Researchers report naturally occurring photocurrents in inorganic mineral systems. Solar energy fuels both biological and inorganic processes on Earth. While solar-to-chemical energy conversion via photosynthesis in biological systems is well understood, a corresponding nonbiological light-harvesting system has not been previously observed. Anhuai Lu, Kaihui Liu, Michael Hochella, and colleagues performed physical and chemical analyses of naturally occurring varnishes from desert rock surfaces in northern China, as well as surface coatings from karst and red soil particles in southern China. Electron microscopy and X-ray spectroscopy revealed that the coatings consist mainly of iron (Fe) and manganese (Mn) (oxyhydr)oxides, and are particularly enriched in Mn relative to the underlying rock. In situ photoelectric measurements in rock varnish samples yielded detectable photocurrents in the Fe-rich or Mn-rich mineral coatings, but not in the rock substrate. Currents were stable under steady illumination but shifted rapidly in response to illumination changes and exhibited constant photon-to-electron conversion efficiency. Irradiation-dependent photocurrents were also observed in coatings from karst and red soil. The authors attribute the photoelectric response of the coatings to naturally occurring Fe and Mn (oxyhydr)oxide minerals, such as birnessite, hematite, and goethite, which are solar light-responsive semiconductors. The photoelectric behavior of the mineral coatings may play important roles in biogeochemical processes, according to the authors.
Article #19-02473: "Photoelectric conversion on Earth's surface via widespread Fe- and Mn-mineral coatings," by Anhuai Lu et al.
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