Passivation of hematite nanorod photoanodes with a phosphorus overlayer for enhanced photoelectrochemical water oxidation

Abstract

Hematite (i.e., α-Fe2O3) nanorod photoanodes passivated with a phosphorus overlayer have been fabricated by decomposing sodium hypophosphite (NaH2PO2) at a low temperature over the hematite nanorod surface. Extensive scanning electron microscopy, transmission electron microscopy, x-ray diffractometry and UV-vis spectroscopy characterizations confirm that conformal deposition of an amorphous phosphorus overlayer does not change the crystal structure, morphology, and optical absorption properties of hematite photoanodes. X-ray photoelectron spectroscopy reveals that phosphorus in the deposited overlayer exists in an oxidized state. Comprehensive steady-state polarization, transient photocurrent response, and impedance spectroscopy measurements as well as Mott-Schottky analysis manifest that the phosphorus overlayer is able to effectively passivate surface states and suppress electron-hole recombination, substantially enhancing the photocurrent for water oxidation. Combining the phosphorization treatment with two-step thermal activation, a photocurrent density of 1.1 mA cm(-2) is achieved at 1.23 V versus reversible hydrogen electrode under illumination of 100 mW cm(-2), ca 55 times higher than that of the non-activated pristine hematite photoanode measured under the same conditions. The simple and fast phosphorization strategy we present here can be readily applied to passivate surfaces of other semiconductor photoelectrodes to improve their photoelectrochemical performance.

Key Strategies to Advance the Photoelectrochemical Water Splitting Performance of α‐Fe2O3 Photoanode

“…As mentioned, the passivation of the α‐Fe 2 O 3 surface by depositing a thin overlayer of metal oxides, metal hydroxides, graphene, g‐C 3 N 4 , etc ., has a great impact on the charge separation as well as charge transfer enhancement across the solid‐liquid interface. There are different theories to explain the role of the passivation layer in improving the PEC water oxidation performance , ,. Yang et al .…”

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

“…[13][14][15] However, PEC performance of a-Fe 2 O 3 has been hindered by intrinsic material limitations resulting in al ate onsetp otential [0.9-1.1 Vv s. the reversible hydrogen electrode (V RHE )], [16] short hole-diffusion length (L h % 2-4 nm), [17] poor OER performance, [18] and high charge-recombination rate. [3,21] For instance,v arious layers containing TiO 2 , [22] Al 2 O 3 , [23] Ti-doped SiO x , [9] Ga 2 O 3 , [24] and amorphous phosphorus [25] enhanced PEC performance by passivating the surface states of a-Fe 2 O 3 . Various strategies, including the use of passivation layers and/or co-catalysts have shown to reduce the onset potential of a-Fe 2 O 3 .…”

“…Various strategies, including the use of passivation layers and/or co-catalysts have shown to reduce the onset potential of a-Fe 2 O 3 . [3,21] For instance,v arious layers containing TiO 2 , [22] Al 2 O 3 , [23] Ti-doped SiO x , [9] Ga 2 O 3 , [24] and amorphous phosphorus [25] enhanced PEC performance by passivating the surface states of a-Fe 2 O 3 . Furthermore, diverse co-catalysts including FeOOH, [26] IrO 2 , [14] Ni(OH) 2 , [27,28] and CoÀPi [29,30] (Co-Pi = Cobalt-phosphate) have been widely studied for decreasing onset potentialb yi ncreasing the OER performance of a-Fe 2 O 3 .Overall, the picture resulting from these studies suggest that ac omplementary contributionf rom surface-state passivation and enhanced surface catalysis is responsible for the observedg ain in onset potential for functionalized a-Fe 2 O 3 .…”

Surface Modification of Hematite Photoanodes for Improvement of Photoelectrochemical Performance

“…TA spectroscopy shows that the enhanced PEC responses of heterostructured photoanodes are mainly due to an enhanced charge separation. Additionally, non-metal surface passivation of hematite using e.g., phosphorus [57], or bromide [58] were previously presented. It was shown that amorphous phosphorus is able to passivate surface states of a hematite nanorod photoanode and to suppress electron-hole recombination, significantly enhancing the photocurrent for water oxidation [57].…”

“…They found that partial HCl wet-etching Additionally, non-metal surface passivation of hematite using e.g., phosphorus [57], or bromide [58] were previously presented. It was shown that amorphous phosphorus is able to passivate surface states of a hematite nanorod photoanode and to suppress electron-hole recombination, significantly enhancing the photocurrent for water oxidation [57]. It is proposed that cetyltrimethylammonium bromide (CTAB) modification of the hematite surface leads to an increase of carrier density favoring the transport of the charge carriers [58].…”