Extended Data Fig. 1.xlsx

unprocessed impedance data gathered from additional sample A

Figure3.xlsx

Single-frequency impedance transients (unprocessed raw data over time

Figure2.xlsx

temperature dependent conductivity (fits of data from Figure1.xlsx

Figure4.xlsx

intensity-modulated photocurrent spectroscopy (IMPS, measured raw data and fitted time constant

Sample A1.tif

Arrhenius plot of additional sample A1. As guide, the measurements of the epitaxial and polycrystalline samples from the main text are also plotted as full lines in the same color code as in the main text (same Figure appears in the ESI, Figure S9).</i

Figure1.xlsx

unprocessed impedance dat

Empirical Analysis of the Photoelectrochemical Impedance Response of Hematite Photoanodes for Water Photo-oxidation

Photoelectrochemical impedance spectroscopy (PEIS) is a useful tool for the characterization of photoelectrodes for solar water splitting. However, the analysis of PEIS spectra often involves <i>a priori</i> assumptions that might bias the results. This work puts forward an empirical method that analyzes the distribution of relaxation times (DRT), obtained directly from the measured PEIS spectra of a model hematite photoanode. By following how the DRT evolves as a function of control parameters such as the applied potential and composition of the electrolyte solution, we obtain unbiased insights into the underlying mechanisms that shape the photocurrent. In a subsequent step, we fit the data to a process-oriented equivalent circuit model (ECM) whose makeup is derived from the DRT analysis in the first step. This yields consistent quantitative trends of the dominant polarization processes observed. Our observations reveal a common step for the photo-oxidation reactions of water and H₂O₂ in alkaline solution

Different Roles of Fe_1–<i>x</i>Ni_<i>x</i>OOH Cocatalyst on Hematite (α-Fe₂O₃) Photoanodes with Different Dopants

Transparent Fe_1–<i>x</i>Ni_<i>x</i>OOH overlayers (∼2 nm thick) were deposited photoelectrochemically on (001) oriented heteroepitaxial Sn- and Zn-doped hematite (α-Fe₂O₃) thin film photoanodes. In both cases, the water photo-oxidation performance was improved by the cocatalyst overlayers. Intensity modulated photocurrent spectroscopy (IMPS) was applied to study the changes in the hole current and recombination current induced by the overlayers. For the Sn-doped hematite photoanode, the improvement in performance after deposition of the Fe_1–<i>x</i>Ni_<i>x</i>OOH overlayer was entirely due to reduction in the recombination current, leading to a cathodic shift in the onset potential. For the Zn-doped hematite photoanode, in addition to a reduction in recombination current, an increase in the hole current to the surface was also observed after the overlayer deposition, leading to a cathodic shift in the onset potential as well as an enhancement in the plateau photocurrent. These results demonstrate that Fe_1–<i>x</i>Ni_<i>x</i>OOH cocatalysts can play different roles depending on the underlying hematite photoanode. The effect of the cocatalyst is not always limited to changes in the surface properties but may also cause an increase in hole current from the bulk to the surface that indicates a possible cross-link between surface and bulk processes

Influence of Ti Doping Levels on the Photoelectrochemical Properties of Thin-Film Hematite (α-Fe₂O₃) Photoanodes

Doping with Ti enhances the electron conductivity and photoelectrochemical properties in hematite (α-Fe₂O₃) photoanodes with respect to those of undoped hematite photoanodes. However, the optimal doping level is unknown. This work examined the influence of the Ti doping level on the photoelectrochemical properties of thin-film (∼50-nm) hematite photoanodes. The films were deposited by pulsed laser deposition (PLD) on glass substrates coated with transparent electrodes (fluorinated tin oxide, FTO) from Ti-doped Fe₂O₃ targets with different Ti concentrations: 0 (undoped), 0.25, 0.8, 1, and 7 cation %. The film thicknesses, morphologies, microstructures, and optical properties were nearly the same for all of the photoanodes, thereby enabling systematic comparison of the effect of the doping level without spurious side effects related to morphological variations. The photoelectrochemical performances of all of the Ti-doped photoanodes were considerably higher than that of the undoped photoanode. Among the doped photoanodes, the performance of the heavily doped (7 cation %) photoanode was found to be lower than those of the photoanodes with doping levels of ≤1 cation %. Complementary measurements with a hole scavenger (H₂O₂) and intensity-modulated photocurrent spectroscopy (IMPS) analysis showed that, for the doped photoanodes, both the charge-separation and charge-transfer efficiencies improved with decreasing doping levels and were considerably lower for the heavily doped photoanode than for the lightly doped photoanodes

Effect of Orientation on Bulk and Surface Properties of Sn-doped Hematite (α-Fe₂O₃) Heteroepitaxial Thin Film Photoanodes

The orientation dependence on the photoelectrochemical properties of Sn-doped hematite photoanodes was studied by means of heteroepitaxial film growth. Nb-doped SnO₂ (NTO) was first grown heteroepitaxially on <i>c</i>, <i>a</i>, <i>r</i>, and <i>m</i> plane single crystal sapphire substrates in three different orientations. Hematite was then grown in the (001), (110), and (100) orientations on the NTO films. The structural, morphological, optical, and photoelectrochemical properties of the photoelectrodes were studied. The hematite photoanodes possessed high crystallinity and smooth surfaces. Hole scavenger measurements made in H₂O₂-containing electrolyte revealed that the flux of photogenerated holes arriving at the surface was not significantly affected by orientation. Cathodic shifts in the onset potential for water photo-oxidation of up to 170 mV were observed for (110) and (100) oriented hematite photoanodes as compared to (001) oriented films. These results suggest that varying the orientation of heteroepitaxial thin film Sn-doped hematite photoelectrodes primarily affects charge transfer into the electrolyte arising from the surface properties of the different crystal faces rather than affecting hole transport through the bulk under illumination. Electrochemical techniques were then used to probe the existence of surface states which were found to vary with both exposed crystal face as well as foreign dopant inclusion. Kelvin probe force microscopy (KPFM) measurements revealed correlation between the work function of the hematite films (measured in air) and the flat-band and onset potentials for water photo-oxidation (in alkaline aqueous solution)