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Jung, Jin-Young,Yu, Jin-Young,Wehrspohn, Ralf B.,Lee, Jung-Ho American Chemical Society 2019 The Journal of Physical Chemistry Part C Vol.123 No.3
<P>Photoelectrochemical (PEC) water splitting performed by an electrocatalyst integrated with a semiconducting photoelectrode is advantageous with improvements in both charge-transfer kinetics and interface energetics because of the electrocatalyst/semiconductor junction. In general, interface energetics has been considered to arise from differences in the intrinsic electronic energy levels between the electrocatalyst and the semiconductor. Here, we demonstrated that when a NiO<SUB><I>x</I></SUB> thin film with porous and nanocrystalline structures is integrated with a Si photoelectrode, the interface energetics is developed by an electrochemical energy level extrinsically formed by the pseudocapacitive surface reaction (a redox reaction of NiO<SUB><I>x</I></SUB> for electrochemical charge storage). This new type of junction, named a pseudocapacitive NiO<SUB><I>x</I></SUB>/Si junction, revealed two intriguing features: the interface energetics is dynamically changed as charging/discharging progresses, and the developed electrochemical energy level and the electronic energy level of Si are abnormally misaligned under equivalent circuit conditions. With these features, the open circuit potential (<I>V</I><SUB>oc</SUB>) of the PEC device was determined by the degree of misalignment (i.e., the electrochemical energy level). The electrochemical energy level was maximized by ∼1 V through the insertion of a SiO<SUB>2</SUB> interfacial layer thick enough to suppress discharge and 1 h of PEC operation for sufficient charging by the transfer of light-induced electrons. As a result, the highest <I>V</I><SUB>oc</SUB> of ∼1 V, surpassing the theoretical limit of 0.85 V in Si photovoltaics, was achieved. This finding demonstrated a new paradigm for self-powered PEC reactions.</P> [FIG OMISSION]</BR>
Photoelectrochemical hydrogen evolution of tapered silicon nanowires
Li, Xiaopeng,Xiao, Yanjun,Zhou, Keya,Wang, Junna,Schweizer, Stefan L.,Sprafke, Alexander,Lee, Jung-Ho,Wehrspohn, Ralf B. The Royal Society of Chemistry 2015 Physical chemistry chemical physics Vol.17 No.2
<P>The origin of the photocurrent enhancement and the overpotential reduction in solar water splitting employing nanostructured silicon is still a matter of debate. A set of tapered Si nanowires (SiNWs) has been designed for clarifying the impact of nanostructured Si on the hydrogen evolution reaction (HER) while precisely tailoring several interference factors such as surface area, light absorption and surface defect density. We find that defect passivation by KOH achieved by tapering is much more beneficial than the optical gain. Surfactant-mediated modification of SiNWs is capable of engineering the band structure. As a result, we suggest a guideline for nanostructured Si photoelectrodes optimized for the HER.</P> <P>Graphic Abstract</P><P>The photoelectrochemical performance of Si nanowires can be greatly improved both in photocurrent and photovoltage through a tapering process. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4cp04396k'> </P>
Li, Xiaopeng,Xiao, Yanjun,Bang, Jin Ho,Lausch, Dominik,Meyer, Sylke,Miclea, Paul‐,Tiberiu,Jung, Jin‐,Young,Schweizer, Stefan L.,Lee, Jung‐,Ho,Wehrspohn, Ralf B. WILEY‐VCH Verlag 2013 ADVANCED MATERIALS Vol.25 No.23
<P><B>Through metal‐assisted chemical etching (MaCE), superior purification of dirty Si is observed</B>, from 99.74 to 99.9884% for metallurgical Si and from 99.999772 to 99.999899% for upgraded metallurgical Si. In addition, large area of silicon nanowires (SiNW) are fabricated. The purification effect induces a ∼35% increase in photocurrent for SiNW based photoelectrochemical cell.</P>