通讯单位:上海交通大学机械与动力工程学院燃烧与环境技术研究中心
论文DOI:
https://doi.org/10.1016/j.apcatb.2023.123155
本研究提出了一种新型的多局域梯度掺杂技术,即利用“纳米胶囊”的离子源预先储存掺杂离子并通过离子扩散将它们持续且非均匀地释放进入半导体材料中。分散的烧绿石构型K2Ta2O6(KTOpyr)纳米颗粒因其独特的孔道结构具备K+存储和释放能力,缓释的K+随后扩散到基底聚合物氮化碳(PCN)中。K+浓度层面的多局域梯度分布及其诱导的三维势阱被限域在KTOpyr颗粒周围,这不仅可以加速光生电子和空穴的分离,还可以为光生载流子提供更多抵达表面的迁移通道。通过半导体材料将太阳能直接转化为电能或化学能是实现资源可持续性和碳中和的极具前途的途径,但仍面临光生电子和空穴分离效果欠佳的问题,而此问题被视作达到目标能量转换效率的主要抑制因素。内置电场的产生和增强与内部对称性破缺和能带结构工程密切相关,已被广泛用于改善半导体材料的电荷分离。学者们目前提出了各种策略来诱导内置电场,包括结工程构建,形貌及晶面调控和非对称助催化剂组装。其中,合理设计的梯度掺杂策略被视作通过构建连续的能带弯曲结构来构建沿浓度梯度定向的内置电场的有效方法,其在光催化、光伏、光电化学等技术领域得到了深入研究和广泛应用。梯度掺杂引起的非对称电场增强有利于光生载流子的提取和迁移,因此相较于典型的均匀掺杂,其分离效率更高。KTOpyr晶体结构由共顶点TaO6八面体的基本单元沿[100]方向Z型排列组成。KO4四面体嵌入在顶角之间构成的六边形空腔内。独特的邻接网格结构有助于形成可储存和输运K+的3D离子通道。碱金属K+掺杂是调控PCN降低导带的常用手段。由于K+掺杂在PCN内呈现限域的辐射状的梯度分布,因而诱导形成了连续弯曲的导带位置,进而在导带势能面内构建出多局域的势阱。起伏的势能面能更有效地捕获光生电子,从而抑制载流子对的快速复合,以达到更高效的光生电子-空穴对的分离效果。离子缓释效应有效控制扩散的K+掺杂水平位于合适范围内,避免了Nv杂质能级提供的复合中心的形成。本研究通过在PCN层中嵌入KTOpyr颗粒设计了多局部梯度K+掺杂PCN样品(简记为(K)PCN&KTO)。图1a, b所示的机理可阐释为由掺杂源(KTOpyr颗粒)内部的K+提取引发的点扩散控制的K+离子释放,随后离子在基底(PCN层)中扩散, K+梯度分布被限制在颗粒附近。因此,随着K+浓度梯度的增大,导带内相应的连续弯曲得以构建。由此产生的点源辐射K+分布在每个颗粒的附近创造了一个内建电场并在颗粒之间相互叠加,进而形成局部不均匀的CBM(导带底)电位表面起伏不定(图1e)。在三维势阱的驱动下,光生载流子倾向于反向分离,即电子倾向于向更正电势处迁移和积累,从而降低了复合概率并提高了迁移效率。此外,多局部势阱还可以为光生电子和空穴提供更多的迁移通道。▲Figure 1. a-b, e, Schematic illustrations of K+ ion release controlled via point-diffusion occurring on PCN substrate with the resulting gradient K+ doping distribution, which exhibits an outstanding photocatalytic performance in hydrogen evolution. The potential contour of CBM level with ups and downs is introduced by the point-source radiate K+ doping distribution (e), which facilitates to the directional migration of photo-induced electrons and inhibition of charge recombination. ci-cii, The atomic arrangement of KTOpyr crystalline structure, which displays a unique coadjacent network with 3D-channels for K+ storing and transfer. di-dii, The morphology characterization and microstructure observed in SEM imaging. The scale bars are 200 nm in di and 2 μm in dii, respectively. Octahedral KTOpyr particles (di) are embedded into the substrate and tightly wrapped by PCN layers (dii). Spectral measurements (f-g) are the comparison of pristine PCN and (K)PCN&KTO-x sequence to investigate the relationship between the doping source addition and the doping effects. f, X-ray diffraction patterns. The zoomed-in images on the right side show characteristic peaks for KTOpyr (blue arrow) and PCN (yellow arrow) respectively in the low-angle region. g, XPS core-level spectra of K 2p after peak-differentiating and imitating. Two doublets correspond to surface K-Ox associated with the oxygen from KTOpyr lattice (K(L)) and from adsorbates (K(A)), respectively. The matching between the sample sequence and line colors is normalized to all figures in this manuscript.图1dii和图2aiv显现了由PCN层紧密包裹的KTOpyr八面体颗粒(图1di)。KTOpyr颗粒具有稳定的离子释放能力并在热作用下形成了受控K+点扩散。烧绿石结构(KTOpyr)的理论原子排布如图1ci, cii所示。KTOpyr是由沿[100]方向的Z形排列的共角TaO6八面体单元构成的,每个角周围的六边形腔嵌入了一个KO4四面体。独特的共邻网络有助于形成可存储K+(紫色)的3d连接通道和离子的输运。XRD结果中明显的峰移和K/Ta原子比的显著下降进一步证实了离子的释放。由于烧绿石结构可以容忍A位阳离子空位并具有良好的结构稳定性,KTOpyr基本晶体结构在部分K+流出后仍然保持不变。对于(K)PCN& KTO样品,元素K明显分布在PCN中,而无元素Ta掺杂(图2ai, aiii)。K/N从内到外的原子比大致从25%逐渐下降到5%,下降趋势符合扩散梯度的对数趋势。作为对比实验,对于以KOH颗粒作为掺杂源的(K)PCN&KOH样品,其K+分布较为均匀。通常KOH在360 °C时熔化并且晶体结构坍塌,自由离子(K+和OH-)在PCN层中平稳扩散。两种K+掺杂源(KTOpyr和KOH)的区别可以从本质上解释为KTOpyr颗粒在稳定的结构中保持离子释放能力,导致稳定的连续K+点扩散和K+辐射梯度分布,而相反,均匀掺杂归因于不受约束的自由离子迁移。