New insights into water photooxidation on reductively pretreated hematite photoanodes

Abstract

It has been recently demonstrated that the photoactivity toward oxygen evolution of a number of n-type metal oxides can be substantially improved by a reductive electrochemical pretreatment. Such an enhancement has been primarily linked to the formation of low valent metal species that increase electrode conductivity. In this work, we report new insights into the electrochemical doping using highly ordered (110)-oriented hematite nanorods directly grown on FTO. The reductive pretreatment consists in applying negative potentials for a controlled period of time. Such a pretreatment was optimized in both potentiostatic and potentiodynamic regimes. We show that the optimized pretreatment enhances electrode conductivity due to an increase in charge carrier density. However, it additionally triggers changes in the morphologic, catalytic and electronic properties that facilitate the separation and collection of the photogenerated charge carriers causing an up to 8-fold enhancement in the photocurrent for water oxidation. The reductive pretreatment can be considered as a highly controllable electrochemical n-type doping with the amount of generated Fe/polaron species and the change in film morphology as the main factors determining the final efficiency for water photooxidation of the resulting electrodes.

Suppressed Charge Recombination in Hematite Photoanode via Protonation and Annealing

“…XPS spectra show that binding energies of Fe 2p3/2 and Fe 2p1/2 are 710.3 eV and 723.6 eV, respectively, along with a typical satellite peak at 718.5 eV, which is characteristic of Fe 3+ . 40,52 However, no shoulder close to the Fe 2p3/2 peak corresponding to the presence of Fe 2+ was detected after the protonation or annealing treatments. Raman and Mössbauer spectra are also virtually the same for as-prepared, protonated and annealed hematite, only showing the characteristics of Fe2O3.…”

“…Furthermore, previous reports on electrochemical reduction of hematite used sample prepared under a relatively low temperature of 550 o C 34 or 600 o C [39][40] , which have very low photoactivity.…”

“…Still, photocurrent densities of less than 1 mA cm -2 are reported for optimized pretreated hematite at 1.23 VRHE in these works. 34,[39][40] In our work, however, an additional activation treatment by sintering hematite at 800 o C for 10 min was adopted to enable the diffusion of Sn from fluorinedoped SnO2 (FTO) substrate to hematite. [4][5][16][17][18] Therefore, this work used highly-doped hematite, which is also verified by the much higher photocurrents in our case.…”

“…30 On the other hand, a negative enough polarization can also reduce hematite to more conductive phases such as Fe3O4 or even Fe to enhance the performances. 34,[39][40] This study aims at understanding the influence of electrochemical doping on the PEC performance of hematite nanorods photoanode prepared by hydrothermal method. 4-5, 16-18, 41 Protons are electrochemically introduced to hematite, followed by annealing treatments at an optimized temperature of 120 o C. Indeed, the protonation and annealing processed hematite shows a photocurrent density of 1.8 mA cm -2 at 1.23 VRHE, which is higher than the values reported in literature for hematite nanorods photoanode without cocatalysts.…”

Photoelectrocatalytic production of solar fuels with semiconductor oxides: materials, activity and modeling

“…Therefore, it is possible to take advantage of this process to promote their photocatalytic activity, as it causes a non-permanent electrochemical doping of the material. 67,73,74 Furthermore, in the case of nanoporous electrodes, the voltammetric measurements in the dark can also be combined with open circuit photopotential decay measurements. 75 Such a combination allows to evaluate the recombination and transfer (to solution) rate constants of charge carriers accumulated upon illumination at the metal oxide SC electrode.…”