## 3 Results and Discussion  
### Preparation of the Electrodes and Characterization  
Photocathodes based on nanocrystalline NiO were prepared according to procedures adapted from the literature [19]. Two different methodologies were employed, leading to NiO electrodes of type A and type B. For NiO electrodes of type A, a NiO paste made of pre-formed NiO nanoparticles was employed and deposited by spin coating onto FTO electrodes, while for NiO electrodes of type B, a precursor solution based on NiCl\({}_{2}\) was used, which was deposited onto FTO by doctor blading (see Section 2.2). In both cases, calcination at 550 \({}^{\circ}\)C following a programmed ramping procedure was performed to attain the desired NiO electrodes. Film thickness was measured using a profilometer and found to be 1.1 and 0.6 \(\upmu\)m for electrodes of types A and B, respectively (Figure S3). The morphology of the NiO films was then characterized by SEM and AFM analyses. Representative SEM micrographs and the corresponding AFM images are shown in Figure 2.  
For both electrode types, the SEM analysis shows a homogeneous distribution of the nanocrystalline NiO support onto the FTO substrate. The surface is, however, more porous and rougher for NiO electrodes of type A (Figure 2A) with respect to NiO electrodes of type B, for which a flatter structure is clearly apparent (Figure 2B). Both AFM images (Figure 2C,D) and the corresponding section analyses (Figure S4) further confirm these observations. Increased surface roughness was indeed measured for electrodes of type A (Rq = 48.4 nm, Ra = 40.9 nm) when compared to electrodes of type B (Rq = 13.7 nm, Ra = 10.6 nm). These data agree with the higher porosity, as measured by gas adsorption isotherms, and the smaller crystallite size, as measured by X-ray diffraction (XRD) analysis, expected for NiO electrodes of type A rather than those of type B [19]. Finally, the AFM analysis (Figures 2C,D and S4) suggests improved connectivity among NiO nanoparticles in the case of electrodes of type A.  
Figure 2: SEM micrographs (**A**,**B**) and AFM images (**C**,**D**) of NiO electrodes of type A (**A**,**C**) and type B (**B**,**D**). The NiO films were then sensitized by overnight soaking into a 0.5 mM solution of 4-(bis-[4-[5-(2,2-dicyano-vinyl)-thiophene-2-yl]-phenyl]-amino)-benzoic acid (**P1**) in acetonitrile. Two exemplars of the resulting electrodes are reported in Figure 1C,D. Since electrodes of type A are not sufficiently transparent to enable reliable measurement of the absorption spectrum using spectrophotometry in transmission mode, determination of the dye loading was attained through a desorption procedure performed by treating a sensitized electrode with tetrabutylammonium hydroxide (TBAOH) in methanol followed by registration of the absorption spectrum (see Supplementary Materials, Figure S5). Then, considering the molar extinction coefficient of the **P1** dye in acetonitrile at the absorption maximum (\(\varepsilon\) = 58,000 M\({}^{-1}\)cm\({}^{-1}\)) [17] and assuming similar \(\varepsilon\) values for the protonated and deprotonated species, the amount of dye was quantified using the Lambert-Beer equation. A surface concentration of 11.7 (\(\pm\)1.7) nmol-cm\({}^{-2}\) was estimated for sensitized NiO electrodes of type A. On the other hand, determination of the dye loading for sensitized electrodes of type B was possible by direct measurement of the absorption spectrum (Figure S6). Using the Lambert-Beer equation for thin films (Equation (1)) and considering the maximum absorption and the extinction coefficient [17], a surface loading of 6.9 (\(\pm\)0.3) nmol-cm\({}^{-2}\) was calculated. This result was further confirmed by desorption measurements with TBAOH in methanol (surface concentration of 6.7 (\(\pm\)0.9) nmol-cm\({}^{-2}\), Figure S7). The larger dye loading observed for NiO electrodes of type A can be attributed to the higher porosity of the NiO film as well as to the larger film thickness, both leading to an increased surface area for attachment of the **P1** dye.