Page - 136 - in Photovoltaic Materials and Electronic Devices

isanobviously increasedadsorptionathighpressurewith increaseddepositionof BiOInanosheetsonTiO2 nanofibers forBiOI/TiO2-C20andBiOI/TiO2-C30,along withincreasedspecificsurfaceareas, indicatingthemoreandmoreabundantporosity structures. It isacceptedthat theporosity isrelativetotheamountofBiOInanosheets depositing on TiO2 nanofibers. Hence, the close arrangements of BiOI nanosheets on TiO2 nanofibers (see SEM and TEM images) have resulted in the hierarchical porosity with wide pore size distributions, which are further confirmed by the correspondingporesizedistributions in the insetofFigure5. Theseresults suggest that the BiOI/TiO2 nanofibers with abundant porosity and large specific surface areas will increase the assessable surface areas of the catalyst with dye solution to achieve good photocatalytic activity. Particularly, the large amount of BiOI nanosheets depositing on TiO2 nanofibers without independent nucleation will benefit the formation of more p-n heterojunctions as well as rapid charge transfer duringthephotocatalysis. Materials  2016,  9,  90  6  of  12    Figure  5.  Typical  N2  gas  adsorption  desorption  isotherms  of  different  samples  and  their  corresponding  pore‐size  distributions  (inset).  Table  1.  SILAR  cycles,  BET  specific  surface  area  and  photocatalysis  reaction  rates  of  different  samples.  Samples Cycles BET  Specific  Surface  Area  (m2/g) Kapp  (h−1)  TiO2  NFs  0  15.88  0.000  ±  0.000  BiOI/TiO2‐C10  10  7.27  0.141  ±  0.002  BiOI/TiO2‐C20  20  19.19  0.197  ±  0.009  BiOI/TiO2‐C30  30  38.44  0.724  ±  0.095  M‐BT  (Bi:Ti  =  0.4:1)  ‐  ‐  0.267  ±  0.024  2.5.  Optical  Properties  Figure  6  shows  the  UV‐vis  absorption  spectra  of  TiO2,  BiOI/TiO2‐C10,  BiOI/TiO2‐C20,  BiOI/TiO2‐C30  and  BiOI  converted  from  corresponding  diffuse  reflectance  spectra  by  means  of  the  Kubelka–Munk  function  [28]:  F(R)=(1‐R)2/2R=α/S  (1) R=RSample/RBaSO4  (2) where  R,  α,  and  S  are  the  reflectance,  absorption  coefficient  and  scattering  coefficient,  respectively.  It  can  be  seen  that  TiO2  exhibited  a  typical  absorption  characteristic  of  the  wide  band  gap  semiconductor  with  an  edge  about  380  nm,  while  pure  BiOI  with  a  strong  absorption  at  about    630  nm  in  the  visible  light  region,  indicates  that  it  is  a  narrow  band  gap  semiconductor  according  to  the  equation  Eg  =  1240/λ,  where  Eg  is  the  band  gap  (eV)  and  λ  (nm)  is  the  wavelength  of  the  absorption  edge  in  the  spectrum.  The  band  gap  of  TiO2  and  BiOI  are  estimated  to  be  3.2  eV  and    1.9  eV,  respectively.  It  is  noted  that  the  absorption  edge  of  p‐BiOI/n‐TiO2  NFs  show  significant  red‐shift  from  393  to  500  nm  with  the  increased  amount  of  BiOI  in  the  composite  nanofibers.  Based  on  the  above,  the  increased  amount  of  BiOI  in  p‐BiOI/n‐TiO2  NFs  extends  light  absorbing  range,  Figure5. Typical N2 gas adsorption desorption isotherms of different samples and theircorrespondingpore- izedistributions (inset). Table1. SILARcycles,BETspecificsurfaceareaandphotocatalysis reactionrates ofdifferentsamples. Samples Cycles BETSpecificSurfaceArea(m2/g) Kapp (h´1) TiO2 NFs 0 15.88 0.000˘0.000 BiOI/TiO2-C10 10 7.27 0.141˘0.002 BiOI/TiO2-C20 20 19.19 0.197˘0.009 BiOI/TiO2-C30 30 38.44 0.724˘0.095 M-BT(Bi:Ti= 0.4:1) - - 0.267˘0.024 136