## 2 Materials and Methods  
### Apparatus and Procedures  
All reagents were purchased from Sigma Aldrich (St. Louis, MO, USA) and used as received. Milli-Q water was obtained using a Millipore (Burlington, MA, USA) apparatus, equipped with 0.22 \(\upmu\)m filters. TEC 8 Fluorine-doped Tin Oxide (FTO) conductive glass slides were purchased from Pilkington (Lathom, UK).  
UV-Vis absorption spectra were measured with an Agilent Technologies Cary 300 spectrophotometer (Santa Clara, CA, USA). SEM micrographs and EDS analysis were obtained using an FE-SEM Jeol JSM 7600f scanning electron microscope with an accelerating voltage of 15 kV. Atomic force microscopy (AFM) images were collected using a Digital Instruments Nanoscope III scanning probe microscope (Digital Instruments, Santa Barbara, CA, USA). The instrument was equipped with a silicon tip (RTESP-300 Bruker, Billerica, MA, USA) and operated in tapping mode. Surface topographical analysis of raw AFM  
Figure 1: (**A**) Pictorial representation of the NiO photocathode and the related mechanistic pathways towards hydrogen evolution; (**B**) molecular structure of the **P1** dye; pictures of a sensitized NiO electrode of (**C**) type A and (**D**) type B.  
images was carried out with both Nanoscope 1.5 and Gwyddion analysis software. The film thickness was measured using an Alpha Step D-500 Profilometer (KLA instruments, Milipitas, CA, USA). Data were obtained in step-up/down mode with a scan length of 2 mm at a speed of 0.07 mm\(\cdot\)s\({}^{-1}\) and a stylus force of 5.0 mg. Photoelectrochemical measurements were carried out on a PGSTAT 302N potentiostat in a three-electrode configuration, using an SCE and a Pt wire as the reference and the counter electrode, respectively. A LOT-Oriel solar simulator, equipped with an AM 1.5 G filter, was used as the illumination source and set to 0.1 W\(\cdot\)cm\({}^{-2}\) power irradiance by means of a power meter (Newport 1918-C). Current-voltage and chronoamperometry curves under shuttered illumination were acquired by manually chopping the excitation source. IPCE was measured in a three-electrode configuration under the monochromatic illumination generated by an air-cooled Luxetl 175 W Xe lamp coupled to an Applied Photophysics monochromator. The photocurrent, recorded at the constant potential of \(-\)0.2 V vs. SCE, was acquired on an Eco Chemie PGSAT 101 Potentiostat. Incident irradiance was measured with a calibrated silicon photodiode. Determination of the Faradaic efficiency (FE) was performed through a generator-collector method, adapted from the literature [20]. Briefly, the photocathode was illuminated under an applied bias, thus acting as a H\({}_{2}\) generator. A Pt-covered FTO electrode, prepared by doctor blading of a 5 mM H\({}_{2}\)PtCl\({}_{6}\) solution in ethanol (with 37.5 mg\(\cdot\)mL\({}^{-1}\) of ethyl cellulose as a thickener) followed by calcination at 500 \({}^{\circ}\)C for 15 min, was sandwiched onto the photocathode and used as the collector, i.e., the electrode at which the oxidation of the evolved H\({}_{2}\) takes place. The sandwiched device was held together by a layer of unstretched parafilm (ca. 200 \(\upmu\)m spacing), sealed under pressure at 60 \({}^{\circ}\)C for 50 s. The parafilm was designedly cut to form an L-shaped chamber to minimize H\({}_{2}\) loss while still allowing for the electrolyte to access by capillary forces. Both the photocathode generator and the FTO-Pt collector were contacted using Cu tape and connected to the two working electrodes of a bipotentiostat (PGSTAT 30), while an SCE and a Pt wire were used as the reference and the counter electrode, respectively. The generator electrode was held at a bias of \(-\)0.2 V vs. SCE, while the collector was biased at +0.2 V vs. SCE, which was identified as the optimum H\({}_{2}\) oxidation potential (controlled by bubbling H\({}_{2}\) gas in aqueous solution).