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A photoelectrochemical (PEC) method based on the etching reaction of F ions on the surface of TiO 2 nanorod arrays (TNRs) was proposed for the high sensitivity and selectivity detection of F ions. With the increase of F ion concentration, the surface etching reaction on TNR becomes more intense, resulting in the increased number of surface active sites, the reduction of electron transfer resistance, and the increase of photocurrent density. The prepared TNRs as a PEC probe exhibits a good linear relationship between photocurrent increment and the logarithm of F ion concentration in the range from 0.05 to 1000 nM with an ultra-trace detection limit of 0.03 nM for F ion detection., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Up to now, the maximum coordination number of iodine is seven in neutral iodine heptafluoride (IF 7 ) and eight in anionic octafluoride (IF 8 - ). Here, we explore pressure as a method for realizing new hypercoordinated iodine compounds. First-principles swarm structure calculations have been used to predict the high-pressure and T → 0 K phase diagram of binary iodine fluorides. The investigated compounds are predicted to undergo complex structural phase transitions under high pressure, accompanied by various semiconductor to metal transitions. The pressure induced formation of a neutral octafluoride compound, IF 8 , consisting of eight-coordinated iodine is one of several unprecedented predicted structures. In sharp contrast to the square antiprismatic structure in IF 8 - , IF 8 , which is dynamically unstable under atmospheric conditions, is stable and adopts a quasi-cube molecular configuration with R 3[combining macron] symmetry at 300 GPa. The metallicity of IF 8 originates from a hole in the fluorine 2p-bands that dominate the Fermi surface. The highly unusual coordination sphere in IF 8 at 300 GPa is a consequence of the 5d levels of iodine coming down and becoming part of the valence, where they mix with iodine's 5s and 5p levels and engage in chemical bonding. The valence expansion of iodine under pressure effectively makes IF 8 not only hypercoordinated, but also hypervalent.

Nitrogen-doped carbon materials (NCs) are extensively studied for the oxygen reduction reaction (ORR). However, the nature of active sites of pyridinic nitrogen (N P ) or graphitic nitrogen (N G ) on NCs is still under debate. Herein, we demonstrated that the ORR activity of NCs in alkaline media depended on the electronic synergism between N P and N G , rather than any single type of N. We measured the transferable electrons of NCs by absorption spectroscopy using 7'7'8'8-tetracyanoquinodimethane as an electron acceptor. The transferable electron amount of NCs is relevant to either N P or N G , leading to the [N P ] : [N G ] ratio as an electron transfer descriptor of NCs in a reverse volcano curve manner across nineteen NCs. The similar dependence of ORR activity on the [N P ] : [N G ] ratio of NCs was also discovered, demonstrating the synergistic effect of N P and N G . These results provide a new angle to understand the nature of ORR activity of NCs and optimize the ORR catalyst.