated titanium, also gave the NPSERS band at 580 cm-1. Therefore, the band at 580 cm-1 is assigned to the symmetric stretching vibration of Ti-OH-Ti (bridged OH).
The onset potential of 580 cm-1 band increases as TiO2(100) < TiO2(110) ≤ TiO2(111), whereas the order of the ORR activity is TiO2(100) < TiO2(111) < TiO2(110). No correlation is found between the onset potential and the ORR activity. This result suggests that adsorbed water, which cannot be observed with NPSERS, is involved in the ORR activity.

Acknowledgements

This study was supported by New Energy and Industrial Technology Development Organization (NEDO).

References

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Figure 1