Morcos Chérif, Seron Alain, Maubec Nicolas, Ignatiadis Ioannis, Betelu Stéphanie
BRGM, French Geological Survey, 3 Avenue Claude Guillemin, CEDEX 02, 45060 Orleans, France.
LGC, Chemical Engineering Laboratory, University of Toulouse III, 118 Route de Narbonne, CEDEX 09, 31062 Toulouse, France.
Nanomaterials (Basel). 2022 May 6;12(9):1570. doi: 10.3390/nano12091570.
Co/Fe-based layered double hydroxides (LDHs) are among the most promising materials for electrochemical applications, particularly in the development of energy storage devices, such as electrochemical capacitors. They have also been demonstrated to function as energy conversion catalysts in photoelectrochemical applications for CO2 conversion into valuable chemicals. Understanding the formation mechanisms of such compounds is therefore of prime interest for further controlling the chemical composition, structure, morphology, and/or reactivity of synthesized materials. In this study, a combination of X-ray diffraction, vibrational and absorption spectroscopies, as well as physical and chemical analyses were used to provide deep insight into the coprecipitation formation mechanisms of Co/Fe-based LDHs under high supersaturation conditions. This procedure consists of adding an alkaline aqueous solution (2.80 M NaOH and 0.78 M Na2CO3) into a cationic solution (0.15 M CoII and 0.05 M FeIII) and varying the pH until the desired pH value is reached. Beginning at pH 2, pH increases induce precipitation of FeIII as ferrihydrite, which is the pristine reactional intermediate. From pH > 2, CoII sorption on ferrihydrite promotes a redox reaction between FeIII of ferrihydrite and the sorbed CoII. The crystallinity of the poorly crystalized ferrihydrite progressively decreases with increasing pH. The combination of such a phenomenon with the hydrolysis of both the sorbed CoIII and free CoII generates pristine hydroxylated FeII/CoIII LDHs at pH 7. Above pH 7, free CoII hydrolysis proceeds, which is responsible for the local dissolution of pristine LDHs and their reprecipitation and then 3D organization into CoII4FeII2CoIII2 LDHs. The progressive incorporation of CoII into the LDH structure is accountable for two phenomena: decreased coulombic attraction between the positive surface-charge sites and the interlayer anions and, concomitantly, the relative redox potential evolution of the redox species, such as when FeII is re-oxidized to FeIII, while CoIII is re-reduced to CoII, returning to a CoII6FeIII2 LDH. The nature of the interlamellar species (OH−, HCO3−, CO32− and NO3−) depends on their mobility and the speciation of anions in response to changing pH.
钴/铁基层状双氢氧化物（LDHs）是电化学应用中最具前景的材料之一，尤其在储能设备的开发中，如电化学电容器。它们还被证明在光电化学应用中作为能量转换催化剂，可将二氧化碳转化为有价值的化学品。因此，了解此类化合物的形成机制对于进一步控制合成材料的化学成分、结构、形态和/或反应活性至关重要。在本研究中，结合使用X射线衍射、振动和吸收光谱以及物理和化学分析，以深入了解高过饱和条件下钴/铁基层状双氢氧化物的共沉淀形成机制。该过程包括将碱性水溶液（2.80 M NaOH和0.78 M Na2CO3）加入阳离子溶液（0.15 M CoII和0.05 M FeIII）中，并改变pH值直至达到所需的pH值。从pH为2开始，pH值升高会导致FeIII以水铁矿形式沉淀，水铁矿是初始反应中间体。当pH > 2时，CoII在水铁矿上的吸附促进了水铁矿中的FeIII与吸附的CoII之间的氧化还原反应。随着pH值升高，结晶性较差的水铁矿的结晶度逐渐降低。这种现象与吸附的CoIII和游离CoII的水解相结合，在pH为7时生成初始羟基化的FeII/CoIII层状双氢氧化物。在pH > 7时，游离CoII发生水解，这导致初始层状双氢氧化物局部溶解并重新沉淀，然后三维组装成CoII4FeII2CoIII2层状双氢氧化物。CoII逐渐掺入层状双氢氧化物结构可解释两种现象：正表面电荷位点与层间阴离子之间的库仑吸引力降低，以及氧化还原物种的相对氧化还原电位演变，例如当FeII重新氧化为FeIII时，CoIII重新还原为CoII，恢复为CoII6FeIII2层状双氢氧化物。层间物种（OH−、HCO3−、CO32−和NO3−）的性质取决于它们的迁移率以及阴离子在pH值变化时的形态。
