National Renewable Energy Laboratory, Golden, Colorado 80401, United States.
Korea Advanced Institute of Science and Technology, Daejeon 305-338, South Korea.
Nano Lett. 2015 Apr 8;15(4):2517-25. doi: 10.1021/acs.nanolett.5b00086. Epub 2015 Mar 11.
Stable and high-performance nanoporous "black silicon" photoelectrodes with electrolessly deposited Pt nanoparticle (NP) catalysts are made with two metal-assisted etching steps. Doubly etched samples exhibit an ∼300 mV positive shift in photocurrent onset for photoelectrochemical proton reduction compared to oxide-free planar Si with identical catalysts. We find that the photocurrent onset voltage of black Si photocathodes prepared from single-crystal planar Si wafers by an Ag-assisted etching process increases in oxidative environments (e.g., aqueous electrolyte) owing to a positive flat-band potential shift caused by surface oxidation. However, within 24 h, the surface oxide layer becomes a kinetic barrier to interfacial charge transfer that inhibits proton reduction. To mitigate this issue, we developed a novel second Pt-assisted etch process that buries the Pt NPs deep into the nanoporous Si surface. This second etch shifts the onset voltage positively, from +0.25 V to +0.4 V versus reversible hydrogen electrode, and reduces the charge-transfer resistance with no performance decrease seen for at least two months. PEC performance was stable owing to Pt NP catalysts that were buried deeply in the photoelectrode by the second etch, below a thick surface layer comprised primarily of amorphous SiO2 along with some degree of remaining crystalline Si as observed by scanning and transmission electron micrographs. Electrochemical impedance studies reveal that the second etch leads to a considerably smaller interfacial charge-transfer resistance than samples without the additional etch, suggesting that burying the Pt NPs improves the interfacial contact to the crystalline silicon surface.
采用两步金属辅助刻蚀法制备了具有非电沉积 Pt 纳米颗粒（NP）催化剂的稳定、高性能纳米多孔“黑硅”光电阴极。与具有相同催化剂的无氧化层平面 Si 相比，双刻蚀样品的光电化学质子还原光电流起始的正移约为 300 mV。我们发现，由单晶平面 Si 晶片通过 Ag 辅助刻蚀工艺制备的黑 Si 光阴极的光电流起始电压在氧化环境（例如，水溶液电解质）中增加，这是由于表面氧化引起的平带电位正移。然而，在 24 小时内，表面氧化物层成为抑制质子还原的界面电荷转移的动力学障碍。为了解决这个问题，我们开发了一种新的第二 Pt 辅助刻蚀工艺，将 Pt NPs 深埋入纳米多孔 Si 表面。该二次刻蚀使起始电压从相对于可逆氢电极的+0.25 V 正移至+0.4 V，并降低了电荷转移电阻，至少两个月内没有性能下降。由于 Pt NP 催化剂在第二蚀刻过程中被深埋在光电阴极中，因此 PEC 性能稳定，该表面层主要由非晶态 SiO2 组成，同时还存在一定程度的残余晶态 Si，这一点通过扫描和透射电子显微镜观察到。电化学阻抗研究表明，与没有额外刻蚀的样品相比，第二刻蚀导致界面电荷转移电阻显著减小，这表明埋藏 Pt NPs 改善了与结晶硅表面的界面接触。
