This paper details a successful synthesis and comparison of a range of tri-metal hydrotalcite-like layered double hydroxides (LDHs) using urea hydrolysis.

Hydrocalumite is a layered double hydroxide (LDH) that is finding increased application in numerous scientific fields. Typically, this material is produced through environmentally polluting methods such as co-precipitation, sol-gel synthesis and urea-hydrolysis. Here, the hydrothermal green (environmentally friendly) synthesis of hydrocalumite (CaAl-OH) from Ca(OH)₂ and Al(OH)₃ in water and the parameters that influence its formation are discussed. The parameters investigated include the reaction temperature, reaction time, molar calcium-to-aluminium ratio, the morphology/crystallinity of reactants used, mixing and the water-to-solids ratio. Hydrocalumite formation was favoured in all experiments, making up between approximately 50% and 85% of the final crystalline phases obtained. Factors that were found to encourage higher hydrocalumite purity include a low water-to-solids ratio, an increase in the reaction time, sufficient mixing, the use of amorphous Al(OH)₃ with a high surface area, reaction at an adequate temperature and, most surprisingly, the use of a calcium-to-aluminium ratio that stoichiometrically favours katoite formation. X-ray diffraction (XRD) and Rietveld refinement were used to determine the composition and crystal structures of the materials formed. Scanning electron microscopy (SEM) was used to determine morphological differences and Fourier-transform infrared analysis with attenuated total reflectance (FTIR-ATR) was used to identify possible carbonate contamination, inter alia. While the synthesis was conducted in an inert environment, some carbonate contamination could not be avoided. A thorough discussion on the topic of carbonate contamination in the hydrothermal synthesis of hydrocalumite was given, and the route to improved conversion as well as the possible reaction pathway were discussed.

The influence of reaction temperature and time on the hydrothermal dissolution-precipitation synthesis of hydrotalcite was investigated. Untreated MgO, Al(OH)₃ and NaHCO₃ were used. An industrially beneficial, economically favourable, environmentally friendly, zero effluent synthesis procedure was devised based on green chemistry principles, in which the salt-rich effluent typically produced was eliminated by regenerating the sodium bicarbonate in a full recycle process. It was found that the formation of hydromagnesite dominates at low temperatures independent of reaction time. With an increase in reaction time and temperature, hydromagnesite decomposes to form magnesite. At low temperatures, the formation of hydrotalcite is limited by the solubility of the Al(OH)₃. To achieve a hydrotalcite yield of 96%, a reaction temperature of 160°C for 5 h is required. A yield higher than 99% was achieved at 180°C and 5 h reaction time, producing an layered double hydroxide with high crystallinity and homogeneity. GRAPHICAL ABSTRACT

Comparison of the effect of transition metal modification on the material properties of quintinite synthesised using co-precipitation under the same conditions.

This work highlights the use of Fe-modified MgAl-layered double hydroxides (LDHs) to replace dye and semiconductor complexes in dye-sensitized solar cells (DSSCs), forming a layered double hydroxide solar cell (LDHSC). For this purpose, a MgAl-LDH and a Fe-modified MgAl LDH were prepared. X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray (EDX) spectroscopy were used to analyze the structural properties, morphology, and success of the Fe-modification of the synthesized LDHs. Ultraviolet-visible (UV-Vis) absorption spectroscopy was used to analyze the photoactive behavior of these LDHs and compare it to that of TiO₂ and dye-sensitized TiO₂. Current-voltage (I–V) solar simulation was used to determine the fill factor (FF), open circuit voltage (V_OC), short circuit current (I_SC), and efficiency of the LDHSCs. It was shown that the MgFeAl-LDH can act as a simultaneous photoabsorber and charge separator, effectively replacing the dye and semiconductor complex in DSSCs and yielding an efficiency of 1.56%.

The use of layered double hydroxides (LDHs) in advanced photoactive applications requires tailoring their light absorption, the stabilisation of separated charges and the manipulation of charge recombination behaviour. However, understanding of these processes in LDHs is mostly lacking. Here, we dissect the UV-to-mid-infrared absorption and UV-visible luminescence (at 83 K to 298 K) of MgAl-LDH and discuss plausible charge recombination pathways with the aid of Mg(OH)₂, Al(OH)₃ and calcined MgAl-LDH (373 K to 673 K). Excitation of electrons into the conduction band (CB) is accompanied by prominent intragap absorption: excitation of fundamental vibrations and their overtones to 1.7 eV, and a long band tail to the CB edge, from which a bandgap of 3.12 eV was determined. Luminescence spectra reveal strong, broad, overlapping, multi-band emission between 2 eV and 4.5 eV dependent on temperature. Removal of adsorbed water increases luminescence. At 83 K, nearly 50% is emitted above the CB edge—contradicting common behaviour. Luminescence occurs mainly from the band edge at 298 K, or with the removal of hydroxyl- and carbonate groups by calcination. The temperature dependence reveals competition between radiative and non-radiative deactivation pathways, indicating meta-stable CB states and possible polaron formation. Below-band-edge luminescence is believed to be dominated by the effects of vibrational broadening, coordination defects, access to band-tail states, and exciton formation and its binding to defects/Al³⁺ islands in addition to the interlayer carbonate facilitating charge separation.