Modification of Layered Double Hydroxides Using First-Row Transition Metals for Superior UV-Vis-NIR Absorption and the Influence of the Synthesis Method Used

Abstract

<div>Layered double hydroxides (LDHs) with high and tailorable UV-Vis-NIR absorption were prepared through transition</div><div>metal (TM) modification. The synthesis method used and amount of TM present were found to influence the UV-Vis-</div><div>NIR absorption intensity, -range, and the optical bandgap.</div><div><div>It was found that the incorporation of TMs in MgAl-LDH results in the existence of a "UV-Vis-NIR absorption fingerprint", the intensity of which can be tuned by the amount of TM present. There also exist differences in the UV-Vis-NIR absorption spectra and bandgaps obtained for MgAl-LDH synthesised using different synthesis conditions and methods, but these are not as visible when including transition metals. Further, the materials exhibit very complex spectra for which adequate explanation is lacking in literature. Finally, standard methods to determine the bandgap of materials, did not give conclusive results for all materials, only for some, and indicate that some of the materials might have multiple different transition types.</div></div>

Preparation of Photoactive Transition-Metal Layered Double Hydroxides (LDH) to Replace Dye-Sensitized Materials in Solar Cells

“…Coumarin 153 sensitization led to an increase in absorption between approximately 350 nm and 500 nm, as expected because of the absorption range of the dye itself. All LDHs showed a “background” absorbance, as has been previously noted [ 16 ].…”

“…The simplest preparation possible was chosen for the cells. While calcination is a frequent tool in the preparation of solar cells in order to reduce contact resistance between the semiconductor and the conductive material it is deposited on, calcination of the material onto the substrate was excluded in this work in order to retain the photo-response of the MgFeAl–LDH in the UV-Vis region as previously identified [ 16 ] and to exclude any possible changes in the material characteristics resulting from calcination. Two types of cells were prepared: dye-sensitized cells and cells making use only of the photoabsorptive capacity of the LDHs.…”

“…UV-Vis absorption spectra of all the samples were obtained using a Lambda 800 from Perkin Elmer (Hamburg, Germany). Bandgap values of all the samples were determined using the absorption spectrum fitting (ASF) method [ 16 , 34 ]. The pellets were prepared by grinding a mixture of KBr and the LDHs or TiO 2 (70% KBr and 30% LDH, dye sensitized TiO 2 and TiO 2 ) for 60 s. The powder was then pressed into pellets using a pellet press with a pressure of 8 t that was applied for 2 min.…”

“…The study of LDHs has increased widely in different applications because of their wide range of applications and tunable properties [ 15 ]. LDHs could also be potential materials for photovoltaic applications because of their promising tailorability with respect to light absorption [ 16 ]. Especially Zn- and Ti-based LDHs are good UV-absorbers [ 17 ].…”

“…One of the tailoring options we have found to remain unexplored in solar cells using liquid electrolytes is the use of photoactive LDHs as a combined photoabsorber and separated charge generator and conductor. In previous work [ 16 ], we have shown that it is possible to modify and tailor the absorbance of simple MgAl-LDHs based on the transition metals (Fe, Co, Ni, Cu, and Zn) used. Different synthesis methods for these transition-metal modified MgAl-LDHs were considered (co-precipitation and urea hydrolysis).…”