Hematite Nanorod Electrodes Modified with Molybdenum: Photoelectrochemical Studies

Abstract

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.

Metal Doping to Enhance the Photoelectrochemical Behavior of LaFeO3 Photocathodes

“…As a caveat, the quantitative information obtained from the Mott–Schottky analysis should be taken with caution given the rather stringent conditions that need to be met for its full validity. The existence of frequency dispersion is attributable to frequency‐dependent values of the dielectric constant and/or the contribution of surface states or to the electrode surface being rough, which may yield significant errors in the values of the flat band potential and N A …”

“…As ac aveat,t he quantitative information obtained from the Mott-Schottky analysiss hould be taken with cautiong iven the rather stringent conditions that need to be met for its full validity.T he existence of frequency dispersion is attributable to frequency-dependentv alues of the dielectric constant and/or the contribution of surface states or to the electrode surface being rough, which may yield significant errors in the values of the flat band potentialand N A . [52,53] Nyquistp lots under illumination at À0.25 Vi nt he presence of O 2 are shown in Figure 6C and Df or the different electrodes studied. One semicircle is observed in all cases.Ageneralized decreasei nt he impedance values (semicircle radius) is noticeable with the increment of the Zn and Mg content.T he decrease in impedance is correlated with the increment in the stationaryp hotocurrent with x in Figure 5B and it can also be associated with ah igher photogenerated electron flow toward the surfacea sa dvanced in modelsd eveloped to understandt he electrochemical impedance spectroscopy (EIS) response of photoelectrodes.…”

New insights into water photooxidation on reductively pretreated hematite photoanodes

“…In this regard, some of us have recently calculated more accurately the charge carrier density for nanorod films by using a corrected Mott-Schottky equation that takes into the account the real surface area (Eq. (1))32 .…”

Improvement of sol-gel prepared tungsten trioxide photoanodes upon doping with ytterbium

“…The slopes of the linear regions increase with frequency and for each frequency, the slope decreases in the order WO3 < WO3:Yb. This behavior in the slope might be directly related with a change not only in donor density but also in roughness of the electrode material, as it has been proposed by Gómez and coworkers [11]. In other words, since the real interfacial area for WO3 electrodes is much larger than the geometric one and, as long as characteristic dimension of the nano-objects is larger than that of the space charge region, the Mott-Schottky equation…”

“…Photoanodes used in PEC water splitting are n-type semiconductors that under illumination can photogenerate holes that oxidize water into O2. They mainly include various binary and ternary oxides [5], for instance, α-Fe2O3 [10,11], WO3 [12][13][14][15], ZnO [16,17], In2O3 [18], SnO2 [19], TiO2 [20][21], BiVO4 [22][23][24], CuWO4 [24,25], Bi2WO6 [25], and bismuth molybdates (Bi2O3•nMoO3) [26]. Among them, tungsten oxide (WO3) has been extensively studied as a photoanode for use in solar water splitting since its valence band maximum position can provide sufficient overpotential for…”