过氧化氢作为重要的化工原料，在能源、环境、医疗领域均中发挥着重要的作用。在众多过氧化氢合成技术中，以氧气与水作为原料，太阳光作为能量源的人工光合成技术显示出巨大的环保优势以及经济价值，因此受到越来越多科研工作者的重视。在人工光合成双氧水的过程中，粉末光催化剂首先受光激发而产生电子空穴对。在光催化剂内建电场的作用下，这些光生电子空穴对会被部分分离，迁移到光催化剂表面而进行反应。然而目前的体系中，大部分光生电子空穴对在被分离前进行了复合，导致当前人工光合成过氧化氢的太阳能转化效率依然低下而无法满足工业化应用。因此在该研究课题上亟需开发有效增强电子空穴分离效率的策略。然而在反应中，粉末光催化剂的表面在纳米尺度内同时富集电子和空穴，这对电荷富集位点的精确调控造成了困难。近日，浙江大学环资学院褚驰恒团队基于光电化学-半导体物理理论，提出了一个通过调节光催化剂表面能带结构来增强电荷分离的普适性策略。以通过BiVO4进行光催化合成双氧水为例，具有（银/钯）核/壳结构纳米颗粒(Ag/Pd)和氧化钴颗粒（CoOx）分别被选择性地负载在晶面化的BiVO4颗粒的还原性和氧化性晶面上。相比单独的Pd作为还原端助催化剂，Ag/Pd能够有效降低还原性晶面的肖特基势垒，—同时提供足够的表面反应活性点位生成双氧水；CoOx则快速捕捉氧化性晶面上的空穴的同时，提供表面反应活性点位生成氧气。在保证表面催化性能的前提下，通过调节还原性和氧化性晶面间的能带结构，，有效地增强了BiVO4中光生电子空穴对分离效率，将太阳能转化过氧化氢效率提高到0.73 %，突破了无机材料合成过氧化氢记录。上述工作近日发表在Nature Communications上发表。作者通过光沉积的方式选择性调控BiVO4不同晶面的内建电场，如同电化学沉积一样实现氧化与还原位点的能带结构的精准调控。利用晶面化BiVO4的光生电子与空穴会选择性地富集在还原性{010}与氧化性{110}晶面上，作者以AgNO3与PdCl42-分别作为Ag与Pd的前驱体，成功通过两步光化学沉积将金属Ag与Pd选择性的沉积在{010}晶面上形成核/壳结构的Ag/Pd纳米颗粒。同时，以Co(NO3)2为前驱体在{110}晶面上被负载CoOx（Fig.1a）。TEM-EDS的线扫与面扫确认了Ag/Pd和CoOx纳米颗粒分别选择性地负载在BiVO4的{010}和{110}晶面上（Fig.1b-d）晶面上。BiVO4中的内建电场受到与其接触的金属影响，功函数高的金属会形成高肖特基势垒从而阻碍电子往界面迁移，反之则利于电子迁移。由于Ag的功函数较低，产生的肖特基势垒也较低（Fig. 1e），从而促进电子往{010}面，空穴往{110}面进行迁移(Fig. 1f)。光生电子空穴最终会分别移动到Pd和CoOx上，进行双氧水生成和氧气生成的反应。
Fig. 1. Facet-selective loading of cocatalysts on BiVO4 and interfacial energetics tuning with Ag/Pd core/shell cocatalyst. (a) Stepwise and facet-selective photodeposition of Co, Ag and Pd on BiVO4. (b) Energy-dispersive X-ray spectroscopy (EDS) elemental mapping and line profile along with the white arrow of CoOx/BiVO4/(Ag/Pd). We increased Co (2 wt%), Ag (1 wt%), and Pd (1 wt%) loadings for more clear observation. (c)-(d) Scanning transmission electron microscopy (STEM)-EDS elemental mapping of Ag/Pd particles loaded on BiVO4.We increased Ag (1 wt%) and Pd (1 wt%) loadings for more clear observation. (e) Ultraviolet photoelectron spectroscopy (UPS) spectra of BiVO4, BiVO4/Ag, BiVO4/Pd and BiVO4/(Ag/Pd). (f) Schematic illustration of {010} reduction facet interfacial energetics tuning through Ag/Pd core/shell cocatalyst construction on BiVO4.为了考察能带结构变化与光催化性能的关系，接下来作者对比了不同催化剂的H2O2产出效果进行评估。作者之前的研究表明，CoOx/BiVO4/Pd具备较为优越光催化生成双氧水性能。新开发的CoOx/BiVO4/(Ag/Pd)的活性则是CoOx/BiVO4/Pd的2.1倍。由于二者的表面反应都在Pd和CoOx上进行，表面催化性能十分相近，活性的提升归功于Ag增强电荷分离效率。UPS测试显示Ag拉开了价带（Evb）到费米能级（Efermi）的能级差值，表明低功函的Ag降低了BiVO4还原性{010}晶面上的肖特基势垒，进而增强光生电荷分离。在最优状态下，CoOx/BiVO4:Mo/(Ag/Pd)的全光谱量子产率达到3 %，其中420 nm处的量子产率超过了12 %，而太阳能转化效率达到了0.73 %，远高于现阶段报道的无机光催化剂。
