Photoelectric conversion on Earth's surface via widespread Fe- and Mn-mineral coatings

Abstract

Sunlight drives photosynthesis and associated biological processes, and also influences inorganic processes that shape Earth's climate and geochemistry. Bacterial solar-to-chemical energy conversion on this planet evolved to use an intricate intracellular process of phototrophy. However, a natural nonbiological counterpart to phototrophy has yet to be recognized. In this work, we reveal the inherent "phototrophic-like" behavior of vast expanses of natural rock/soil surfaces from deserts, red soils, and karst environments, all of which can drive photon-to-electron conversions. Using scanning electron microscopy, transmission electron microscopy, micro-Raman spectroscopy, and X-ray absorption spectroscopy, Fe and Mn (oxyhydr)oxide-rich coatings were found in rock varnishes, as were Fe (oxyhydr)oxides on red soil surfaces and minute amounts of Mn oxides on karst rock surfaces. By directly fabricating a photoelectric detection device on the thin section of a rock varnish sample, we have recorded an in situ photocurrent micromapping of the coatings, which behave as highly sensitive and stable photoelectric systems. Additional measurements of red soil and powder separated from the outermost surface of karst rocks yielded photocurrents that are also sensitive to irradiation. The prominent solar-responsive capability of the phototrophic-like rocks/soils is ascribed to the semiconducting Fe- and Mn (oxyhydr)oxide-mineral coatings. The native semiconducting Fe/Mn-rich coatings may play a role similar, in part, to photosynthetic systems and thus provide a distinctive driving force for redox (bio)geochemistry on Earth's surfaces.

Keywords: birnessite; mineral coatings; phototrophic; redox (bio)geochemistry; solar energy.

Fig. 1.

(A) Representative landscape of the Gobi Desert in northwest China (Upper) with optical micrograph (Lower Left) showing mineral coatings and hand specimens (Lower Right) showing the color contrast between the surface and underside of rocks. (B) Backscattered electron (BSE) image of a rock thin section collected from the Gobi Desert and the corresponding elemental mapping of Mn, Fe, and Si by energy-dispersive X-ray spectroscopy (EDX). (C) HRTEM image of an Mn-rich coating from Gobi rock. The EDX spectrum (Upper Inset) corresponds to a spot in an Mn-rich region. Inverse fast Fourier transform image (Lower Inset) shows the clear lattice fringe. (D) Representative landscape of karst in southwest China (Upper) with an optical micrograph (Lower Left) and hand specimens (Lower Right) showing the color contrast between the surface and inside of rocks. (E) Normalized Mn K-edge XANES spectrum of a karst Mn coating and the least-squares fits using linear combination fitting of the spectra of δ-Mn^IVO₂ and (Ca_0.999, Mn^II_0.001)CO₃. (F) FT magnitude (Upper) and imaginary part (Lower) derived from Mn K-edge EXAFS spectra of a karst Mn coating, synthetic δ-MnO₂ (layer structure), and β-MnO₂ (tunnel structure). (G) Representative landscape of the red soil region in southern China (Upper) with optical micrographs (Lower Left in polarized optical microscope and Lower Right in stereoscope). (H) BSE of a soil particle thin section collected from the red soil region and corresponding elemental mapping of Fe, Si, and Al by EDX.

Fig. 2.

Photoelectric measurement results of mineral coatings. (A) Schematic diagram of in situ photoelectronic measurement on a rock varnish sample. (B) Photocurrent and EDX micromapping of Fe-rich and Mn-rich varnish samples (the mapping region corresponds to the area marked by the black dashed line in A). (C) Photocurrent time curves collected from varnish samples and recorded at a selected bias or light intensity. (D) Good linear relationships between photocurrents collected from varnish samples and light intensity. (E) Photocurrent time behavior of electrodes fabricated by mineral coating powders from red soil and karst rock. ON and OFF represent the state of the light source.

Fig. 3.

The bandgap structure and redox potential of some semiconducting minerals in mineral coatings. (A) Oxygen K-edge absorption (thin lines) and emission spectra (bold lines) of Fe (oxyhydr)oxides and Mn oxides in the desert varnish coatings. The bandgap (E_g) of each mineral is determined by calculating the energy difference between the top of the valence band and the bottom of the conduction band given by the inflection points on the adsorption spectra and at half-peak height on the emission spectra. (B) Band edge positions of Fe (oxyhydr)oxides and Mn oxides with respect to the normal hydrogen electrode (NHE, V) and the absolute vacuum scale (AVS, eV). The upper rectangles represent the bottoms of the conduction bands, and the lower rectangles represent the tops of the valence bands (pH = 7). The dashed line indicates the redox potentials of HA (27) and H₂O/O₂ redox couples.

Fig. 4.

Schematic model showing the topmost layer of Earth’s semiconducting mineral coating together with PS II for harvesting and transforming solar energy. This coating lies parallel to the core, mantle, and crust, but clearly represents a vanishingly small amount of mass compared with these major Earth components/layers. Nevertheless, the average abundances of Mn in the core, mantle, and crust have low values, as indicated in this figure, while Mn is extraordinarily concentrated in the ultrathin layer of mineral coatings. These coatings, together with the PS II protein complex in oxygenic photosynthetic organisms, are believed to be responsible for harvesting and transforming solar energy in the geosphere and biosphere, respectively, which broadens the pathways for utilizing solar energy from the well-known organic world to the newly discovered mineral semiconductor world.