Crystallographic Effects of GaN Nanostructures in Photoelectrochemical Reaction.

Tuning the surface structure of the photoelectrode provides one of the most effective ways to address the critical challenges in artificial photosynthesis, such as efficiency, stability, and product selectivity, for which gallium nitride (GaN) nanowires have shown great promise. In the GaN wurtzite crystal structure, polar, semipolar, and nonpolar planes coexist and exhibit very different structural, electronic, and chemical properties. Here, through a comprehensive study of the photoelectrochemical performance of GaN photocathodes in the form of films and nanowires with controlled surface polarities we show that significant photoelectrochemical activity can be observed when the nonpolar surfaces are exposed in the electrolyte, whereas little or no activity is measured from the GaN polar c-plane surfaces. The atomic origin of this fundamental difference is further revealed through density functional theory calculations. This study provides guideline on crystal facet engineering of metal-nitride photo(electro)catalysts for a broad range of artificial photosynthesis chemical reactions.

Direct Deposition of Crystalline Ta3N5 Thin Films on FTO for PEC Water Splitting.

Tantalum nitride is a promising photoanode material for solar water splitting, but further study and practical use are constrained by the harsh conditions of the synthesis from Ta metal. Here, we report the direct deposition of crystalline Ta3N5 on fluorine-doped tin oxide (FTO) substrate via a custom-built atomic layer deposition (ALD) system. A combination of TaCl5 (Ta precursor) and ammonia (N source) was sequentially pulsed into the ALD reactor with the substrate heated to 550 °C to deposit compact and thin films of Ta3N5 with controllable thicknesses on FTO substrates. Importantly, it is shown that the FTO is chemically and structurally stable under the reducing conditions of ammonia at 550 °C. These electrodes produced an exceptional photocurrent onset potential of ∼0.3 V versus reversible hydrogen electrode (RHE) with a maximum photocurrent of ∼2.4 mA cm-2 at 1.23 V versus RHE. Results of photoelectrochemical investigations as a function of film thickness and illumination direction reveal that the performance of Ta3N5 is controlled by a hole diffusion length of ∼50 nm. These results are crucial for the successful integration of Ta3N5 in efficient unassisted water-splitting applications.

Direct in Situ Measurement of Charge Transfer Processes During Photoelectrochemical Water Oxidation on Catalyzed Hematite.

Electrocatalysts improve the efficiency of light-absorbing semiconductor photoanodes driving the oxygen evolution reaction, but the precise function(s) of the electrocatalysts remains unclear. We directly measure, for the first time, the interface carrier transport properties of a prototypical visible-light-absorbing semiconductor, α-Fe2O3, in contact with one of the fastest known water oxidation catalysts, Ni0.8Fe0.2O x , by directly measuring/controlling the current and/or voltage at the Ni0.8Fe0.2O x catalyst layer using a second working electrode. The measurements demonstrate that the majority of photogenerated holes in α-Fe2O3 directly transfer to the catalyst film over a wide range of conditions and that the Ni0.8Fe0.2O x is oxidized by photoholes to an operating potential sufficient to drive water oxidation at rates that match the photocurrent generated by the α-Fe2O3. The Ni0.8Fe0.2O x therefore acts as both a hole-collecting contact and a catalyst for the photoelectrochemical water oxidation process. Separate measurements show that the illuminated junction photovoltage across the α-Fe2O3|Ni0.8Fe0.2O x interface is significantly decreased by the oxidation of Ni2+ to Ni3+ and the associated increase in the Ni0.8Fe0.2O x electrical conductivity. In sum, the results illustrate the underlying operative charge-transfer and photovoltage generation mechanisms of catalyzed photoelectrodes, thus guiding their continued improvement.

Selective Electrodeposition of Tantalum(V) Oxide Electrodes.

Thin films of TaOxHy were cathodically electrodeposited from an aqueous solution containing Ta-IPA precursor and KNO3 as a sacrificial agent. It was shown that the deposition resulted from a precipitation reaction triggered by the local change of pH at the surface of working electrode. Combined structural and compositional analysis revealed that during the electrodeposition the oxidation state of tantalum remained constant, Ta(V). The as-deposited films are mesoporous amorphous tantalum oxide hydrate films, which can be converted to either pure Ta2O5 or Ta3N5 by high-temperature annealing in either air (or Ar) or ammonia, respectively. The Ta3N5 electrodes exhibited promising PEC activity for water oxidation. These results open the door for the reduced temperature synthesis of Ta3N5 electrodes on TCO substrates which would allow for efficient overall solar water splitting.

Tantalum nitride films integrated with transparent conductive oxide substrates via atomic layer deposition for photoelectrochemical water splitting.

Tantalum nitride, Ta3N5, is one of the most promising materials for solar energy driven water oxidation. One significant challenge of this material is the high temperature and long duration of ammonolysis previously required to synthesize it, which has so far prevented the use of transparent conductive oxide (TCO) substrates to be used which would allow sub-bandgap light to be transmitted to a photocathode. Here, we overcome this challenge by utilizing atomic layer deposition (ALD) to directly deposit tantalum oxynitride thin films, which can be fully converted to Ta3N5via ammonolysis at 750 °C for 30 minutes. This synthesis employs far more moderate conditions than previous reports of efficient Ta3N5 photoanodes. Further, we report the first ALD of Ta-doped TiO2 which we show is a viable TCO material that is stable under the relatively mild ammonolysis conditions employed. As a result, we report the first example of a Ta3N5 electrode deposited on a TCO substrate, and the photoelectrochemical behavior. These results open the door to achieve efficient overall water splitting using a Ta3N5 photoanode.