Photoanodes with Fully Controllable Texture: The Enhanced Water Splitting Efficiency of Thin Hematite Films Exhibiting Solely (110) Crystal Orientation

Abstract

Hematite, α-Fe2O3, is considered as one of the most promising materials for sustainable hydrogen production via photoelectrochemical water splitting with a theoretical solar-to-hydrogen efficiency of 17%. However, the poor electrical conductivity of hematite is a substantial limitation reducing its efficiency in real experimental conditions. Despite of computing models suggesting that the electrical conductivity is extremely anisotropic, revealing up to 4 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. Here, we present a strategy for controlling the crystal orientation of very thin hematite films by adjusting energy of ion flux during advanced pulsed reactive magnetron sputtering technique. The texture and effect of the deposition mode on the film properties were monitored by XRD, conversion electron Mössbauer spectroscopy, XPS, SEM, AFM, PEC water splitting, IPCE, transient photocurrent measurements, and Mott-Schottky analysis. The precise control of the synthetic conditions allowed to fabricate hematite photoanodes exhibiting fully textured structures along (110) and (104) crystal planes with huge differences in photocurrents of 0.65 and 0.02 mA cm(-2) (both at 1.55 V versus RHE), respectively. The photocurrent registered for fully textured (110) film is among record values reported for thin planar films. Moreover, the developed fine-tuning of crystal orientation having a huge impact on the photoefficiency would induce further improvement of thin hematite films mainly if cation doping will be combined with the controllable texture.

Hematite Photoanode with Complex Nanoarchitecture Providing Tunable Gradient Doping and Low Onset Potential for Photoelectrochemical Water Splitting

“…The average film thickness of H1 (Figure 1c)a nd H2 (Figure 1d)i s2 53 and 480 nm, respectively,i ndicating that the twice-grown process is effective to increase the thickness and lengtho ft he rods. [47] After ah igh temperature annealing process (750 8Cf or 16 min), Ti 4 + ions from oxidized TiCl 3 ,u sed during the wet chemicals ynthesis, and Sn 4 + ions that can diffuse into the lattice of a-Fe 2 O 3 from underneath fluorine-doped tin oxide Table S1). [47] After ah igh temperature annealing process (750 8Cf or 16 min), Ti 4 + ions from oxidized TiCl 3 ,u sed during the wet chemicals ynthesis, and Sn 4 + ions that can diffuse into the lattice of a-Fe 2 O 3 from underneath fluorine-doped tin oxide Table S1).…”

Evolution of sputtering target surface composition in reactive high power impulse magnetron sputtering

“…The ion assistance in reactive HiPIMS has been successfully employed for deposition of various compound thin films with improved properties. For instance to control the phase formation in TiAlN 4 or TiO 2 5 ; reducing the growth temperature of thermochromic VO 2 ; 6,7 controlling the crystal orientation of α-Fe 2 O 3 ; 8 or increasing the value of refractive index of Ta 2 O 5 .…”

The potential versus current state of water splitting with hematite

“…126.Another important factor which has been overlooked in the literature is possible anisotropic hole transport in the hematite bulk. Anisotropic electron transport in hematite is well-known and has been experimentally demonstrated 65,[129][130][131][132]. The water oxidation photocurrent, however, is limited by hole transport through the valence band, which would produce completely different behavior.…”