Fig. 2. Overall H2O2 photosynthesis activities. (a) Time courses of photocatalytic H2O2 generation. Reaction conditions: photocatalyst, 1 mg/mL; 50 ml DI water saturated with O2; light source, LED visible light, 300 mW cm−2, λ > 400 nm. (b) Selectivity of H2O2 production for BiVO4, BiVO4/Ag, BiVO4/Pd, and BiVO4/(Ag/Pd). Reaction conditions: photocatalyst, 1 mg/ml; 50 mL DI water with 0.1 M H3BO3and 0.075 M ScCl3 saturated with O2 (pH 6.8), 10 v/v% methanol as electron donor; light source, LED visible light, 100 mW/cm2, λ > 400 nm. H2O2 selectivity is defined as the ratio of electrons utilized for H2O2 synthesis to the total number of electrons consumed (i.e., electrons donated by methanol). (c) Preparation of various core/shell cocatalyst and correlation between H2O2photosynthesis performance and core junction metal work function. Reaction conditions: photocatalyst, 1 mg/ml; 50 mL DI water saturated with O2(pH 6.8); light source, LED visible light, 300 mW/cm2, λ > 400 nm; reaction time, 20 min. (d) Decay in H2O2 photosynthesis activity of CoOx/BiVO4/(Ag/Pd) and C3N4/Pd under •OH-rich conditions. Aging conditions: photocatalyst, 1 mg/mL; 50 ml DI water with 25 mM H2O2; light source, 254 nm ultraviolet radiation light; •OH was generated via the reaction H2O2 + hν→2•OH. (e) Repetitive use of CoOx/BiVO4/(Ag/Pd) for H2O2 photosynthesis. Reaction conditions: photocatalyst amount, 1 mg/ml; 50 ml DI water saturated with O2; light source, LED visible light, 300 mW cm−2, λ > 400 nm. (f) Time courses of photocatalytic H2O2 generation over CoOx/Mo:BiVO4/(Ag/Pd) and the corresponding STH efficiency. Reaction conditions: photocatalyst, 10 mg; photocatalyst, 1 mg/ml; 10 mL DI water with 0.1 M H3BO3and 0.075 M ScCl3 saturated with O2 (pH 6.8); light source, xenon lamp solar simulator, 100 mW cm−2, AM 1.5 G; irradiation area, 4.5 cm−2. (g) Apparent quantum yield (AQY) of H2O2photosynthesis over CoOx/Mo:BiVO4/(Ag/Pd) as a function of the incident light wavelength. Reaction conditions: photocatalyst, 10 mg; photocatalyst, 1 mg/ml; 10 mL DI water with 0.1 M H3BO3and 0.075 M ScCl3 saturated with O2 (pH 6.8); light source, monochromatic LED light.在活性评估之后，作者进一步使用瞬态吸收光谱追踪空间电荷信号衰减并探查空间电荷传输与能带结构之间的关系。通过对比CoOx/BiVO4/Pd与CoOx/BiVO4(Ag/Pd)的光生载流子的瞬态吸收光谱信号衰减情况可知，CoOx/Mo:BiVO4/(Ag/Pd)的光生载流子的寿命远大CoOx/BiVO4/Pd，表明Ag/Pd核壳结构的引入提高了空间电荷的分离能力 （图3a 和b）。[PZ1] CoOx与Pd虽然提供了较高的表面反应动力学但是结合先前的工作分析，反应的限速步仍然是空穴端的电荷传输，Ag/Pd纳米颗粒的引入显著改善了光生空穴的在CoOx表面的富集，同时在一定程度上提高了光生电子在Pd表面的富集，两种电荷传输增益效果共同作用提升了催化效率。
Fig. 3. Charge-carrier dynamics. (a)-(b) Transient profiles of (a) free/shallowly trapped electrons probed at 2000 nm and (b) trapped holes probed at 505 nm. Photoexcitation of the samples was performed using 470 nm laser pulses (duration: 6 ns, fluence: 3 mJ/pulse, frequency: 1 Hz). Measurements were carried out in vacuum (base pressure: ~ 10−5Torr). (c)-(d) The transient profiles of trapped holes probed at 505 nm for (c) CoOx/BiVO4/Pd and (d) CoOx/BiVO4/(Ag/Pd) in vacuum and in the presence of HCOOH vapor. Photoexcitation of the samples was performed using 470 nm laser pulses (duration: 6 ns, fluence: 3 mJ/pulse, frequency: 1 Hz). Measurements were carried out in vacuum or in the presence of 20 Torr HCOOH. (e) Schematic illustration of the charge-separation process enhanced by surface energetics tuning.