为了进一步验证点扩散对梯度掺杂的贡献,使用另一相似的K+掺杂源,即钙钛矿型KTaO3颗粒(KTOper),它具有丰富的K+和良好的结构热稳定性。有趣的是,在(K)PCN&KTOper样品中没有观察到K+掺杂(图2bi),因为K/Ta比几乎保持恒定在1且K/N可以忽略不计(图2d)。两者差异的根源在于KTOper颗粒具有规则的原子排列和有序的晶体结构,因而K+在其内部运动受到抑制,即便两者具有相同的基本组成和组成单元。本研究使用拉曼-电镜联用RISE技术对K+梯度分布的表征结果进一步验证了上述结论(图3),在靠近掺杂源颗粒的位置,掺杂效应增强,而在远处逐渐减弱,最终回归本征PCN。▲Figure 2. ai-aiv for (K)PCN&KTO sample, bi-biv for (K)PCN&KTOper sample, ci-civ for (K)PCN&KOH sample. ai-ci and aii-cii are the corresponding EDS mapping of K and N respectively, whilst aiii-biii are representative of Ta with ciii for C. aiv-civ, Bright field TEM images. All scale bars are 200 nm. d, For the comparison of three K+ sources, the semiquantitative concentration distributions are displayed as atomic ratios of K/N collected from 10 concentric rings with corresponding radiuses in arithmetic sequence and for clarity 5 of them are displayed at intervals as marked yellow in aiv-civ. The average K/N within each ring (R1 to R10) is statistically analyzed from inside to outside and the source particles (KTOpyr and KTOper) are enclosed by the innermost circle on purpose.▲Figure 3. a-b, The BSE image (b) with the scale bar of 1 μm and five tagged collected points are schematized via RISE measurement (a), which is recorded on (K)PCN&KTO sample. c-d, Raman spectra (c) at five points (#1-5) compared to pristine PCN with corresponding magnified windows of two dotted sections after smoothing and the variation trend of Raman intensity with the location (d).根据实验和模拟结果推断出CBM沿K+浓度梯度向上连续向下移动。K+浓度分布的不均匀导致了CBM电位分布的上下起伏(图1e),水平各向异性增强,叠加的内建电场增强,相较于传统典型的一维方向的能带弯曲,点扩散的梯度掺杂策略从三个维度上促进载流子分离。而光生载流子分离性能的提升由荧光寿命实验和光电实验(光电流和阻抗测试)以及光催化分解水制氢实验得到验证(图4)。▲Figure 4. a-c for the comparison of pristine PCN, (K)PCN&KOH and (K)PCN&KTO samples. a, Time-resolved transient photoluminescence (PL) decay spectra. b, Electrochemical and photoelectrochemical impedance spectroscopy (EIS and PEIS). c, Transient photocurrent response under visible light illumination. d, Activity contrasts of different K+ doping sources with regard to photocatalytic hydrogen evolution from water splitting. Error bars are noted with black line. e-f, The apparent quantum yield (e) and cycle stability tests (f) of photocatalytic hydrogen generation performance on (K)PCN&KTO sample.本研究进一步探究了此多局部梯度掺杂技术的通用性和可控性。XRD峰位移(图5a)说明A位离子(K+和Na+)在烧绿石结构中的插入为,即Na+取代K+,随后K+取代Na+。因此,通过将KTOpyr颗粒在NaOH溶液中充分混合,通过离子交换可以得到烧绿石型Na2Ta2O6 (NTO)。(Na)PCN&NTO样品也表现出优于(Na)PCN&NaOH样品的性能。K+在PCN中的扩散程度与反应温度和时间密切相关(图5b)。与原始PCN相比,550-1h和550-2h样品由于在一定扩散时间内多局部梯度掺杂分布,性能提高了6倍以上,而样品550-4h由于在充分扩散条件下均匀分布,性能仅提高了3倍。针对KTOpyr单独加热情况, K+的释放挥发(图5c)并没有发生,表明KTOpyr在温度低于600 ℃具有热稳定性。相反,KTOpyr衍射峰(图5b, d)随着时间和温度的增加而向更大的角度逐渐增加,表明K+释放引起的晶格不可逆收缩。▲Figure 5. a-b, Ex-situ XRD patterns for the substitution of A-site ions in the sequence K+-Na+-K+ (a), and for the ion release of K+ in terms of diffusion temperature (T) and time (t) for the secondary-annealing T-t samples (b). c-d, In-situ XRD patterns recorded during the calcination of KTOpyr solely (c) and KTOpyr embedded into PCN (d), respectively.本工作首次采用离子存储材料作为掺杂源,通过缓释效应控制离子点扩散,实现了颗粒半导体内的多局域梯度掺杂效果。虽然本工作是在PCN基底中实现了该技术,但它有望推广到利用其他不同种类离子缓释纳米胶囊的半导体中,这将在光催化和其他光电转换领域表现出广泛的意义。https://me.sjtu.edu.cn/teacher_directory1/shanggguanwenfeng.htmlhttps://www.sciencedirect.com/science/article/pii/S0926337323007981?via%3Dihub
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