Defect Engineering of Titania and Hematite Thin Films By Advanced Plasma Deposition Triggers High Photoelectrochemical Water Splitting Activity
Authors: Kment Štěpán , Naldoni Alberto , Hubička Zdeněk , Zbořil Radek 
Published: February 2020

The development of highly efficient energy conversion/storage devices has attracted attention from scientific and technological researchers due to the increasing concerns about the environment and the depletion of fossil fuels. In this context, the development of cost-competitive materials capable of producing fuels or electricity directly from the energy harvested from sunlight offers a desirable approach towards fulfilling the need of clean energy. Semiconductor metal oxides (e.g., TiO2, α-Fe2O3, WO3 or ZnO) are the most widely adopted materials for the conversion of solar energy into storable and transportable chemical energy such as hydrogen (H2) via the photo-electrochemical water splitting (PEC-WS), where they serve as so-called photoanodes. Especially hematite (α-Fe2O3) and titania (TiO2) based photoanodes have attracted great amount of interest in the field of PEC-WS, however their overall efficiency for solar-driven applications still remain rather low for practical application.
The defect engineering (DE) has become an important research direction for improving the optical and electronic properties of these materials towards highly efficient PEC processes. For example the discovery of black TiO2 by Chen and Mao in 2011 [1] with its substantially enhanced solar absorption has recently triggered a worldwide research interest. In spite of their remarkable enhancement in visible light absorption black TiO2 has demonstrated expected photocatalytic and PEC performance. The introduction of oxygen vacancies has been shown as an effective approach to improve the activity of α-Fe2O3 photoelectrodes [2].The main limitation related to the current DE approaches is that they are predominantly realized via a high-pressure high-temperature gas reduction. However, this methodology severely reduces the possibility to influence the DE process itself and the final properties of the material in a very fine manner, which is undoubtedly the key to precisely understand all the induced electronic changes and related phenomena.
In the presented work, we address this significant drawback by utilizing a novel deposition method based on high-impulse magnetron sputtering (HiPIMS) for the deposition of titania and hematite films under the DE conditions. This method offers the desired capability of high process controllability due to the unique deposition plasma properties. By adjusting the deposition conditions, we can regulate the extent of induced defects and under significantly reduced temperature. [3, 4]
Hematite, α-Fe2O3, is considered as one of the most promising materials for PEC-WS with a theoretical solar-to-hydrogen efficiency of 17%. However, the poor electrical conductivity and short diffusion length of photogenerated holes are substantial limitations reducing hematite’s efficiency in real experimental conditions. Here, we report on addressing these two drawbacks by deposition of very thin nanocrystalline hematite films (~ 30 nm) HiPIMS. Despite of computing models suggesting that the electrical conductivity is extremely anisotropic, revealing up to four orders of magnitude higher electron transport with conduction along the (110) hematite crystal plane, synthetic approaches allowing the sole growth in that direction have not been reported yet. We present a new strategy based on HiPIMS for tuning the crystal orientation of 2D thin hematite films by carefully controlling the energy of particles bombarding the substrate during the plasma assisted deposition procedure. [3] The PEC activity can be even boosted when commonly used fluorine doped tin oxide (FTO) conductive substrate is substituted by platinized substrate. Oriented platinum substrate can induce crystalline defects in hematite film during its growth under very high-energy bombardment, which are thermodynamically difficult to obtain. Such anomalies further considerably increase final PEC activity of hematite photoanodes. The enhanced photoactivity of α-Fe2O3@Pt is attributed to several synergistically contributing features ;namely: (i) a higher electrical conductivity due to self-doping oxygen vacancies; (ii) the formation of an intrinsic heterojunction between the distorted and typical hematite phases that ensures highly efficient separation of photogenerated charge carriers, and (iii) increased optical absorption due to enhance intensity in metal-to-ligand charge-transfer (MLCT). According to our best knowledge, the achieved photocurrent density of 1.1 mA/cm2 at 1.5 V vs. RHE represents the champion values among ultrathin undoped hematite based photoanodes.
Acknowledgments
The authors gratefully acknowledge the support by the Operational Programme Research, Development and Education - European Regional Development Fund, project no. CZ.02.1.01/0.0/0.0/15_003/0000416 of the Ministry of Education, Youth and Sports of the Czech Republic.
References

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