context: The ATR-IR spectra (Fig. 2 for TiO2 based photoanode and Fig. S3† for NiO based photocathode) show that the intensity of the azido stretching bands has completely disappeared after heating, while that of the alkyne is visible meaning that there are some unreacted alkyne groups in the film. On the other hand, the stretching bands of the ester groups of the crosslinking agent 3 and that of the lactam of the DPP 1 or 2 (both around 1700 cm−1) are still clearly visible, proving that these moieties are present on the surface of the film. This result is consistent with the presence of four alkyne groups in 3 relative to only two azides on the dyes 1 or 2. The ATR-IR spectrum of the DPP 2 grafted on the electrode was not modified after heating in the crosslinking conditions (15 min at 140 °C in orthodichlorobenzene) in absence of the cross-linker 3 (Fig. S4†). This indicates that the azido groups are stable in these conditions and do not degrade upon heating at 140 °C for 15 min.
answer: NiO 