Jun NAKAMURA (中村 淳)

<p>This research used solid waste from sugarcane production, named bagasse, as raw material for a functional carbon electrode. The bagasse was carbonized to produce carbon powder and, following activation with water vapor at 700°C. The activated carbon was doped with N and S to improve its electrochemical properties by treating it with thiourea precursor and heating it at 850°C under nitrogen flow to produce N/S-doped carbon (NSCE). The produced carbon was then characterized to understand the specific diffraction pattern, molecular vibrations, and surface morphology. The result found that the NSCE showed two broad diffraction peaks at 23° and 43°, corresponding to [002] and [100] crystal planes following JCPDS75-1621. FTIR spectra showed some O–H, C–H, C–O, and C=C peaks. Peaks of C=N, C–N, and S–H demonstrate the presence of N/S within the NSCE. Raman analysis revealed that N/S doping caused structure defects within the single C6 layer networks, providing carbon vacancies (<inline-formula><mml:math id="m1" display="inline"><mml:semantics id="sm1"><mml:mrow><mml:msubsup><mml:mrow><mml:mi>V</mml:mi></mml:mrow><mml:mi>C</mml:mi><mml:mrow><mml:mo>•</mml:mo><mml:mo>•</mml:mo><mml:mo>•</mml:mo><mml:mo>•</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:semantics></mml:math></inline-formula>) because of C replacement by <inline-formula><mml:math id="m2" display="inline"><mml:semantics id="sm2"><mml:mrow><mml:mtext>N </mml:mtext><mml:mo stretchy="false">(</mml:mo><mml:msubsup><mml:mrow><mml:mi>N</mml:mi></mml:mrow><mml:mi>C</mml:mi><mml:mo>′</mml:mo></mml:msubsup><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:semantics></mml:math></inline-formula> and <inline-formula><mml:math id="m3" display="inline"><mml:semantics id="sm3"><mml:mrow><mml:mtext>S </mml:mtext><mml:mo stretchy="false">(</mml:mo><mml:msubsup><mml:mrow><mml:mi>S</mml:mi></mml:mrow><mml:mi>C</mml:mi><mml:mo>″</mml:mo></mml:msubsup><mml:mo stretchy="false">)</mml:mo></mml:mrow></mml:semantics></mml:math></inline-formula>. Meanwhile, XPS analysis showed N/S introduction to the C network by revealing peaks at 168.26 eV and 169.55 eV, corresponding to S2p_3/2 and S2p_1/2, and 171.95 eV corresponds to C–SO₃–C, indicating the presence of S within the thiol group attached to the carbon. Meanwhile, N1s are revealed at 402.4 eV and 405.5 eV, confirming pyrrolic nitrogen (N-5) and quaternary nitrogen (N-Q). The electrochemical analysis found that the reaction within the prepared-NSCE/NaClO₄/Na was reversible, with an onset potential of 0.1 V vs. Na/Na⁺, explaining the intercalation and deintercalation of sodium ions. The sodium battery full cell showed an excellent battery performance with an initial charging-discharging capacity of 720 mAh g⁻¹ and 570 mAh g⁻¹, respectively, at 0.2C. Meanwhile, a cycling test showed the average Coulombic efficiency of 84.4% and capacity retention of 57% after 50 cycles.</p>

Abstract

We present a combined experimental and theoretical study of the Se-treated GaAs(001)-($$2\times 1$$) surface. The ($$2\times 1$$) structure with the two-fold coordinated Se atom at the outermost layer and the three-fold coordinated Se atom at the third layer was found to be energetically stable and agrees well with the experimental data from scanning tunneling microscopy, low energy electron diffraction, and x-ray photoelectron spectroscopy. This atomic geometry accounts for the improved stability of the Se-treated surface against the oxidation. The present result allows us to address a long-standing question on the structure of the Se-passivated GaAs surface, and will leads us to a more complete understanding of the physical origin of the electrical and chemical passivation of Se-treated GaAs surface.