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The electronic properties of hematite were investigated by means of synchrotron radiation photoemission (SR-PES) and X-ray absorption spectroscopy (XAS). Hematite samples were exposed to trimethyl aluminum (TMA) pulses, a widely used Al-precursor for the atomic layer deposition (ALD) of Al2O3. SR-PES and XAS showed that the electronic properties of hematite were modified by the interaction with TMA. In particular, the hybridization of O 2p states with Fe 3d and Fe 4s4p changed upon TMA pulses due to electron inclusion as polarons. The change of hybridization correlates with an enhancement of the photocurrent density due to water oxidation for the hematite electrodes. Such an enhancement has been associated with an improvement in charge carrier transport. Our findings open new perspectives for the understanding and utilization of electrode modifications by very thin ALD films and show that the interactions between metal precursors and substrates seem to be important factors in defining their electronic and photoelectrocatalytic properties.

Abstract:The preparation of hematite nanorod electrodes modified with molybdenum and their photoelectrochemical behavior for water photooxidation have been addressed in the quest for improved electrodes for water splitting. The hematite nanorods were synthesized by chemical bath deposition, while Mo was added by following two variants of a drop casting method based on ammonium heptamolybdate solutions. FE-SEM, TEM, XRD and XPS were employed for electrode structural and morphological characterization. The reported results reveal that the impregnation method does not cause significant changes in the hematite structure and nanorod morphology. Importantly, the modification with Mo triggers a significant improvement in the photoactivity of the electrodes, obtaining a photocurrent increase of up to 43x. A specific MottSchottky analysis applicable to nanostructured electrodes was performed, revealing that the modification with Mo leads to an increase in electron concentration and to a shift of the flat band potential toward more positive values. A second role of Mo as a passivating agent needs to be invoked to explain the experimental observations. It is worth noting that this modification method allows a precise control of the amount of Mo contained in the samples while maintaining the morphology of the electrode.

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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.