Acta Phys. -Chim. Sin. ›› 2021, Vol. 37 ›› Issue (11): 2007067.doi: 10.3866/PKU.WHXB202007067

Special Issue: Energy and Materials Chemistry

• COMMUNICATION • Previous Articles     Next Articles

Microscopic Mechanism on Giant Photoeffect in Proton Transport Through Graphene Membranes

Liming Guan1, Beidou Guo1,2, Xinrui Jia1,2, Guancai Xie1,2, Jian Ru Gong1,2,*()   

  1. 1 Chinese Academy of Sciences (CAS) Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and Hierarchy Fabrication, National Center for Nanoscience and Technology, Beijing 100190, China
    2 University of Chinese Academy of Sciences, Beijing 100049, China
  • Received:2020-07-25 Accepted:2020-09-07 Published:2020-09-11
  • Contact: Jian Ru Gong E-mail:gongjr@nanoctr.cn
  • About author:Jian Ru Gong, Email: gongjr@nanoctr.cn; Tel.: +86-10-82545649
  • Supported by:
    the Strategic Priority Research Program of CAS(XDB36030000);the National Natural Science Foundation of China(21422303);the National Natural Science Foundation of China(21573049);the National Natural Science Foundation of China(21872043);the National Basic Research Plan, China(2016YFA0201600);the Beijing Natural Science Foundation, China(2142036);the Youth Innovation Promotion Association and the Special Program of "One Belt One Road" of CAS

Abstract:

Graphene monolayers are permeable to thermal protons and impermeable to other atoms and molecules, exhibiting their potential applications in fuel cell technologies and hydrogen isotope separation. Furthermore, the giant photoeffect in proton transport through catalytically activated graphene membranes was reported by Geim et al. Their experiment showed that the synergy between illumination and the catalytically active metal plays a key role in this photoeffect. Geim et al. suggested that the local photovoltage created between metal nanoparticles and graphene could funnel protons and electrons toward the metal nanoparticles for the production of hydrogen, while repelling holes away from them, causing the giant photoeffect. However, based on static electric field theory, this explanation is not convincing and the work lacks an analysis on the microscopic mechanism of this effect. Herein, we provide the exact microscopic mechanism behind this phenomenon. In semi-metal pristine graphene, most photon excited hot electrons relax to lower energy states within a timescale of 10−12 s, while the typical timescale of a chemical reaction is 10−6 s. Thus, hot electrons excited by incident photons relax to lower energy states before reacting with protons through the graphene. When graphene is decorated with metal, electron transfer between the graphene and the metal, induced by different work functions, would result in the formation of interface dipoles. When using metals such as Pt, Pd, Ni, etc., which can strongly interact with graphene, local dipoles form. Protons are trapped around the negative poles of the local dipoles, while electrons are around the positive poles. Upon illumination, the electrons are excited to metastable excited states with higher energy levels. Due to the energy barriers around them, the free electrons in the metastable excited states will have a relatively longer lifetime, which facilitates the production of hydrogen through their effective reaction with protons that permeated through the graphene. The concentration of high-energy electrons under illumination was estimated, and the results showed that more electrons are energized to the excited state with strong illumination. According to the analysis, the giant photoeffect in proton transport through the catalytically activated graphene membrane is attributed to long-lived hot electrons and a fast proton transport rate. Since there is no change in the activation energy of the reaction, the metal catalyst increases the rate of the reaction by increasing the number of successful collisions between the reactants to produce the significant photoeffect. This might lead to a new microscopic mechanism that clarifies the role of the catalyst in improving the efficiency of photo(electro)catalytic reactions.

Key words: Graphene, Proton transport, Dipole, Hot electron, Hydrogen

MSC2000: 

  • O649