Ta3N5是一种具有2.1 eV直接带隙的n型半导体,其带隙跨越水的氧化还原电位.此外,Ta3N5的理论太阳能制氢效率(STH)高达15.9％,超过商业化应用的效率门槛(10％),是一种理想的光电化学分解水制氢光阳极材料.采用Ta2O5作为前驱体,在氨气气氛下高温氮化制备Ta3N5是一个由表及里的非均相氮化过程,该过程会产生大量的低价钽和氮空位等本征深能级缺陷,导致费米能级钉扎效应的产生,从而使得光生电压显著降低和光电流起始电位较高.因此,开发能够进行体相均相氮化的前驱体,以抑制Ta3N5深能级缺陷的产生,具有重要意义.本文采用气相溶剂热法,在钽箔上制备了一种六氟钽酸氨((NH4)2Ta2O3F6)化合物,并以其多面体锥阵列薄膜作为前驱体,通过可控的氮化过程将前驱体结构拓扑转变为低深能级缺陷含量的Ta3N5多孔阵列薄膜.在高温氮化过程中,(NH4)2Ta2O3F6会释放含氮、氢和氟的气体小分子并形成贯穿体相的多孔通道,有利于氨气及氮化过程中产生的其他小分子物质的渗透,促进体相均匀氮化过程,避免生成大量的本征深能级缺陷.同时,(NH4)2Ta2O3F6中的高电负性氟离子可以减弱Ta-O键,进一步促进氮化反应.扫描电镜和透射电镜(TEM)结果表明,制备的(NH4)2Ta2O3F6是具有实心结构的多面体锥阵列薄膜,而拓扑转变所得的Ta3N5多面体锥薄膜具有多孔结构.X射线光电子能谱(XPS)、紫外-可见漫反射光谱和稳态/瞬态光电压谱表征结果表明,通过(NH4)2Ta2O3F6拓扑转变制备Ta3N5可有效抑制Ta3N5薄膜中深能级缺陷的形成.采用两种产氧反应助催化剂依次修饰后,XPS和TEM结果显示出助催化剂的双壳层结构与化学组成.光电化学分解水测试结果表明,所制得的Ta3N5光阳极在AM1.5G模拟太阳光的照射下,可展现出0.2 VRHE(vs.RHE)的极低光电流起始电位,且在1.23 VRHE时的光电流密度可达3.28 mA cm-2,经过连续5 h的稳定性测试,仍能保持初始值的85％.此外,稳定性测试前后助催化剂的XPS和TEM结果表明,Ta3N5光阳极光电流下降的原因可能是产氧助催化剂中硼物种的消耗.而通过减小(NH4)2Ta2O3F6多面体锥前驱体的尺寸,可以进一步减少Ta3N5薄膜中的本征深能级缺陷的含量,修饰助催化剂后可在0 VRHE下展现出光电催化水氧化活性.综上所述,通过(NH4)2Ta2O3F6新型前驱体拓扑转变制备了低深能级缺陷含量的Ta3N5光阳极,表现出极低的光电流起始电位,为构建无偏压下自发全分解水的低深能级缺陷浓度的Ta3N5光电极提供了一种新途径,该方法也可拓展至其他过渡金属氮化物的可控制备与缺陷调控.
A Ta3N5 photoanode with few deep-level defects derived from topologic transition of ammonium tantalum oxyfluoride for ultralow-bias photoelectrochemical water splitting
An open challenge for developing solar-driven Ta3N5-based photoanodes with the ability to induce low-bias photoelectrochemical(PEC)water splitting is that their deep-level defects originated from low-valent tantalum cations(Ta3+)and nitrogen vacancies(VN)seriously reduce the photovoltage and thus increase the bias for water splitting.Herein,we developed an effective topotactic transition synthesis route of producing few deep-level defects porous Ta3N5 film from the precursor film of ammonium tantalum oxyfluoride compound((NH4)2Ta2O3F6)pyramids on the Ta foil.The highly electronegative fluoride ions in(NH4)2Ta2O3F6 could weaken the Ta-O bonds and the accompanied porous structure facilitates reactant diffusion,which favors the complete nitridation.Consequently,the resulting porous Ta3N5 film has very few deep-level defects,enabling an ultralow photocurrent onset potential at 0.2 V(vs.RHE)and a short-circuit photocurrent density(Jsc)of 3.28 mA cm-2 after decorating oxygen evolution reaction(OER)cocatalysts under AM 1.5 G irradiation.Moreover,the Jsc can retain 85％of the initial value for a 5 h continuous stability test.By reducing the particle size of(NH4)2Ta2O3F6 pyramid precursor,the deep-level defects could be further lowered in the Ta3N5 film,achieving the photoactivity for water oxidation at 0 V(vs.RHE)after modifying the OER co-catalyst.
(NH4)2Ta2O3F6Topologic transitionLess defective Ta3N5Onset potentialPhotoelectrochemical water splitting
NETL
NSTL
万方数据