作者进一步在COMSOL中搭建了BiVO4颗粒的3D模型，对电荷分离过程进行分析。粉体光催化剂集成了氧化与还原活性位点，分别对应了电化学的阳极与阴极，作者因此从电化学角度解析空间电荷分离以及Ag/Pd的能带结构调控设计，从电势差的角度解析晶面控制与核壳结构相结合的空间电荷分离路径设计，展示了如何产生更大的电压。通过有限元模拟结合电化学测试模拟了CoOx/BiVO4/Pd与CoOx/BiVO4/(Ag/Pd)的电荷传输过程。模拟结果表明，Ag/Pd核壳结构的引入提升空间电荷分离效率，对于单一BiVO4颗粒来说大幅度增强了其{010}与{110}晶面之间的光电压，在{010}与{110}晶面上分别富集了更多的电子与空穴进而促进Pd上的双氧水生成和CoOx上的氧气生成反应，最终提高H2O2的全合成效率。
Fig. 4. Simulations of photocarrier distributions. (a)-(b) Schematic model and band diagram of BiVO4as a solar cell. (c) Current density vs. applied potential with barrier heights at cathodic sites (F010) varied from 0 to 0.4 V (grey dotted arrow). The barrier height of the anodic side was fixed at 1.23 V. Two dash lines labeled with 0.0081 and 0.0257 mA cm-2 were used to indicate the photocurrent densities converted from experimental H2O2 generation rates of CoOx/BiVO4/Pd and CoOx/BiVO4/(Ag/Pd), respectively. The operating conditions of CoOx/BiVO4/Pd and CoOx/BiVO4/(Ag/Pd) were marked with two void circles. Their detailed optoelectronic properties are presented in the following figures. (d)-(i) 2D cross-sectional plots of the optoelectronic properties of a BiVO4 particle, including conduction band energy (eV) (d)-(e), electron concentration (f)-(g), and hole concentration (h)-(i). (j)-(m) 1D plots of energy band diagram (j)-(k) and mobile charge carrier density (l)-(m).基于以上结果（图1-图4），我们提出了一种通用的能带调控方法(图5c) : (i) 应用晶面工程将表面能带结构分离实现电子和空穴的空间分离; (ii) 选择性地在氧化还原所需晶面上沉积具有相应功能的金属/金属氧化物物种来调节电荷分离的表面能带结构; 以及 (iii) 选择性地在金属/金属氧化物物种上沉积助催化剂来调节表面反应动力学。
Fig. 5.Generality of interfacial-energetics-tuning strategy for enhancing artificial photosynthesis. Time courses of photocatalytic H2O2generation by (a) C3N4/Pd and C3N4/(Ag/Pd) and (b) TiO2/Pd and TiO2/(Ag/Pd). Reaction conditions: photocatalyst, 1 mg/mL; 50 ml DI water saturated with O2; light source, LED visible light (300 mW cm−2, λ > 400 nm) for C3N4and UVA light (λ = 365 nm) for TiO2. (c) A general approach for effective interfacial-energetics-tuning and enhanced artificial photosynthesis.本研究利用晶面控制手段精确调节光催化剂表面能带结构进而增强光生电荷分离效率，将能带弯曲调控手段从光电催化领域引入光催化领域，大幅度地提升了光催化剂的活性。将该策略应用在光催化合成过氧化氢领域中，显著提高了太阳能到过氧化氢化学能的转化效率，为过氧化氢绿色合成提供了新的思路。考虑到人工光合成反应的共同性，该策略也可能适用于光催化分解水以及二氧化碳还原。
褚驰恒：浙江大学百人计划研究员，从事环境微界面化学与污染控制方向研究。先后在北京大学、东京大学、苏黎世联邦理工学院获得学士、硕士、博士学位，2016年博士毕业后在耶鲁大学从事博士后研究，2019年入职浙江大学资环学院。课题组主页https://person.zju.edu.cn/chihengchu
潘振华：日本中央大学助教。多年来从事光催化反应以及相关半导体物理的研究，然而鲜有建树。因沉迷游戏而工作懒散拖沓，备受诟病却屡教不改。
刘添：中国科学技术大学苏州高等研究院，特任副研究员，2020年博士毕业于湖南大学化学化工学院，2022年从浙江大学环境与资源学院博士后出站，主要从事环境功能材料设计与应用方面的研究。
