Photoelectrochemical (PEC) water splitting promises a solution to the problem of large-scale solar energy storage. However, its development has been impeded by the poor performance of photoanodes, particularly in their capability for photovoltage generation. Many examples employing photovoltaic modules to correct the deficiency for unassisted solar water splitting have been reported to-date. Here we show that, by using the prototypical photoanode material of haematite as a study tool, structural disorders on or near the surfaces are important causes of the low photovoltages. We develop a facile re-growth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) is achieved. This result permits us to construct a photoelectrochemical device with a haematite photoanode and Si photocathode to split water at an overall efficiency of 0.91%, with NiFeOx and TiO2/Pt overlayers, respectively.

Zusätzliche Lage: Die photoelektrochemische Aktivität von Hämatit in der Wasserspaltung wurde durch Modifikation der Oberfläche mit amorphem NiFeOx deutlich gesteigert (siehe Auftragung; FTO=fluordotiertes Zinnoxid; RHE=reversible Wasserstoffelektrode). Die gemessene Photospannung erhöht sich von 0.24 auf 0.61 V, woraus sich ein rekordverdächtig niedriges Einschaltpotential von 0.62 V ergibt.

Hematite (α-Fe(2)O(3)) was grown on vertically aligned Si nanowires (NWs) using atomic layer deposition to form a dual-absorber system. Si NWs absorb photons that are transparent to hematite (600 nm < λ < 1100 nm) and convert the energy into additional photovoltage to assist photoelectrochemical (PEC) water splitting by hematite. Compared with hematite-only photoelectrodes, those with Si NWs exhibited a photocurrent turn-on potential as low as 0.6 V vs RHE. This result represents one of the lowest turn-on potentials observed for hematite-based PEC water splitting systems. It addresses a critical challenge of using hematite for PEC water splitting, namely, the fact that the band-edge positions are too positive for high-efficiency water splitting.

The performance of Ta 3 N 5 as a photoelectrode for solar water splitting is compromised by the low photovoltage and poor stability. Wang and colleagues reveal that these issues are caused by the growth of a thin oxide layer on the surface. Although self-limiting in nature, this layer pins the Fermi level and leads to almost complete suppression of the photoactivity. The effect is quantitatively measured via X-ray spectroscopy and photoelectrochemical studies. The information sheds light on strategies for improving photoelectrode performance. SUMMARYTantalum nitride (Ta 3 N 5 ) is a promising photoelectrode for solar water splitting. Although near-theoretical-limit photocurrent has already been reported on Ta 3 N 5 , its low photovoltage and poor stability remain critical challenges. In this study, we used Ta 3 N 5 nanotubes as a platform to understand the origins of these issues. Through a combination of photoelectrochemical and high-resolution electron microscope measurements, we found that the self-limiting surface oxidation of Ta 3 N 5 resulted in a thin amorphous layer (ca. 3 nm), which proved to be effective in pinning the surface Fermi levels and thus fully suppressed the photoactivity of Ta 3 N 5 . X-ray core-level spectroscopy characterization not only confirmed the surface composition change resulting from the oxidation but also revealed a Fermi-level shift toward the positive direction by up to 0.5 V. The photoactivity degradation mechanism reported here is likely to find applications in other solar-to-chemical energy-conversion systems.

Ultrathin TiO2 was grown on hematite surface by atomic layer deposition (ALD). Obvious photoelectrochemical water oxidation performance improvement was observed for samples treated with as few as a single cycle of TiO2 deposition. Up to 100 mV cathodic shift of the turn on potential was measured on samples treated by 20-cycle ALD TiO2. Photocurrent improvement was also measured on samples treated by ALD TiO2. Systematic studies ruled out possibilities that the improvement was due to electrocatalytic or bulk doping effects. It was shown that the surface treatment led to better charge separation, less surface charge recombination and, hence, greater photovoltage by hematite. The facile surface treatment by ultrathin TiO2 may find broad applications in the development of stable and high-performance photoelectrodes.