Modulating Surface Redox Reactions and Solvated Electron Emission on Boron-Doped Diamond by (Photo)electrochemistry

The interplay between photochemical and electrochemical reactions fundamentally influences charge transfer processes at solid-liquid interfaces. Nevertheless, chemical processes at semiconductor surfaces triggered by light excitation under an applied potential remain poorly explored. This work deciphers the synergistic effect of potential and light excitation on boron-doped diamond electrodes in producing either surface redox reactions or emission of solvated electrons in water. The effect of diamond surface termination on electron affinity, band bending, and charge extraction is identified in a photoelectrochemical cell. While photocurrent is observed for excitation as low as 3.5 eV, we show that it is induced mostly by surface redox reactions, whereas solvated electrons are detected only for excitation above the band gap (5.47 eV). Solvated electrons are generated irrespective of band bending, which affects only the emission yield. Depending on the surface band bending, photoreduction of the hydroxylated surface groups and photooxidation of the C—<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mrow><a:mi mathvariant="normal">H</a:mi></a:mrow></a:mrow></a:math> surface groups can be induced by direct photoexcitation in the range of 4.2–4.8 eV. The surface of the diamond can be electrochemically reduced when the Fermi level of the oxidized surface decreases below the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline"><d:msup><d:mrow><d:mrow><d:mi mathvariant="normal">H</d:mi></d:mrow></d:mrow><d:mo>+</d:mo></d:msup></d:math>/<g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:msub><g:mrow><g:mrow><g:mi mathvariant="normal">H</g:mi></g:mrow></g:mrow><g:mn>2</g:mn></g:msub></g:math> redox potential. On the other hand, the hydrogenated surface oxidizes spontaneously for potentials at which the Fermi level drops below the occupied <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mstyle displaystyle="false" scriptlevel="0"><j:mtext>C---H</j:mtext></j:mstyle></j:math> surface states, depending on both the pH and the electron affinity of the surface. This work provides insights into (photo)redox processes on diamond materials, which may find applications in photoelectrochemical solar fuel generation or energy storage.