Semiconductor Electrodes XXXIII . Photoelectrochernistry of n - Type WSe in Acetonitrile

The photoe lee t rochemica l behavior of n t ype WSe2 single c rys ta l e lectrodes in acetoni t r i le solut ions containing severa l redox couples (I/Is, B r / B r 2 , C1-/Ct2, th i an th rene ~ R u ( b p y ) 8 2+/a+ was invest igated. Elect rodes wi th discontinui t ies in the van der Waals ' surface show la rge da rk currents a n d recombina t ion of electrons wi th photo-oxid ized solution species. P r e t r e a t m e n t of such a surface wi th C1passivates these d a r k active sites and increases the pho tocur ren t density. The observed photopotent ia ls a t WSe2 for redox couples wi th potent ials , Vredox, more posi t ive than 0.5V vs. SCE show behavior cons is tent wi th F e r m i level pinning; the onset potent ia l of photocur ren ts increases l inea r ly wi th increas ing Vredox while the photopoten t ia l remains constant. The energy posi t ion at which pinning occurs depends on the densi ty of surface states and the concentra t ion of solut ion species. The character is t ics of a photovol ta ie cell based on the n W S e 2 / C l , C12/P t sys tem is also d e scr ibed . The per formance of photoelec t rochemical devices for conversion of solar to chemical and e lect r ica l energy can be cr i t ica l ly l imi ted bY processes occurr ing via in te rmedia te energy levels at the semiconductor / l iqu id in ter face (surface s ta tes) . Surface recombina t ion and Fe rmi level pinning are two such processes tha t gene ra l ly are de t r imen ta l to efficient energy conversion. In e i ther case the photoelec t rochemical resul ts a re somewhat different f rom expecta t ions based on the ideal ized model (1) of the semiconductor e lec t rode / solut ion interface. A number of invest igat ions of the dependence of potent ia l d i s t r ibu t ion (2) and charge t ransfe r kinet ics (3) on surface s tates have been car r ied out. The surface p r e t r e a t m e n t or modification procedures can influence the d is t r ibut ion and dens i ty of surface states a t energies located wi th in the bandgap and thus cause changes in the e lec t rochemical behavior (4, 5). Recently, Tr ibutsch and co-workers have in t roduced photoelec t rochemical cells based on l a y e r t y p e t r an si t ion meta l dichalcogenides (5). These and la te r invest igat ions (6-11) have shown tha t the observed photopotent ia ls in aqueous (5f, 9) and nonaqueous (6-8) solutions are smal le r than the expectat ions based on the ideal junc t ion model. Lewerenz (~0) and Tr i butsch (11) and co-workers have also recent ly invest iga ted the role of surface morphology on conversion efficiencies of l aye red semiconductors in aqueous solutions. We repor t here an inves t igat ion of n t y p e WSe2 electrodes in acetoni t r i le (MeCN) solutions. The effect of the na ture of the surface and p r e t r ea tme n t procedures on the e lec t rochemical response and a photovol ta ic cell based on the photogenera t ion of ch lor ine at n t y p e WSe2 immersed in an MeCN solut ion conta ining C1-/C12 is described. Experimental Semiconductor e~ectrodes.--N-type WSe2 single crystals were generous ly dona ted b y Dr. Ba r ry Mil ler and Dr. F r a n k DiSalvo of Bell Laborator ies . Elect r ica l contacts were made to the back of each crys ta l by rubb ing I n / G a a l loy into the crys ta l to which a copper lead was contacted wi th s i lver epoxy cement (Al l ied P r o d ucts Corporat ion, New Haven, Connect icut) . A clean new crys ta l face ( • C-axis) was exposed b y st icking adhesive tape on the f ront side and gent ly pul l ing off the top surface layer . To ob ta in e lect rodes wi th only * Electrochemical Society Active Member.