The photocatalytic overall water splitting reaction provides a pathway to hydrogen fuel from sunlight. Photocatalysts must achieve the reaction without the application of an external bias, which requires an effective charge separation mechanism. Photolabeling studies and electrostatic simulations for the well-known CoOOH/Al:SrTiO₃/Rh/Cr₂O₃ photocatalyst suggest that charge separation is driven by work function differences at the (100) and (110) facets of SrTiO₃, which are electron and hole selective, respectively. Here we use hydrogen annealed SrTiO_3−x single crystals to obtain the first quantitative assessment of the charge separation ability of the (100), or (110), or (111) facets during oxygen evolution. Under UV illumination (60 mW cm⁻²), the crystals exhibit variable water oxidation photocurrents (0.34, 0.82, 1.36 mA cm⁻² at 1.23 V versus RHE) and photovoltage values of 1.40, 1.52 and 1.52 V for (100), (110), and (111) SrTiO_3−x, respectively. A surface photovoltage increase in that same order (0.31 V < 0.57 V < 0.67 V) is confirmed independently with vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPV) under 375 nm (1.91 mW cm⁻²) illumination. Mott Schottky measurements in aqueous K_3/4[Fe(CN)₆] reveal facet-dependent flatband positions of −0.58, −0.71, and −0.74 V RHE for the (100), (110), and (111) crystals respectively. This confirms that the photoelectrochemical water oxidation performance of SrTiO_3−x crystals is controlled by the work function of each facet, which determines the electron transfer barrier height of the respective solid–liquid junctions. After correcting for differences in electron donor concentrations, barriers are found to increase in the order (100) < (111) < (110) and differ by as much as 0.16 eV, similar to an earlier prediction. Overall, these results explain the charge separation mechanism in SrTiO₃ photocatalysts and highlight the need for faceted semiconductor crystals as light absorbers in particle-based photocatalysts.

中文翻译：

---

晶面控制钛酸锶 (SrTiO3) 单晶光电化学水氧化过程中的电荷分离

光催化整体水分解反应提供了从阳光中获取氢燃料的途径。光催化剂必须在不施加外部偏压的情况下实现反应，这需要有效的电荷分离机制。对著名的 CoOOH/Al:SrTiO ₃ /Rh/Cr ₂ O _{3光催化剂的光标记研究和静电模拟表明，电荷分离是由 SrTiO 3}的 (100) 和 (110) 面的功函数差异驱动的，这些差异是分别是电子和空穴选择性。在这里，我们使用氢退火的 SrTiO _{3− x}单晶来首次定量评估析氧过程中 (100)、(110) 或 (111) 面的电荷分离能力。在紫外光照（60 mW cm ^-2 ）下，晶体表现出可变的水氧化光电流（ 1.23 V vs RHE 下为0.34、0.82、1.36 mA cm ^-2），（100）的光电压值为 1.40、1.52 和 1.52 V，（分别为(110)和(111)SrTiO _{3− x}。在375 nm (1.91 mW cm ^-2 ) 照明下，用振动开尔文探针表面光电压光谱(VKP-SPV) 独立地证实了相同数量级的表面光电压增加(0.31 V < 0.57 V < 0.67 V)。在 K _3/4 [Fe(CN) ₆ ]水溶液中的莫特肖特基测量揭示了 (100)、(110) 和 (111) 晶体的面相关平带位置分别为 -0.58、-0.71 和 -0.74 V RHE 。这证实了SrTiO _{3− x}晶体的光电化学水氧化性能是由每个面的功函数控制的，这决定了各自固液结的电子转移势垒高度。在校正电子供体浓度的差异后，发现势垒按 (100) < (111) < (110) 的顺序增加，差异高达 0.16 eV，与之前的预测类似。总的来说，这些结果解释了SrTiO ₃光催化剂中的电荷分离机制，并强调了在颗粒光催化剂中需要多面半导体晶体作为光吸收剂。