A Study On Doping And Surface Modification Effects Of Hematite Nanorods For Efficient PEC Water Splitting

Solar energy conversion using a photoelectrochemical?PEC?water splitting system has been considered as one of the strategic ways to solve the global energy problem.As the semiconductor photoelectrodes which absorb solar light,Hematite??-Fe₂O₃?has been considered as one of promising photoanonde materials for water splitting due to its broad visible light adsorption and high stability under alkaline conditions.However,some factors such as low electrical conductivity and slow oxygen evolution reaction kinetics have to be considered in preparation of hematite photoanodes.Therefore,the improvements which can eliminate these inherent defects become important.In this study,three methods have been adopted:in-situ doping,mid-situ doping and surface modification.Moreover,doping and surface modification have been combined together for observing the synergistic effect.On the other hand,three elements?Sn,Mo and Co?have been attempted for investigating the doping or modification effect.First of all,we have synthesized Sn-doped hematite films on FTO substrate by soaking FeOOH film in SnCl₂ or SnCl₄ aqueous solution via mid-situ method.The sample which was soaked by SnCl₂ aqueous solution shows a well retained morphology and a high photocurrent.On the contrary,the sample Fe₂O₃/SnCl₄-H₂O appears to be etched.We choose the former to do further surface modification with SnCl₄ through ex-situ process and the photocurrent has been improved,they gave a photocurrent density of 1.54 mA cm^-2 at 1.23 V _RHE.Firstly,Na₂MoO₄ was introduced to mediate hematite thin films via different synthesis routes?namely in-situ mediation and ex-situ modification?to look into the effect of Mo doping and surface modification on the physical and chemical properties as well as the PEC performance of hematite thin films.During in-situ mediation process,it was found that the morphology and film thickness could be changed significantly due to the addition of MoO₄^2-,while for the ex-situ modification,the PEC performance of the hematite have been greatly improved by simply soaking in Na₂MoO₄ solutionSecondly,we have explored the influence of the different concentration of MoO₄^2-.During in-situ mediation process,it was found the morphology and film thickness could be changed significantly due to the addition of MoO₄^2-.Under optimized concentration,the photocurrent reached 0.98 mA cm^-2@1.23 V_RHE?For the surface modification,the PEC performance of the hematite have been greatly improved by soaking in Na₂MoO₄ followed by annealing at low temperature and a photocurrent of 1.1 mA cm^-2@1.23 V_RHEHE has been achieved.Thirdly,in order to improve the sluggish water oxidation reaction kinetics,we explored the feasibility of improving the charge transfer at the semiconductor/electrolyte interface by in-situ growth of Co-MOFs through a facile ion-exchanging method.Co?NO₃?₂ and2-methylimidazole were used as precursors for growth of Co-MOF and varied Co/MIm ratios led to different morphologies.Under optimized conditions,the photocurrent density and the onset potential of the Co-MOF modified hematite showed a significant increase to 2.0mAcm^-2 at 1.23 V vs RHE and a cathodic shift of 180 mV,as relative to bare hematite.Compared with the bulk CoO or Co²⁺,the atomically distributed Co²⁺existing in Co-MOF exhibited excellent hole storage capability and charge transfer efficiency as evidenced by the high surface capacitance and extremely low surface charge transfer resistance.