Page - 138 - in Photovoltaic Materials and Electronic Devices

Figure S1) on the degradation of methyl orange (MO) under visible-light irradiation (ě420 nm). Before irradiation, the adsorption-desorption equilibrium of MO in the dark is established within 30 min over different samples. The time-dependent absorbancespectraofdifferentsamplesareshowninFigureS1. Theadsorptionof BiOI/TiO2-C30 increasessignificantlycomparedtoothersamples,whichmightbe attributedtothehighspecificsurfacearea. After3hirradiation,thephotodegradation efficiencies of MO for BiOI/TiO2-C30 are about 92%, in comparison to 60%, 66%, 38% and almost none for M-BT, BiOI/TiO2-C20, BiOI/TiO2-C10 and TiO2 nanofibers, respectively. InFigure7b,thekineticlinearfittingcurvesoverdifferentphotocatalysts show that the photocatalytic degradation of MO followed a Langmuir-Hinshelwood apparentfirst-orderkineticsmodel: InC{C0 “´kKt“´kappt (3) where C0 is the initial concentration (mg/L) of the reactant; C is the concentration (mg/L); t is the visible-light irradiation time; k is the reaction rate constant (mg/(L¨min)); and K is the adsorption coefficient of the reactant (L/mg); kapp is the apparent first-order rate constant (min´1). The kapp of different samples are shown in Table 1. It is indicated that the photocatalytic activities is in the order of BiOI/TiO2-C30 > BiOI/TiO2-C20 > M-BT > BiOI/TiO2-C10 > TiO2. The above illuminates that the construction of p-n heterojunctions can effectively enhance the photocatalytic properties. Furthermore, the increased of the specific surface area and the amount of p-n heterojuctions obviously enhance the photocatalytic activity. Furthermore, the photocatalysis under UV-light irradiation (Figure S2) also demonstrates theabovepoint. To understand the photocatalytic properties of p-BiOI/n-TiO2 NFs, a schematic diagram is proposed (Scheme 1). When p-type BiOI contacts n-type TiO2, the diffusion of electrons and holes create an inner electric field where a space-charge region is formed at the interfaces of p-n heterojunction. Under visible-light irradiation, the photogenerated electrons transfer from the conduction band of BiOI to that of TiO2, while the photogenerated holes stay at the valence band of BiOI.Therecombinationofphotogeneratedchargecarrier is inhibitedgreatly in the heterojunctionsofp-BiOI/n-TiO2 NFs. Thus, thephotogeneratedelectronsandholes caneffectively takepart in thephotodegradationofMOundervisible light. Onthe otherhand, thenanofiberstructuresofTiO2 canprevent theagglomerationofBiOI nanosheets and facilitate the transfer of the dye molecules during photocatalytic process. Moreover, the exposed surface of BiOI is mainly {001} facet, which is very active for photocatalytic reactions under visible-light irradiation [32]. Thus, the nanosheet structure of BiOI might also improve the surface reaction rates and contributetothephotocatalysis. Itshouldbenotedthatthep-BiOI/n-TiO2 NFscanbe 138