Mao-Chia Huang

Influence of sodium halides (NaF, NaCl, NaBr, NaI) on the photocatalytic performance of hydrothermally synthesized hematite photoanodes.

It has been suggested that a high concentration of Fe(3+) in solution, a low pH, and noncomplexing ions of high ionic strength are all essential for developing a high-quality hematite array. Our curiosity was piqued regarding the role of the electrolyte ions in the hydrothermal synthesis of hematite photoanodes. In this study, we prepared hematite photoanodes hydrothermally from precursor solutions of 0.1 M FeCl3 at pH 1.55 with a background electrolyte of 1.0 M sodium halide (NaF, NaCl, NaBr, or NaI). We compared the structures and properties of the as-obtained hematite photoanodes with those of the material prepared in 1.0 M NaNO3, the most widely adopted electrolyte in previous studies. Among our studied systems, we found that the hematite photoanode prepared in NaCl solution was the only one possessing properties similar to those of the sample obtained from the NaNO3 solution-most importantly in terms of photoelectrochemical performance (ca. 0.2 mA/cm(2) with +0.4 V vs SCE). The hematites obtained from the NaF, NaBr, and NaI solutions exhibited much lower (by approximately 2 orders of magnitude) photocurrent densities under the same conditions, possibly because of their relatively less ordered crystallinity and the absence of rodlike morphologies. Because the synthetic protocol was identical in each case, we believe that these two distinct features reflect the environments in which these hematite photoanodes were formed. Consistent with the latest studies reported in the literature of the X-ray photoelectron spectra of fast-frozen hematite colloids in aqueous solutions, it appears that the degree of surface ion loading at the electrolyte-hematite interface (Stern layer) is critical during the development of hematite photoanodes. We suspect that a lower ion surface loading benefits the hematite developing relatively higher-order and a rodlike texture, thereby improving the photoelectrochemical activity.

Interfacial phenomena in hematite photoanodes fabricated by directly associating iron oxide suspensions with FTO substrates using a dipping-annealing method

The fabrication of solid semiconductor nanoparticles on conductive substrates while retaining their high photocatalytic activity as they were in the dispersed form remains a challenge. In this study, we adopted the idea of used a dipping–annealing (DA) method to associate solid iron oxide nanoparticles directly onto an FTO substrate. We focused on the interfacial phenomenon of the as-fabricated hematite photoanodes to evaluate factors that may affect their photocatalytic performance. A significant sintering effect occurs during calcination; this process converts the iron (hydro)oxide precursor into a hematite structure and removes binder molecules. This sintering effect is more severe for the nanocubes than the nanospheres. However, the sintering effect would induce a size effect, further compromising the photocatalytic performance of the prepared photoanodes. Based on EIS analyses, the deteriorated photocatalytic performance arises from the deactivation that occurs at the exposed facet, increasing the open circuit potential and the resistance of hematite bulk, as well as decreasing the capacitance at the hematite–electrolyte interface.

Understanding the role of phosphate in the photoelectrochemical performance of cobalt-phosphate/hematite electrode systems

The complex of cobalt-phosphate (CoPi) is known to be an efficient catalyst that can greatly enhance the photoelectrochemical (PEC) performance of hematite electrodes. However, the complicated role that associated Pi plays in the CoPi catalyst is not yet fully understood. In this study, we noted that the photocurrent density–voltage curves between Co and CoPi associated hematite electrodes are rather different, particularly in the tailings and transient spikes. This means that the reduction in the recombination loss by the associated Pi could probably result from its high electronegativity and tendency to withdraw photoexcited electrons from hematite. The results from additional surface complexation modeling and FTIR analyses further indicate that the conformation of the associated CoPi complexes also directly affect the efficiency of the withdrawal. Interactions between Co and the neighboring Pi would, on the other hand, induce the development of the CoPi catalyst to fine particles or continuous CoPi layers, which would indirectly influence their PEC performance due to the size effect. Based on our results, associated Pi in CoPi catalysts mainly influences PEC performance by withdrawing photoexcited electrons and reducing the size of CoPi catalysts.